Neuron Structure & Function

1. Cell Body

The cell body, also called the soma, contains the nucleus and most of the cytoplasmic organelles. It integrates incoming signals from the dendrites and plays a central role in protein synthesis. Nissl bodies, which are abundant rough endoplasmic reticulum fragments, are prominent in the soma and are responsible for synthesis of neurotransmitter molecules and structural proteins.

2. Dendrites

Dendrites are branching cytoplasmic projections that increase the receptive surface area of the neuron. They receive chemical signals from other neurons through synapses and convert them into graded potentials. These signals travel toward the cell body where they are processed. The number and complexity of dendrites vary depending on the type of neuron and its functional role.

3. Axon

The axon is a long process that conducts impulses away from the cell body. It originates from a region called the axon hillock, which is the site where action potentials are initiated when threshold is reached. The axon may branch to form collaterals and terminates in synaptic boutons, which release neurotransmitters.

A helpful mnemonic for remembering the functional flow in a typical neuron is “D S A T” which stands for Dendrite receives, Soma integrates, Axon conducts, Terminal transmits.

4. Myelin Sheath

Some axons are wrapped by myelin, a lipid rich insulating covering formed by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system. Myelin increases the velocity of impulse conduction by allowing current to leap between nodes of Ranvier in a process called saltatory conduction. Myelinated fibers conduct impulses more rapidly than unmyelinated ones, making myelination essential for fast reflexes and coordinated movement.

5. Synaptic Terminals

Synaptic terminals form junctions with other neurons, muscles, or glands. They contain synaptic vesicles filled with neurotransmitters. When an action potential reaches the terminal, it triggers release of neurotransmitters into the synaptic cleft, enabling communication with target cells.

Definition and Importance

The resting membrane potential is the electrical potential difference across the neuron's membrane when it is not transmitting a signal. It is usually about -70 millivolts, meaning the interior of the neuron is more negative than the exterior. This electrical potential sets the stage for generation of action potentials.

Factors Responsible for Resting Potential

Selective membrane permeability

Potassium ions move more freely across the membrane than sodium ions because there are more potassium leak channels. As potassium diffuses out, it leaves behind negatively charged proteins, contributing to the negative internal charge.

Sodium potassium pump

The sodium potassium pump continuously moves three sodium ions out and two potassium ions in. This maintains ionic gradients and helps sustain the negative resting potential.

A simple mnemonic is “3 out, 2 in makes the inside thin” to remember that more positive charges leave the cell than enter it.

Graded Potentials

Graded potentials are small changes in membrane potential that vary in size depending on stimulus strength. They occur mainly in dendrites and the soma. They are not all or none and do not propagate long distances. Instead, they decay over space and time. Graded potentials determine whether the axon hillock will reach threshold to initiate an action potential.

Overview

An action potential is a rapid and brief reversal of membrane potential that travels along the axon. It allows long distance communication within the nervous system. It obeys the all or none principle, meaning it either occurs fully or not at all.

Phases of the Action Potential

1. Depolarization

During depolarization, voltage gated sodium channels open causing sodium to rush into the cell. The membrane potential becomes less negative and eventually positive.

2. Repolarization

As the membrane depolarizes, sodium channels become inactivated and voltage gated potassium channels open. Potassium flows out of the cell restoring the negative membrane potential.

3. Hyperpolarization

Potassium channels remain open slightly longer, causing the membrane potential to become more negative than the resting value before stabilizing.

Refractory Periods

Absolute refractory period

During this time no stimulus can trigger another action potential because sodium channels are inactivated.

Relative refractory period

A stronger than usual stimulus is required to initiate another action potential because the membrane is still hyperpolarized.

A mnemonic often used is “Depo In, Repo Out” referring to sodium ions entering during depolarization and potassium ions exiting during repolarization.

Propagation of Action Potentials

Continuous Conduction

In unmyelinated axons, each segment of the membrane must depolarize sequentially. This process is slower and occurs along the entire length of the axon.

Saltatory Conduction

In myelinated fibers, action potentials jump between nodes of Ranvier where voltage gated channels are concentrated. This increases conduction speed significantly. Saltatory conduction is highly energy efficient and critical for rapid neural signaling.

Synaptic Transmission

Types of Synapses

Chemical synapses

These involve release of neurotransmitters from synaptic vesicles. They are the most common type. Neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane.

Electrical synapses

These involve direct transfer of ions through gap junctions. They allow rapid and synchronized communication but are less common in humans.

Events in Chemical Synaptic Transmission

1. Arrival of an action potential at the terminal.
2. Opening of voltage gated calcium channels.
3. Calcium entry triggers fusion of synaptic vesicles with the membrane.
4. Neurotransmitter is released into the cleft.
5. Neurotransmitter binds to receptors on the postsynaptic membrane.
6. Response occurs, either excitation or inhibition.
7. Neurotransmitter is removed by reuptake, enzymatic breakdown, or diffusion.

Neurotransmitters

Common neurotransmitters include acetylcholine, glutamate, gamma aminobutyric acid, glycine, dopamine, serotonin, and norepinephrine. Each neurotransmitter has specific receptors and physiological roles.

Myelin Formation

Schwann cells wrap around peripheral axons while oligodendrocytes myelinate central axons. Each Schwann cell forms a single segment of myelin while oligodendrocytes can myelinate multiple axons. The myelin sheath is composed mainly of lipids which provide electrical insulation.

Functions of Myelin

• Increases conduction velocity.

• Ensures precise timing of neural signals.

• Conserves metabolic energy.

• Protects and supports axons.

Clinical Relevance

Diseases such as multiple sclerosis and Guillain Barré syndrome affect myelin and result in slowed transmission, muscle weakness, and sensory disturbances.