Neurons: Structure and Function

Neurons are the primary functional units of the nervous system, responsible for generating, sending, receiving, and responding to electrical impulses. This page delves into the structure of neurons and their essential components.

Definition: Neurons are specialized cells in the nervous system capable of generating, transmitting, and receiving electrical signals.

The three basic regions of a neuron are:

1. Cell Body $Soma$: This is the wide area of the neuron containing the nucleus and various organelles. It houses a special endoplasmic reticulum that manufactures neurotransmitters.
2. Dendrites: These are thick, branched extensions from the cell body that serve as receptors for impulses and stimuli.
3. Axon: A long, single cytoplasmic extension arising from the axon hillock $a small swelling on the edge of the cell body$. The axon ends in branched structures called axon terminals, each terminating in a synaptic end bulb.

Highlight: The axon always sends impulses away from the cell body region.

Key features of the axon include:

• Synaptic end bulbs: Disc-shaped structures at the end of axon terminals.
• Synaptic vesicles: Membrane-bound cavities within the synaptic end bulbs that contain neurotransmitter molecules.
• Myelin sheath: A layer of myelin that surrounds only the axon part of some neurons, insulating them and increasing the speed of impulse transmission.

Vocabulary: Neurotransmitters - Chemical messengers that transmit signals across synapses from one neuron to another.

Understanding the structure and function of neurons is crucial for grasping how the nervous system processes and transmits information throughout the body.

Neuron Classification

Neurons can be classified in various ways based on their shape, function, myelination, and location within the nervous system. This page provides a detailed overview of these classification systems.

Classification by shape:

1. Unipolar neurons: These have one branched extension from their cell body region that serves as both dendrite and axon.
2. Bipolar neurons: These have one dendrite and one axon extending from the cell body region.
3. Multipolar neurons: These have many dendrites and one axon, and are the most common type.

Example: Motor neurons in the spinal cord are typically multipolar, with numerous dendrites receiving signals from other neurons and a single long axon extending to the target muscle.

Classification by function:

1. Sensory $afferent$ neurons: These carry impulses into the central nervous system $CNS$.
2. Motor $efferent$ neurons: These carry impulses out of the CNS and into muscles and glands.
3. Association neurons: These link sensory and motor neurons within the CNS.

Classification by myelination:

1. Unmyelinated neurons: These lack a myelin coating and appear dark. When grouped together, they form gray matter.
2. Myelinated neurons: These have axons coated in myelin. When grouped together, they form white matter.

Highlight: The presence or absence of myelin affects the speed of impulse transmission, with myelinated neurons conducting signals much faster than unmyelinated ones.

Classification by location:

1. CNS neurons: These are found in the brain and spinal cord. Bundles of their axons are called tracts.
2. PNS neurons: These are found outside the CNS. Bundles of their axons are called nerves.

Vocabulary:

• Nuclei: Groups of cell bodies of CNS neurons.
• Ganglia: Groups of cell bodies in the PNS.

Understanding these classifications is essential for comprehending the diverse types of neurons and their functions within the nervous system.

Study Questions and Key Points

This page provides a review of important concepts related to neuroglial cells and neurons in the nervous system.

Key points to remember:

1. Neuroglial cells do not carry or receive electrical impulses. Their primary function is to support and aid neurons in various ways.
2. The main types of neuroglia in the Central Nervous System $CNS$ are: Astrocytes: These wind around neurons and blood vessels, holding cells and structures together. They also contribute to the blood-brain barrier. Ependymal cells: These line the ventricles in the brain and the central canal of the spinal cord, playing a role in cerebrospinal fluid circulation. Oligodendrocytes: These wrap around selected CNS neurons, providing insulation $myelin$ and increasing the speed of impulse transmission. Microglia: These move through the CNS, engulfing and destroying potentially harmful particles and cellular debris.

Highlight: Each type of neuroglial cell has a specific function that contributes to the overall health and efficiency of the nervous system.

3. In the Peripheral Nervous System $PNS$, the main types of neuroglia are: Satellite cells: These wrap around neurons, forming ganglia and regulating the exchange of substances. Schwann cells: These wrap around PNS neurons, providing myelin insulation.
4. Neurons are specialized cells that can generate, send, receive, and respond to electrical impulses. They consist of three main parts: the cell body $soma$, dendrites, and axon.
5. Neurons can be classified based on their shape $unipolar, bipolar, multipolar$, function $sensory, motor, association$, myelination $myelinated, unmyelinated$, and location $CNS, PNS$.

Example: A motor neuron in the spinal cord would be classified as a multipolar, efferent, myelinated, CNS neuron.

Understanding these key points is crucial for grasping the structure and function of the nervous system and the roles played by its various cellular components.

Neuroglial Cells in the Nervous System

Neuroglial cells, also known as neuroglia, play a crucial supporting role in the nervous system. Unlike neurons, they do not carry information or impulses but are essential for maintaining and aiding neuronal function. This page provides an overview of the various types of neuroglial cells and their specific functions in the central nervous system $CNS$ and peripheral nervous system $PNS$.

Definition: Neuroglial cells are support cells in the nervous system that do not carry information or impulses but aid neurons in various ways.

The main types of neuroglial cells in the CNS include:

1. Astrocytes: These star-shaped cells wrap around neurons and blood vessels, providing structural support and contributing to the blood-brain barrier.
2. Ependymal cells: Cube-shaped cells with cilia that line the ventricles in the brain and the central canal of the spinal cord.
3. Oligodendrocytes: Cells that wrap around selected CNS neurons, providing them with a myelin sheath.
4. Microglia: Tiny cells that move through the CNS, engulfing and destroying potentially harmful particles and cellular debris.

In the PNS, the main types of neuroglial cells are:

1. Satellite cells: These cells wrap around neurons, forming ganglia and regulating the exchange of substances between neurons and their environment.
2. Schwann cells: Similar to oligodendrocytes in the CNS, these cells wrap around PNS neurons, providing myelin insulation.

Highlight: Neuroglial cells are more numerous than neurons and are more mitotically active when mature, meaning they can undergo cell division.

Vocabulary: Myelin - A white, fatty substance that forms an insulating layer around nerve fibers, increasing the speed at which impulses can travel through neurons.

The presence of myelin gives rise to the terms "white matter" and "gray matter" in the nervous system:

• White matter: Areas where myelinated neurons are concentrated, appearing white due to the myelin.
• Gray matter: Areas where unmyelinated neurons are concentrated, appearing gray or dark.

Understanding the types of neuroglial cells and their functions is crucial for comprehending the complex workings of the nervous system and its various components.