Nerve Physiology: Action Potentials

• Nerve physiology encompasses the study of the function and mechanisms of the nervous system.
• It includes electrical and chemical signaling.
• The nervous system uses electrical signals (action potentials) and chemical signals (neurotransmitters) to transmit information.

Action Potentials

• Action potentials are rapid, transient changes in the electrical potential across a neuron's membrane.
• They are essential for communication within the nervous system.
• Resting membrane potential is typically around -70 mV, maintained by ion concentrations and permeability.
• Depolarization occurs when the membrane potential becomes more positive.
• Repolarization is the return of the membrane potential to its resting state.
• Hyperpolarization is when the membrane potential becomes more negative than the resting potential.
• Threshold is the level of depolarization required to trigger an action potential (usually around -55 mV).
• Voltage-gated ion channels open and close in response to changes in membrane potential.
• Sodium (Na+) influx causes depolarization.
• Potassium (K+) efflux causes repolarization.
• The sodium-potassium pump (Na+/K+ ATPase) helps maintain resting ion gradients.
• The action potential propagates down the axon without decrement due to the positive feedback loop of depolarization and Na+ influx.
• The refractory period limits the frequency of action potentials.
• The absolute refractory period is when another action potential cannot be generated.
• The relative refractory period is when a stronger-than-normal stimulus is required to initiate an action potential.

Synaptic Transmission

• Synaptic transmission is the process by which a neuron communicates with another cell across a synapse.
• The presynaptic neuron releases neurotransmitters.
• The postsynaptic neuron receives the neurotransmitters.
• Electrical synapses involve direct connections (gap junctions) between cells, allowing for rapid, bidirectional communication.
• Chemical synapses involve the release of neurotransmitters into the synaptic cleft.
• Neurotransmitters are synthesized in the presynaptic neuron.
• Neurotransmitters are stored in vesicles.
• An action potential reaching the presynaptic terminal triggers the opening of voltage-gated calcium (Ca2+) channels.
• Ca2+ influx causes vesicles to fuse with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft.
• Neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane.
• Postsynaptic receptors can be ionotropic (ligand-gated ion channels) or metabotropic (G protein-coupled receptors).
• Ionotropic receptors cause rapid changes in membrane potential.
• Metabotropic receptors activate intracellular signaling pathways, leading to slower, longer-lasting effects.
• Neurotransmitters are removed from the synaptic cleft by reuptake, enzymatic degradation, or diffusion.
• Reuptake involves transporters on the presynaptic neuron or glial cells.
• Enzymatic degradation breaks down neurotransmitters in the synaptic cleft.
• Excitatory postsynaptic potentials (EPSPs) depolarize the postsynaptic membrane, increasing the likelihood of an action potential.
• Inhibitory postsynaptic potentials (IPSPs) hyperpolarize the postsynaptic membrane, decreasing the likelihood of an action potential.
• Synaptic integration is the summation of EPSPs and IPSPs at the axon hillock, determining whether an action potential is generated.
• Temporal summation is the adding up of postsynaptic potentials generated at the same synapse over time.
• Spatial summation is the adding up of postsynaptic potentials generated at different synapses at the same time.

Neurotransmitters

• Neurotransmitters are chemical messengers that transmit signals across synapses.
• Acetylcholine (ACh) is involved in muscle contraction, memory, and attention.
• ACh is synthesized from choline and acetyl-CoA.
• ACh is degraded by acetylcholinesterase (AChE).
• Glutamate is the primary excitatory neurotransmitter in the brain.
• GABA (gamma-aminobutyric acid) is the primary inhibitory neurotransmitter in the brain.
• Dopamine is involved in reward, motivation, and motor control.
• Norepinephrine (noradrenaline) is involved in alertness, arousal, and the fight-or-flight response.
• Serotonin (5-HT) is involved in mood, sleep, and appetite.
• Peptide neurotransmitters include endorphins, substance P, and neuropeptide Y.
• Neuromodulators are substances that modulate the effects of neurotransmitters.
• Examples of neuromodulators include adenosine and nitric oxide.

Nerve Conduction

• Nerve conduction is the process by which action potentials are propagated along the axon.
• Unmyelinated axons conduct action potentials continuously along their length.
• Myelinated axons have myelin sheaths formed by Schwann cells (in the peripheral nervous system) or oligodendrocytes (in the central nervous system).
• Myelin sheaths increase the speed of action potential conduction.
• Nodes of Ranvier are gaps in the myelin sheath where action potentials are regenerated.
• Saltatory conduction is the "jumping" of action potentials from one node of Ranvier to the next.
• Axon diameter affects conduction velocity.
• Larger diameter axons have lower internal resistance and faster conduction velocities.
• Temperature affects conduction velocity.
• Higher temperatures generally increase conduction velocity.

Neural Plasticity

• Neural plasticity is the ability of the nervous system to change its structure and function in response to experience or injury.
• Synaptic plasticity is the change in the strength of synaptic connections.
• Long-term potentiation (LTP) is a long-lasting increase in synaptic strength.
• LTP is often induced by high-frequency stimulation.
• Long-term depression (LTD) is a long-lasting decrease in synaptic strength.
• LTD is often induced by low-frequency stimulation.
• Structural plasticity involves changes in the number and morphology of neurons and synapses.
• Neurogenesis is the formation of new neurons.
• It occurs in limited regions of the adult brain, such as the hippocampus.
• Synaptogenesis is the formation of new synapses.
• Dendritic arborization is the growth and branching of dendrites.
• Plasticity enables learning and memory.
• It allows the nervous system to adapt to changing environments.
• Plasticity is also involved in recovery from brain injury.

Types of Nerve Fibers

• Nerve fibers are classified based on their diameter, myelination, and conduction velocity.
• Group A fibers are large, myelinated fibers with the fastest conduction velocities.
• They include Aα, Aβ, Aγ, and Aδ fibers.
• Aα fibers are motor and proprioceptive fibers.
• Aβ fibers are touch and pressure fibers.
• Aγ fibers are motor fibers to muscle spindles.
• Aδ fibers are pain and temperature fibers.
• Group B fibers are medium-sized, myelinated fibers with intermediate conduction velocities.
• They are preganglionic autonomic fibers.
• Group C fibers are small, unmyelinated fibers with the slowest conduction velocities.
• They are postganglionic autonomic fibers and some sensory fibers.
• Sensory nerve fibers can also be classified as I, II, III, and IV.
• Type I fibers correspond to Aα fibers.
• Type II fibers correspond to Aβ fibers.
• Type III fibers correspond to Aδ fibers.
• Type IV fibers correspond to C fibers.