Topic 2.2: Physiology of the Neuron

Overview

This topic explores the structure and function of neurons, the fundamental units of the nervous system. It covers the regions of the neuron, their roles in nerve impulse transmission, the electrical phenomena underlying neuronal activity, and the mechanisms of synaptic transmission.

Neuronal Structure and Functional Regions

Neurons are highly specialized cells designed to transmit electrical impulses throughout the body. Their unique structure enables them to perform reception, propagation, and transmission of nerve signals.

• Cell Body (Soma): Contains the nucleus and is the biosynthetic center of the neuron. It houses organelles such as rough endoplasmic reticulum (Nissl bodies), Golgi apparatus, and mitochondria, supporting high metabolic activity.

• Dendrites: Short, branched extensions that serve as the receptive region. They receive signals from other neurons and provide a large surface area for synaptic input.

• Axon: A long, singular process that acts as the conducting region. It generates and propagates action potentials away from the cell body.

• Axon Terminals: The distal endings of the axon, forming the secretory region. They release neurotransmitters to communicate with other neurons or effector cells.

Special Properties of Neurons:

• Extreme Longevity: Neurons can live for over 100 years if adequately nourished.

• Amitotic: Most neurons do not divide after maturation, meaning damage is often permanent.

• High Metabolic Rate: Neurons require continuous oxygen and glucose to function.

Additional info: Neurons are among the most complex cells, with intricate interconnections forming vast neural networks.

Neuronal Processes and Terminology

• Tract: Bundle of nerve processes in the central nervous system (CNS).

• Nerve: Bundle of nerve processes in the peripheral nervous system (PNS).

• Nucleus: Cluster of neuron cell bodies in the CNS.

• Ganglion: Cluster of neuron cell bodies in the PNS.

Functional Regions of the Neuron

• Receptive Region: Dendrites and cell body; receives incoming signals.

• Conducting Region: Axon; propagates action potentials.

• Secretory Region: Axon terminals; releases neurotransmitters.

Electrical Activity of Neurons

Neurons communicate via electrical signals generated by the movement of ions across their plasma membrane.

• Resting Membrane Potential (RMP): The voltage difference across the membrane of a resting neuron, typically between -40 to -90 mV.

• Key Ions: Potassium (K+) is found primarily inside the cell; Sodium (Na+) is found primarily outside.

• Na+/K+ ATPase: Maintains RMP by pumping 3 Na+ out and 2 K+ in per cycle.

Fundamental Principles of Electricity:

• Voltage is created by the separation of oppositely charged ions across the plasma membrane.

• Current is the flow of ions, and resistance is the hindrance to ion flow.

Electrochemical Gradient: The combined effect of concentration and electrical gradients that drive ion movement.

Ion Channels in Neuronal Membranes

• Leakage (Passive) Channels: Always open, allowing ions to move down their gradients.

• Gated Channels: Require a signal to open/close; types include:

  • Chemically-gated: Open in response to neurotransmitters or hormones.

  • Voltage-gated: Open in response to changes in membrane potential.

Graded Potentials

Graded potentials are local changes in membrane potential that occur in the receptive region of the neuron. They can be depolarizations (making the inside less negative) or hyperpolarizations (making the inside more negative).

• Depolarization: Increases the probability of generating a nerve impulse.

• Hyperpolarization: Decreases the probability of generating a nerve impulse.

• Graded potentials decrease in magnitude with distance and are proportional to stimulus strength.

Action Potentials

Action potentials are rapid, large changes in membrane potential that propagate along the axon without decreasing in amplitude.

• Threshold: The membrane potential at which an action potential is triggered (typically -55 to -50 mV).

• Phases:

  • Depolarization: Voltage-gated Na+ channels open, Na+ enters the cell.

  • Repolarization: Na+ channels close, K+ channels open, K+ leaves the cell.

  • Hyperpolarization: K+ channels remain open briefly, making the membrane more negative than RMP.

• Absolute Refractory Period: No new action potential can be generated.

• Relative Refractory Period: A stronger stimulus is required to generate another action potential.

All-or-None Principle: An action potential either occurs fully or not at all; its size does not vary with stimulus intensity.

Equation for Na+/K+ Pump:

Propagation of Action Potentials

Action potentials travel along the axon in one direction, from the axon hillock to the axon terminals. The speed of propagation increases with axon diameter and myelination.

• Myelin Sheath: Insulates axons, increasing conduction speed via saltatory conduction (jumping between nodes of Ranvier).

• Nodes of Ranvier: Gaps in the myelin sheath where action potentials are regenerated.

• Oligodendrocytes: Myelinate axons in the CNS.

• Schwann Cells: Myelinate axons in the PNS.

Clinical Note: Multiple Sclerosis

• Autoimmune destruction of myelin sheaths leads to impaired nerve conduction, resulting in symptoms such as muscle weakness and vision problems.

Synaptic Transmission

Neurons communicate with each other and with effector cells at synapses, which can be electrical or chemical.

• Electrical Synapses: Direct current flow via gap junctions; rapid and synchronized transmission, found in some brain regions and muscle tissue.

• Chemical Synapses: Use neurotransmitters to transmit signals across a synaptic cleft; most common type.

Mechanism of Chemical Synaptic Transmission:

1. Action potential arrives at axon terminal.

2. Voltage-gated Ca2+ channels open; Ca2+ enters terminal.

3. Neurotransmitter is released into synaptic cleft.

4. Neurotransmitter binds to receptors on postsynaptic membrane.

5. Ion channels open, generating a postsynaptic potential.

Termination of Synaptic Transmission:

• Enzymatic degradation (e.g., acetylcholine by acetylcholinesterase)

• Reuptake by presynaptic neuron (e.g., norepinephrine)

• Diffusion away from synaptic site (e.g., nitric oxide)

Synaptic Potentials and Integration

• Excitatory Postsynaptic Potentials (EPSPs): Depolarize the postsynaptic membrane, increasing likelihood of action potential.

• Inhibitory Postsynaptic Potentials (IPSPs): Hyperpolarize the postsynaptic membrane, decreasing likelihood of action potential.

• Summation:

  • Temporal Summation: Multiple EPSPs/IPSPs from a single synapse in rapid succession.

  • Spatial Summation: EPSPs/IPSPs from multiple synapses at the same time.

• Axon Hillock: Acts as the neural integrator, summing all incoming signals to determine if threshold is reached for action potential generation.

Summary Table: Neuron Regions and Functions

Key Equations

• Resting Membrane Potential:

• Na+/K+ Pump:

Additional info: These notes provide a comprehensive overview suitable for exam preparation in college-level Anatomy & Physiology courses.