Anatomy Chapter 12

The refractory period is the time after an action potential during which a neuron cannot generate another action potential in response to a normal stimulus. It includes the absolute refractory period, where no stimulus can trigger another action potential, and the relative refractory period, where a stronger-than-normal stimulus is required.

Large-diameter axons have a shorter absolute refractory period (about 0.4 msec), allowing for a higher frequency of impulses (up to 1000 per second), while small-diameter axons have longer refractory periods (up to 4 msec), limiting them to a maximum of 250 impulses per second.

Propagation of action potentials allows for the transmission of information along the neuron from the trigger zone to the axon terminals without decrement, maintaining the strength of the signal as it travels.

1. Resting State: All voltage-gated Na+ and K+ channels are closed. The membrane is at resting potential with negative charges inside and positive charges outside.

2. Depolarizing Phase: Na+ channels open when the threshold is reached, allowing Na+ ions to flow in, causing depolarization.

3. Repolarizing Phase Begins: Na+ channels close and K+ channels open, allowing K+ ions to flow out, starting repolarization.

4. Repolarization Phase Continues: Continued K+ outflow restores resting potential, closing K+ channels returns the membrane to resting state.

The depolarizing phase is initiated when the membrane potential of the axon reaches threshold, causing the activation gates of Na+ channels to open and allowing Na+ ions to flow into the neuron, resulting in a buildup of positive charges inside the membrane.

During the repolarizing phase, the inactivation gates of Na+ channels close and K+ channels open, allowing K+ ions to exit the neuron. This outflow of K+ ions leads to a buildup of negative charges inside the membrane, contributing to the return to resting membrane potential.

The outflow of K+ ions during the repolarization phase increases the negative charge inside the neuron, which helps to restore the resting membrane potential. As more K+ ions leave, the membrane potential decreases back to approximately -70 mV, returning to the resting state.

Action potentials propagate along a neuron by regenerating at adjacent segments of the membrane. When sodium ions flow in, they open voltage-gated Na+ channels in neighboring areas, allowing the action potential to travel in one direction toward the axon terminals. This process is influenced by the absolute refractory period, which prevents the action potential from propagating back toward the cell body.

Neurotoxins such as tetrodotoxin (TTX) block action potentials by inserting themselves into voltage-gated Na+ channels, preventing them from opening. This blockage stops the propagation of action potentials, effectively disrupting communication in the nervous system.

Nervous tissue generates nerve impulses (action potentials) that facilitate communication and regulation of body organs, helping to maintain controlled conditions essential for life.

Both systems aim to keep controlled conditions within life-sustaining limits. The nervous system responds rapidly using nerve impulses, while the endocrine system responds by releasing hormones.

The nervous system is responsible for perceptions, behaviors, memories, and initiating all voluntary movements.

The two main types of cells are neurons (nerve cells) and neuroglia (supporting cells for neurons).

The three basic functions of the nervous system are:

1. Sensory function - Detects internal and external stimuli.
2. Integrative function - Processes sensory information and makes decisions for responses.
3. Motor function - Activates effectors (muscles and glands) to elicit appropriate responses.

The central nervous system (CNS) processes incoming sensory information and is the source of thoughts, emotions, and memories. It consists of the brain and spinal cord.

The somatic nervous system (SNS) conveys output from the CNS to skeletal muscles and is under voluntary control, while the autonomic nervous system (ANS) conveys output to smooth muscle, cardiac muscle, and glands, and is generally involuntary.

The two main branches of the autonomic nervous system (ANS) are the sympathetic nervous system and the parasympathetic nervous system. The sympathetic system prepares the body for 'fight or flight' responses, while the parasympathetic system promotes 'rest and digest' activities.

The sensory function of the nervous system operates by detecting internal and external stimuli through sensory receptors, which then relay this information to the brain and spinal cord via cranial and spinal nerves.

The enteric nervous system (ENS) regulates the activity of the smooth muscle and glands of the gastrointestinal (GI) tract and can function independently while also communicating with the other branches of the autonomic nervous system.

The two main subdivisions of the nervous system are the central nervous system (CNS), which consists of the brain and spinal cord, and the peripheral nervous system (PNS), which consists of all nervous tissue outside the CNS.

The components of the peripheral nervous system (PNS) include:

1. Cranial nerves
2. Spinal nerves
3. Enteric plexuses in the small intestine
4. Sensory receptors in the skin

The peripheral nervous system (PNS) is divided into two main divisions:

1. Sensory Division

   • Somatic senses
   • Special senses

2. Motor Division

   • Somatic nervous system
   • Autonomic nervous system
     • Sympathetic nervous system
     • Parasympathetic nervous system
     • Enteric nervous system

The central nervous system (CNS) is responsible for:

• Processing sensory information
• Integrating and coordinating motor commands
• Higher functions such as thought, memory, and emotion

Nervous tissue is comprised of neurons and neuroglia.

Neurons provide unique functions such as sensing, thinking, remembering, controlling muscle activity, and regulating glandular secretions.

Neuroglia support neurons by nourishing, protecting, and maintaining the interstitial fluid that bathes them.

The three main parts of a neuron are:

1. Cell body
2. Dendrites
3. Axon

Dendrites are the receiving or input portions of a neuron, containing receptor sites for binding chemical messengers from other cells.

The axon propagates nerve impulses toward another neuron, a muscle fiber, or a gland cell.

The axon hillock is the area where nerve impulses arise and is crucial for initiating the action potential that travels along the axon.

Gray matter primarily consists of neuronal cell bodies, while white matter is composed of myelinated axons that facilitate communication between different brain regions.

Nissl bodies are clusters of rough endoplasmic reticulum and ribosomes in the cell body that are involved in protein synthesis for neuron growth and repair.

The structure of neurons, including their long axons and branching dendrites, allows for efficient signal transmission and complex connections necessary for processing information in the nervous system.

• Dendrites: Receive incoming signals from other neurons and transmit them to the cell body.
• Cell Body (Soma): Integrates the signals received from dendrites and generates an outgoing signal if the threshold is met.
• Axon: Conducts the electrical impulse away from the cell body towards axon terminals, where the signal can be transmitted to other neurons or target cells.

Slow axonal transport:

• Moves materials at a rate of 1-5 mm per day.
• Conveys axoplasm in one direction only, from the cell body to the axon terminals.
• Supplies new axoplasm to developing or regenerating axons.

Fast axonal transport:

• Moves materials at a rate of 200-400 mm per day.
• Uses proteins as 'motors' to transport materials along microtubules in both directions (anterograde and retrograde).
• Anterograde transport moves organelles and synaptic vesicles from the cell body to axon terminals.
• Retrograde transport moves materials from axon terminals back to the cell body for degradation or recycling.

Neurotransmitters are molecules released from synaptic vesicles that can either excite or inhibit another neuron, muscle fiber, or gland cell. Many neurons can contain multiple types of neurotransmitters, each having different effects on the postsynaptic cell.

Neurons are classified as follows:

Multipolar neurons have many processes extending from the cell body, while bipolar neurons have two processes (one dendrite and one axon), and unipolar neurons have a single process that splits into two branches (one toward the periphery and one toward the central nervous system).

Structural diversity in neurons allows for specialized functions in different parts of the nervous system. Neurons vary in size, shape, and dendritic branching patterns, which can influence their roles in neural circuits and communication.

1. Astrocytes: Star-shaped, largest and most numerous; involved in contact with blood capillaries and neurons.

2. Oligodendrocytes: Form myelin sheaths around CNS axons.

3. Microglial Cells: Act as immune defense in the CNS.

4. Ependymal Cells: Line the ventricles of the brain and central canal of the spinal cord, involved in cerebrospinal fluid production.

The three functional classes of neurons are:

1. Sensory Neurons - Transmit signals from sensory receptors to the central nervous system.
2. Interneurons - Process signals within the central nervous system.
3. Motor Neurons - Transmit signals from the central nervous system to effectors (muscles or glands).

Interneurons are responsible for integration of signals within the central nervous system, processing information between sensory and motor neurons.

Astrocytes perform several important functions:

1. Provide structural support to neurons.
2. Maintain the blood-brain barrier by regulating the permeability of endothelial cells.
3. Regulate the growth and migration of neurons during development.
4. Maintain the chemical environment for nerve impulse generation by regulating ion concentrations and neurotransmitter uptake.
5. Potentially influence learning and memory by affecting synapse formation.

Microglial cells function as phagocytes, removing cellular debris formed during normal development of the nervous system and phagocytizing microbes and damaged nervous tissue.

Oligodendrocytes myelinate multiple axons in the CNS, while Schwann cells myelinate a single axon in the PNS. Additionally, Schwann cells can enclose multiple unmyelinated axons, whereas oligodendrocytes do not.

Ependymal cells line the ventricles of the brain and central canal of the spinal cord, producing and assisting in the circulation of cerebrospinal fluid. They also form the blood-cerebrospinal fluid barrier.

The two types of neuroglia in the PNS are Schwann cells and satellite cells. Schwann cells myelinate axons, while satellite cells surround the cell bodies of neurons in PNS ganglia, providing structural support and regulating material exchange.

The myelin sheath electrically insulates the axon of a neuron and increases the speed of nerve impulse conduction.

Schwann cells myelinate a single axon segment in the PNS, while oligodendrocytes myelinate parts of several axons in the CNS.

The neurolemma aids regeneration by forming a regeneration tube that guides and stimulates regrowth of the axon.

Nodes of Ranvier are gaps in the myelin sheath that appear at intervals along the axon, found in both the PNS and CNS, but are fewer in number in the CNS.

A ganglion is a cluster of neuronal cell bodies located in the PNS, while a nucleus is a cluster of neuronal cell bodies located in the CNS.

Myelination increases the speed of electrical signal transmission along axons by allowing the action potentials to jump between the nodes of Ranvier, a process known as saltatory conduction. This enhances the efficiency of nerve signal propagation and reduces the energy expenditure required for signal transmission.

Myelinated axons are surrounded by a myelin sheath produced by Schwann cells in the PNS or oligodendrocytes in the CNS, which facilitates faster signal transmission. In contrast, unmyelinated axons are surrounded by a single layer of Schwann cell plasma membrane without the myelin sheath, resulting in slower signal conduction.

A myelinated axon consists of the following components:

1. Axon - the long projection of a neuron that conducts impulses.
2. Myelin sheath - the insulating layer formed by Schwann cells or oligodendrocytes.
3. Neurolemma - the outermost layer of the Schwann cell that surrounds the myelin sheath.
4. Node of Ranvier - the gaps between adjacent myelin sheaths that facilitate rapid signal transmission.

A nerve is a bundle of axons located in the peripheral nervous system (PNS), while a tract is a bundle of axons located in the central nervous system (CNS). Nerves connect the brain and spinal cord to the periphery, whereas tracts interconnect neurons within the brain and spinal cord.

Gray matter is primarily composed of neuronal cell bodies, dendrites, and unmyelinated axons, giving it a gray appearance. White matter, on the other hand, is composed mainly of myelinated axons, which gives it a whitish, glistening appearance due to the myelin sheath.

White matter consists primarily of myelinated axons of many neurons.

Gray matter consists of neuron cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia.

In the spinal cord, white matter surrounds an inner core of gray matter, which is shaped like a butterfly or the letter H in transverse section.

Neurons use graded potentials for short-distance communication and action potentials for long-distance communication.

Ion channels are crucial for the generation of graded potentials and action potentials, allowing for the electrical excitability and communication of neurons.

A neurolemma is the outermost layer of a Schwann cell that surrounds the axon in the peripheral nervous system, playing a key role in regeneration and repair of nerve fibers.

In the nervous system, a nucleus refers to a cluster of neuron cell bodies located within the central nervous system, serving as a functional grouping of neurons.

A nerve action potential is an electrical signal that occurs in a neuron when a graded potential reaches a threshold, leading to the rapid depolarization and repolarization of the neuron's membrane. Graded potentials are small changes in membrane potential that occur in response to stimuli, and if they are strong enough, they can trigger a nerve action potential that travels along the axon to transmit information to the central nervous system (CNS).

Graded potentials develop in sensory receptors when they detect stimuli, such as touch. These potentials can trigger nerve action potentials in sensory neurons, which then transmit the sensory information to the CNS for processing and perception. This process is essential for the relay of sensory stimuli and integrative functions in the nervous system.

When a person touches a pen, the following occurs:

1. A graded potential develops in the sensory receptor in the skin.
2. This graded potential triggers a nerve action potential in the sensory neuron, which travels to the CNS.
3. The action potential causes the release of neurotransmitters at a synapse with an interneuron, stimulating it to form a graded potential.

This sequence demonstrates how graded and action potentials work together to relay sensory information and facilitate perception.

Perception primarily occurs in the cerebral cortex, which processes sensory information and integrates it to form a coherent understanding of the environment.

The axon of the interneuron forms a nerve action potential, which travels along the axon and results in neurotransmitter release at the next synapse with another interneuron.

Neurotransmitters generate a graded potential in a lower motor neuron, which triggers the formation of a nerve action potential and subsequent release of neurotransmitters at neuromuscular junctions with skeletal muscle fibers.

The two basic features are the existence of a resting membrane potential and the presence of specific types of ion channels.

Ion channels allow specific ions to move across the plasma membrane down their electrochemical gradient, creating a flow of electrical current that can change the membrane potential.

The four types of ion channels are:

1. Leak channels - randomly alternate between open and closed positions.
2. Ligand-gated channels - open or close in response to the binding of a ligand.
3. Mechanically-gated channels - open or close in response to mechanical stimulation.
4. Voltage-gated channels - open in response to a change in membrane potential.

Voltage-gated channels are crucial because they participate in the generation and conduction of action potentials in the axons of all types of neurons, allowing for rapid signal transmission.

Mechanically-gated channels are activated by a mechanical stimulus, such as touch.

The resting membrane potential is maintained by:

1. Ion Concentration Gradients: There is a higher concentration of Na+ ions outside the cell and a higher concentration of K+ ions inside the cell.

2. Selective Permeability: The membrane is more permeable to K+ ions than to Na+ ions, allowing more K+ to flow out of the cell, which contributes to the negative charge inside.

3. Sodium-Potassium Pump: This pump actively transports 3 Na+ ions out of the cell and 2 K+ ions into the cell, helping to maintain the concentration gradients.

4. Buildup of Charges: A small buildup of negative ions in the cytosol and positive ions in the extracellular fluid creates an electrical potential difference across the membrane.

A resting membrane potential of -70 mV indicates that the inside of the neuron is 70 millivolts more negative than the outside. This difference in charge is crucial for the neuron's ability to generate action potentials and transmit signals. It reflects the balance of ions across the membrane and the neuron's readiness to respond to stimuli.

1. Unequal distribution of ions in the extracellular fluid (ECF) and cytosol.
2. Inability of most anions to leave the cell.
3. The electrogenic nature of the Na+-K+ ATPase.

The unequal distribution of ions, with more K+ leak channels than Na+ leak channels, causes more K+ ions to diffuse out of the cell than Na+ ions entering, making the inside of the membrane increasingly negative.

If a neuron had more Na+ leak channels than K+ leak channels, the resting membrane potential would become less negative (more positive) because more Na+ ions would enter the cell, reducing the polarization.

Most anions cannot leave the cell because they are attached to nondiffusible molecules like ATP and large proteins. This contributes to the negative resting membrane potential as these trapped anions cannot follow K+ ions out of the cell.

Na+-K+ ATPases help maintain the resting membrane potential by pumping out 3 Na+ ions for every 2 K+ ions they bring into the cell. This process offsets the inward leakage of Na+ and the outward leakage of K+, contributing to the negativity of the resting membrane potential.

The typical resting membrane potential of a neuron is approximately -70 mV.

Hyperpolarizing graded potentials make the membrane potential more negative (e.g., from -70 mV to -80 mV), while depolarizing graded potentials make the membrane potential less negative (e.g., from -70 mV to -60 mV).

Graded potentials vary in amplitude depending on the strength of the stimulus, which influences how many ligand-gated or mechanically-gated channels open or close and how long they remain open.

Decremental conduction refers to the mode of travel of graded potentials, where the electrical signal spreads to adjacent regions along the plasma membrane for a short distance and gradually dies out as charges are lost through leak channels.

Summation is the process by which graded potentials add together, allowing an individual graded potential to become stronger and last longer when combined with other graded potentials.

A mechanical stimulus (pressure) opens mechanically-gated channels, allowing cations (mainly Na+ and Ca2+) to enter the cell, resulting in a depolarizing graded potential.

When acetylcholine binds to ligand-gated channels, it opens cation channels that allow Na+, K+, and Ca2+ to pass through. The inflow of Na+ is greater than the outflow of K+ or the inflow of Ca2+, leading to a depolarizing graded potential.

Glycine binds to ligand-gated channels, opening Cl- channels that allow Cl- ions to enter the cell, resulting in a hyperpolarizing graded potential.

A hyperpolarizing graded potential is a change in the membrane potential that makes the inside of the cell more negative, moving it further away from the threshold for action potential generation.

A depolarizing graded potential is a change in the membrane potential that makes the inside of the cell less negative (more positive), moving it closer to the threshold for action potential generation.

The two types of graded potentials are:

1. Postsynaptic potentials - occur in the dendrites or cell body of a neuron in response to neurotransmitters.
2. Receptor potentials - occur in sensory receptors in response to stimuli.

The amplitude of a graded potential increases with greater stimulus strength. A stronger stimulus results in a larger graded potential, while a weaker stimulus results in a smaller graded potential.

Summation occurs when two or more graded potentials add together, resulting in a larger amplitude than the individual potentials. This can happen when stimuli of the same strength occur close together in time.

The two main phases of an action potential are the depolarizing phase, where the membrane potential becomes less negative and then positive, and the repolarizing phase, where the membrane potential is restored to the resting state of -70 mV.

The threshold for generating an action potential in many neurons is about -55 mV. This is the level that the membrane potential must reach for an action potential to occur.

The greater the stimulus strength above threshold, the greater the frequency of action potentials generated. However, the amplitude of each action potential remains the same regardless of stimulus intensity.

If a stimulus is subthreshold, it is a weak depolarization that cannot bring the membrane potential to threshold, and therefore, an action potential will not occur.

The phases of an action potential include:

1. Depolarizing phase: Voltage-gated Na+ channels open, causing the membrane potential to become more positive.
2. Repolarizing phase: Voltage-gated K+ channels open, allowing K+ to exit the cell, returning the membrane potential to a negative value.
3. After-hyperpolarizing phase: The membrane potential temporarily becomes more negative than the resting potential due to prolonged K+ channel opening.

During the depolarizing phase, voltage-gated Na+ channels are open. During the repolarizing phase, voltage-gated K+ channels are open while Na+ channels are inactivating.

No, an action potential will not occur in response to a hyperpolarizing graded potential. This is because hyperpolarization makes the membrane potential more negative, moving it further away from the threshold required to trigger an action potential.

The all-or-none principle states that an action potential either occurs completely or not at all, similar to how pushing the first domino in a row causes all dominoes to fall or none at all, depending on whether the push is strong enough to reach the threshold.

During the depolarizing phase, voltage-gated Na+ channels open rapidly, allowing Na+ to flow into the cell, causing the membrane potential to change from -55 mV to +30 mV due to the influx of Na+ ions.

Voltage-gated Na+ channels have two gates: an activation gate that opens at threshold and an inactivation gate that closes shortly after. This allows Na+ to flow into the cell, contributing to the depolarization of the membrane.

During the repolarizing phase, voltage-gated K+ channels open more slowly than Na+ channels, allowing K+ to flow out of the cell, which helps to return the membrane potential from +30 mV back to -70 mV.

The speed of action potential propagation is influenced by three main factors:

1. Amount of myelination: Myelinated axons propagate action potentials faster than unmyelinated ones.
2. Axon diameter: Larger diameter axons conduct action potentials more quickly due to increased surface area.
3. Temperature: Cooler temperatures slow down the propagation of action potentials.

The speed of propagation of an action potential is determined by:

1. Myelination: Myelinated axons conduct impulses faster than unmyelinated axons.
2. Axon diameter: Larger diameter axons (like A and B fibers) conduct impulses faster than smaller diameter axons (like C fibers).
3. Temperature: Higher temperatures can increase the speed of conduction.

The frequency of action potentials is crucial for detecting stimulus intensity:

• A light touch generates a low frequency of action potentials.
• A firmer pressure elicits action potentials at a higher frequency.
  Additionally, the number of sensory neurons activated by the stimulus also contributes to the perception of intensity, with firmer pressure activating more neurons than a light touch.

Myelination increases the speed of propagation of an action potential significantly. Action potentials propagate faster along myelinated axons compared to unmyelinated axons due to saltatory conduction, where the impulse jumps from one node of Ranvier to the next, allowing for quicker transmission of signals.

• Presynaptic neuron: The nerve cell that carries a nerve impulse toward a synapse and sends a signal.
• Postsynaptic neuron: The cell that receives the signal, which may be another nerve cell or an effector cell (muscle or glandular cell) that responds to the impulse at the synapse.

Most synapses between neurons are classified as:

• Axodendritic: From axon to dendrite.
• Axosomatic: From axon to cell body.
• Axoaxonic: From axon to axon.

Additionally, synapses can be either electrical or chemical, differing in structure and function.

Synapses are essential for homeostasis as they allow for the filtering and integration of information. They play a crucial role in communication between neurons, enabling processes such as learning and memory.

A synapse is a junction between one neuron and another neuron or an effector cell, allowing for communication. Neurons communicate at synapses by transmitting signals through either electrical or chemical means, which can be influenced by various factors including learning and neurological disorders.

The two main types of synapses are:

1. Electrical Synapses:

   • Action potentials conduct directly through gap junctions.
   • Advantages include:
     • Faster communication due to direct conduction.
     • Synchronization of neuron or muscle fiber activity.

2. Chemical Synapses:

   • Separated by a synaptic cleft filled with interstitial fluid.
   • Involves the release of neurotransmitters that bind to receptors on the postsynaptic neuron, converting electrical signals into chemical signals and vice versa. This process is slower due to a synaptic delay of about 0.5 msec.

The advantages of electrical synapses include:

1. Faster communication: Action potentials pass directly from the presynaptic to the postsynaptic cell through gap junctions, resulting in quicker signal transmission.
2. Synchronization: They can coordinate the activity of multiple neurons or muscle fibers, allowing for simultaneous action potentials, which is crucial for functions like heartbeat regulation and gastrointestinal movement.

In chemical synapses, the presynaptic neuron releases a neurotransmitter in response to a nerve impulse. This neurotransmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic neuron, generating a postsynaptic potential. This process involves converting an electrical signal (nerve impulse) into a chemical signal (neurotransmitter) and then back into an electrical signal (postsynaptic potential).

A presynaptic neuron converts an electrical signal into a chemical signal through the release of neurotransmitter molecules via exocytosis of synaptic vesicles when a nerve impulse arrives at the synaptic end bulb. This process involves the opening of voltage-gated calcium channels, allowing calcium ions to flow in, which triggers the exocytosis of neurotransmitters into the synaptic cleft.

A postsynaptic neuron converts a chemical signal back into an electrical signal by the binding of neurotransmitter molecules to receptors in its plasma membrane. This binding opens ligand-gated channels, allowing ions to flow across the membrane, creating a postsynaptic potential.

1. A nerve impulse arrives at the synaptic end bulb of a presynaptic axon.

2. The depolarizing phase of the nerve impulse opens voltage-gated Ca2+ channels, allowing Ca2+ to flow inward.

3. The increase in Ca2+ concentration triggers exocytosis of synaptic vesicles, releasing neurotransmitters into the synaptic cleft.

4. Neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic neuron's plasma membrane.

5. Binding of neurotransmitters opens ligand-gated channels, allowing ions to flow across the postsynaptic membrane, creating a postsynaptic potential.

Electrical synapses can transmit signals in both directions because they involve direct electrical coupling between neurons through gap junctions, allowing ions to flow freely between cells. In contrast, chemical synapses involve the release of neurotransmitters from the presynaptic neuron to the postsynaptic neuron, which only allows signal transmission in one direction, from presynaptic to postsynaptic neuron.

A postsynaptic potential is a change in membrane voltage that occurs when ions flow through opened channels. It can be a depolarization (excitation) if Na+ channels open, allowing Na+ to enter the cell, or a hyperpolarization (inhibition) if Cl- or K+ channels open, making the inside of the cell more negative.

An action potential in the postsynaptic neuron is triggered when a depolarizing postsynaptic potential reaches the threshold level.

• Excitatory postsynaptic potential (EPSP): Caused by a neurotransmitter that depolarizes the postsynaptic membrane, bringing it closer to threshold.
• Inhibitory postsynaptic potential (IPSP): Caused by a neurotransmitter that hyperpolarizes the postsynaptic membrane, making it more difficult to generate an action potential.

Neurotransmitter receptors are classified as either ionotropic receptors or metabotropic receptors based on whether the neurotransmitter binding site and the ion channel are components of the same protein or different proteins.

Ionotropic receptors contain a neurotransmitter binding site and an ion channel as part of the same protein. When the neurotransmitter binds, the ion channel opens, leading to either an EPSP or IPSP in the postsynaptic cell.

Metabotropic receptors have a neurotransmitter binding site but lack an ion channel as part of their structure. They are coupled to a separate ion channel via a G protein, which can open or close the ion channel indirectly through second messengers.

The same neurotransmitter can be excitatory at some synapses and inhibitory at others, depending on the type of receptor it binds to. For example, acetylcholine (ACh) can generate EPSPs at excitatory synapses and bind to metabotropic receptors at inhibitory synapses.

The ionotropic acetylcholine receptor generates an EPSP by binding acetylcholine (ACh), which opens a cation channel. This allows the influx of sodium (Na+) and calcium (Ca²+) ions into the cell, leading to depolarization and the generation of an EPSP.

The ionotropic GABA receptor produces an IPSP by binding GABA, which opens a chloride (Cl-) channel. This allows chloride ions to enter the cell, leading to hyperpolarization and the generation of an IPSP.

In the metabotropic acetylcholine receptor, the G protein is activated upon binding of acetylcholine (ACh). This activation leads to the opening of a potassium (K+) channel, allowing potassium ions to exit the cell, which results in an inhibitory postsynaptic potential (IPSP).

Ionotropic receptors contain a neurotransmitter binding site and an ion channel that opens directly upon binding, while metabotropic receptors have a binding site but are coupled to a separate ion channel via a G protein, which mediates a slower, more prolonged response.

Acetylcholine (ACh) can be excitatory or inhibitory depending on the type of receptor it binds to. At ionotropic acetylcholine receptors, ACh typically generates an EPSP, while at metabotropic acetylcholine receptors, it can lead to an IPSP by activating K+ channels.

1. Diffusion: Neurotransmitter molecules diffuse away from the synaptic cleft.
2. Enzymatic degradation: Certain neurotransmitters are inactivated by enzymes, such as acetylcholinesterase breaking down acetylcholine.
3. Uptake by cells: Neurotransmitters are actively transported back into the releasing neuron (reuptake) or into neighboring neuroglia (uptake).

• Spatial Summation: Involves the summation of postsynaptic potentials from stimuli occurring at different locations on the membrane of a postsynaptic cell simultaneously.
• Temporal Summation: Involves the summation of postsynaptic potentials from stimuli applied to the same axon in rapid succession, causing overlapping EPSPs that sum over time.

When the summation of EPSPs reaches the threshold, one or more nerve impulses (action potentials) arise in the postsynaptic neuron.

This is an example of spatial summation because it involves simultaneous stimulation from multiple presynaptic neurons.

Temporal summation is the summation of postsynaptic potentials that occur at the same location on the postsynaptic membrane but at different times. It results from the buildup of neurotransmitter released by a single presynaptic neuron in rapid succession, allowing for a greater chance of reaching the threshold for an action potential.

The generation of an action potential in a postsynaptic neuron is determined by the net summation of excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs). If the total excitatory effects exceed the inhibitory effects and reach the threshold level, an action potential will occur.

When the total excitatory effects are greater than the total inhibitory effects but below the threshold level, an excitatory postsynaptic potential (EPSP) occurs. This partial depolarization makes it easier for subsequent stimuli to generate a nerve impulse through summation.

Inhibitory postsynaptic potentials (IPSPs) occur when the total inhibitory effects exceed the excitatory effects, leading to hyperpolarization of the postsynaptic membrane. This results in inhibition of the neuron and prevents the generation of a nerve impulse.

Strychnine poisoning blocks glycine receptors, disrupting the balance between excitation and inhibition in the central nervous system. This leads to uncontrolled nerve impulses in motor neurons, causing sustained contraction of skeletal muscles, including the diaphragm, which can result in suffocation due to inability to inhale.

Similarities:

• Both are changes in the postsynaptic membrane potential due to neurotransmitter binding.
• Both influence the likelihood of generating an action potential.

Differences:

• EPSPs result from excitatory neurotransmitters and lead to depolarization, while IPSPs result from inhibitory neurotransmitters and lead to hyperpolarization.

Action potentials are 'all-or-none' because they either occur fully or not at all once the threshold is reached. In contrast, EPSPs and IPSPs are 'graded' because their magnitude can vary depending on the amount of neurotransmitter released and the number of receptors activated, leading to varying degrees of depolarization or hyperpolarization.

Dendrites receive stimuli through activation of ligand-gated or mechanically-gated ion channels, producing generator or receptor potentials in sensory neurons, and producing excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) in motor neurons and interneurons.

The cell body receives stimuli and produces excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) through activation of ligand-gated ion channels.

The axon hillock serves as a trigger zone in many neurons, integrating EPSPs and IPSPs, and if the sum reaches threshold, it initiates an action potential (nerve impulse).

The axon propagates nerve impulses from the initial segment (or from dendrites of sensory neurons) to axon terminals in a self-regenerating manner, with the impulse amplitude remaining unchanged as it propagates along the axon.

The inflow of Ca²+ caused by the depolarizing phase of a nerve impulse triggers exocytosis of neurotransmitter from synaptic vesicles at axon terminals and synaptic end bulbs (or varicosities).

Neurotransmitters can be divided into two classes based on size: small-molecule neurotransmitters and neuropeptides.

Acetylcholine (ACh) is an excitatory neurotransmitter at some synapses, such as the neuromuscular junction, and an inhibitory neurotransmitter at others, where it binds to metabotropic receptors that open K+ channels, such as in the parasympathetic nervous system.

Glutamate has powerful excitatory effects and is involved in most excitatory neurons in the CNS, communicating via ionotropic receptors that open cation channels, leading to the production of excitatory postsynaptic potentials (EPSPs).

Gamma-aminobutyric acid (GABA) is an important inhibitory neurotransmitter that, when binding to ionotropic receptors, opens Cl- channels, leading to inhibition of postsynaptic neurons.

GABA is the most common inhibitory neurotransmitter in the CNS, found at many brain synapses, and its action is enhanced by antianxiety drugs like diazepam (Valium).

The main biogenic amines prevalent in the nervous system include norepinephrine, epinephrine, dopamine, and serotonin.

Biogenic amines can cause either excitation or inhibition depending on the type of metabotropic receptor present at the synapse.

Catecholamines, which include norepinephrine, dopamine, and epinephrine, are synthesized from the amino acid tyrosine and contain an amino group and a catechol ring.

Dopamine is involved in emotional responses, addictive behaviors, and pleasurable experiences, and it helps regulate skeletal muscle tone and movement.

GABA (Gamma aminobutyric acid) has the structure: H3N+-CH2-CH2-CH2-COO-.

The primary neurotransmitter used in inhibitory synapses in the spinal cord is glycine, along with GABA.

Norepinephrine plays roles in arousal, dreaming, and regulating mood.

Substance P consists of a sequence of 11 amino acids linked by peptide bonds, including arginine (Arg), proline (Pro), lysine (Lys), and others.

The two enzymes that break down catecholamines are catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO).

Serotonin, also known as 5-hydroxy-tryptamine (5-HT), is involved in sensory perception, temperature regulation, control of mood, appetite, and the induction of sleep.

Nitric oxide (NO) is produced by the enzyme nitric oxide synthase (NOS) from the amino acid arginine. It acts as an excitatory neurotransmitter and plays a role in memory and learning. NO is formed on demand and has a brief action due to its high reactivity.

Carbon monoxide (CO) is an excitatory neurotransmitter that is produced as needed and may protect against excess neuronal activity. It is involved in dilation of blood vessels, memory, olfaction, vision, thermoregulation, insulin release, and anti-inflammatory activity.

Neuropeptides are neurotransmitters consisting of 3 to 40 amino acids linked by peptide bonds. They can have excitatory or inhibitory actions depending on the receptor type and are involved in various physiological responses, including pain relief and hormone regulation.

Enkephalins are neuropeptides with a potent analgesic effect that is 200 times stronger than morphine. They are involved in pain relief and are linked to improved memory, euphoria, and regulation of hormones.

Substances can modify neurotransmitter effects by stimulating or inhibiting their synthesis. For example, L-dopa is used in Parkinson's disease to boost dopamine production in affected brain areas.

Angiotensin II stimulates thirst, may regulate blood pressure in the brain, causes vasoconstriction, and promotes the release of aldosterone, which increases the rate of salt and water reabsorption by the kidneys.

Agonists bind to receptors and enhance or mimic the effects of natural neurotransmitters, while antagonists bind to and block neurotransmitter receptors, inhibiting their effects.

Cocaine blocks transporters for dopamine reuptake, allowing dopamine to linger longer in synaptic clefts, which produces excessive stimulation of certain brain regions and results in euphoria.

Divergence in neural circuits occurs when a single presynaptic neuron synapses with several postsynaptic neurons, allowing one neuron to influence multiple neurons simultaneously, amplifying the signal.

Convergence in neural circuits occurs when several presynaptic neurons synapse with a single postsynaptic neuron, allowing for more effective stimulation or inhibition of that postsynaptic neuron.

A neural circuit is a functional group of neurons that processes a specific kind of information.

A diverging circuit allows a single input neuron to branch out and stimulate multiple output neurons, creating a fan-like pattern of signal distribution.

A converging circuit allows multiple input neurons to converge onto a single output neuron, enabling the postsynaptic neuron to receive impulses from several different sources.

A reverberating circuit stimulates a series of neurons in a loop, allowing the output signal to persist for a duration that can last from seconds to hours, contributing to functions like breathing and short-term memory.

A parallel after-discharge circuit involves a single presynaptic neuron stimulating a group of neurons that converge on a common postsynaptic neuron, allowing for rapid and precise output of impulses, useful in activities like mathematical calculations.

The neurolemma plays a crucial role in the regeneration of axons and dendrites in the peripheral nervous system (PNS). It provides a supportive environment for repair by:

1. Facilitating Regeneration: The neurolemma surrounds the axons and dendrites, allowing for the potential repair of damaged nerves if the cell body remains intact.
2. Supporting Schwann Cells: Schwann cells, which are associated with the neurolemma, are essential for myelination and also aid in the regeneration process by guiding the regrowing axon.
3. Preventing Scar Tissue Formation: The presence of the neurolemma helps to prevent rapid scar tissue formation, which can hinder the regeneration process.

Chromatolysis is the destruction of the cell body of a neuron, characterized by granular masses. During this process, the axon distal to the damaged region becomes swollen and breaks into fragments, while the myelin sheath deteriorates. However, the neurolemma remains intact, allowing for potential regeneration.

Wallerian degeneration refers to the degeneration of the distal portion of the axon and myelin sheath following injury. This process involves the breakdown of the axon and myelin sheath, while the neurolemma remains, allowing for potential regeneration of the axon.

Schwann cells multiply by mitosis and grow toward each other to form a regeneration tube across the injured area. This tube guides the growth of a new axon from the proximal area into the distal area, facilitating the reestablishment of sensory and motor connections.

Multiple sclerosis (MS) is an autoimmune disease that causes progressive destruction of myelin sheaths surrounding neurons in the central nervous system (CNS). This leads to slowed and disrupted propagation of nerve impulses, resulting in various neurological symptoms and functional impairments.

Relapsing-remitting multiple sclerosis is characterized by attacks of symptoms such as muscle weakness, abnormal sensations, and double vision, followed by periods of remission where symptoms temporarily disappear. Attacks typically occur every year or two, leading to progressive loss of function over time.

Epilepsy can be caused by various factors including brain damage at birth, metabolic disturbances (like hypoglycemia), infections (such as encephalitis), toxins (like alcohol), vascular disturbances, head injuries, and tumors. However, many seizures have no identifiable cause.

Epileptic seizures can often be managed with anti-epileptic drugs such as phenytoin, carbamazepine, and valproate sodium. In some cases, an implantable device that stimulates the vagus nerve can reduce seizures, and surgical intervention may be considered for severe cases.

Excitotoxicity is the destruction of neurons through prolonged activation of excitatory synaptic transmission, primarily caused by high levels of glutamate in the interstitial fluid of the CNS. The most common cause is oxygen deprivation due to ischemia, such as during a stroke, which leads to glutamate accumulation and neuronal death.

Guillain-Barré syndrome (GBS) is an acute demyelinating disorder where macrophages strip myelin from axons in the PNS. It is the most common cause of acute paralysis in North America and Europe, often resulting from the immune system's response to a bacterial infection.

Neuroblastoma is a malignant tumor consisting of immature nerve cells (neuroblasts), most commonly occurring in the abdomen and frequently in the adrenal glands. It is the most common tumor in infants, although it is rare overall.

The central nervous system (CNS) consists of the brain and spinal cord, while the peripheral nervous system (PNS) includes all nervous tissue outside the CNS, such as nerves and sensory receptors. The PNS is further divided into sensory (afferent) and motor (efferent) divisions.

The autonomic nervous system (ANS) conveys motor output from the CNS to smooth muscle, cardiac muscle, and glands. It is subdivided into:

1. Sympathetic Nervous System: Prepares the body for 'fight or flight' responses.
2. Parasympathetic Nervous System: Promotes 'rest and digest' activities.
3. Enteric Nervous System: Manages the functions of the gastrointestinal system.

Depression is linked to an imbalance of neurotransmitters in the brain, particularly serotonin, norepinephrine, and dopamine. This imbalance may contribute to the symptoms of depression, and medications like selective serotonin reuptake inhibitors (SSRIs) aim to correct this by prolonging the activity of serotonin at synapses.

The primary functions of the nervous system include:

1. Sensing - detecting changes in the environment.
2. Thinking - processing information and making decisions.
3. Remembering - storing and recalling information.
4. Controlling muscle activity - coordinating movements.
5. Regulating glandular secretions - managing the release of hormones and other substances.

The resting membrane potential is the electrical potential difference across the plasma membrane of unstimulated excitable cells, typically around -70 mV. It is determined by:

1. Unequal distribution of ions in the extracellular fluid (ECF) and cytosol.
2. Inability of most cytosolic anions to leave the cell.
3. Electrogenic nature of the Na+/K+ ATPases that maintain ion gradients.

Summation in synaptic transmission refers to the combined effect of multiple presynaptic neurons releasing neurotransmitters at the same time. This can lead to the generation of a nerve impulse if the total excitatory input exceeds the threshold. Summation can be:

1. Spatial summation - multiple signals from different locations.
2. Temporal summation - multiple signals from the same location over time.

Chemical synaptic transmission may be modified by:

1. Affecting the synthesis of a neurotransmitter.
2. Altering the release of a neurotransmitter.
3. Changing the removal of a neurotransmitter.
4. Blocking or stimulating neurotransmitter receptors.

Neurogenesis, the birth of new neurons from undifferentiated stem cells, is normally very limited in the nervous system. Repair of damaged axons does not occur in most regions of the CNS, indicating a limited capacity for regeneration.

For axons and dendrites associated with a neurolemma in the PNS to undergo repair, the following conditions must be met:

1. The cell body must be intact.
2. The Schwann cells must be functional.
3. Scar tissue formation must not occur too rapidly.

Leak channels allow K+ to exit more rapidly than Na+ can enter the axon, which influences the propagation of action potentials. Some mammalian myelinated axons have only a few voltage-gated K+ channels.

The speed of propagation of an action potential is determined by the diameter of the axon, the presence or absence of a myelin sheath, and the temperature.

A synapse is a region of contact between two neurons or between a neuron and an effector.

In electrical synapses (gap junctions), ions can flow equally well in either direction, allowing either neuron to be presynaptic. In chemical synapses, one neuron releases neurotransmitter, and the other neuron has receptors that bind this chemical, allowing the signal to proceed in only one direction.

At excitatory synapses, ACh binds to ionotropic receptors with cation channels that open and generate EPSPs in the postsynaptic cell. At inhibitory synapses, ACh binds to metabotropic receptors coupled to G proteins that open K+ channels, resulting in IPSPs in the postsynaptic cell.

Spatial summation occurs when the buildup of neurotransmitter released simultaneously by several presynaptic end bulbs results in a combined effect on the postsynaptic neuron.

Since -60 mV is below threshold, an action potential will not occur in the postsynaptic neuron.

Biogenic amines are neurotransmitters derived from amino acids that have been chemically modified. Examples include norepinephrine, epinephrine, dopamine, and serotonin.

Convergence refers to a motor neuron receiving input from several other neurons, integrating multiple signals.

The neurolemma provides a regeneration tube that guides the regrowth of a severed axon.