Structure and Function of Neurons and Neuroglia

To maintain structural integrity, neurons utilize neurofibrils, which are composed of neurofilaments (intermediate filaments) that support the dendrites and axons.

Exam Tip: If a question asks about the "site of impulse generation," look for the axon hillock. If it asks about "chemical storage," look for synaptic vesicles in the terminal boutons.

Structural and Functional Classifications

Neurons are not all shaped the same. They are classified structurally based on the number of processes extending from the cell body:

• Unipolar (Pseudounipolar) Neurons: Have one process that divides into two branches (a peripheral and a central process).
• Bipolar Neurons: Possess two processes—one dendrite and one axon. These are specialized and found in the retina, inner ear, and olfactory areas.
• Multipolar Neurons: Feature many dendrites and one axon. This is the most common type of neuron and is found throughout the brain and spinal cord.

They are also classified functionally based on the direction in which they carry impulses:

Neuroglia: The Support System

While neurons do the "talking," neuroglia (glial cells) do the "supporting." Glial cells are smaller, more numerous than neurons, and—unlike neurons—are capable of mitosis (cell division). They are divided into two categories based on their location: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

Central Nervous System (CNS) Glia

The CNS includes the brain and spinal cord. It contains four main types of glia:

1. Astrocytes: The most abundant glial cells. They are star-shaped and come in two forms: protoplasmic (found in gray matter) and fibrous (found in white matter).
2. Ependymal Cells: Ciliated cuboidal cells that line the ventricles of the brain and the central canal of the spinal cord. They help produce and circulate cerebrospinal fluid (CSF).
3. Microglial Cells: Small, phagocytic cells that act as the immune defense, removing debris and damaged tissues.
4. Oligodendrocytes: These cells extend processes that wrap around several axons to form the myelin sheath, which provides electrical insulation to speed up signal transmission.

Peripheral Nervous System (PNS) Glia

The PNS includes cranial and spinal nerves. It contains two main types of glia:

1. Satellite Cells: These surround neuron cell bodies in the PNS to regulate the fluid environment and provide support.
2. Neurolemmocytes (Schwann Cells): These produce the myelin sheath for axons in the PNS. The outermost membrane of the Schwann cell is known as the neurilemma.

Nerve Anatomy and the Synapse

Connective Tissue Organization

A "nerve" is a cable-like bundle of parallel axons. To protect these fibers, the body uses three layers of connective tissue:

• Endoneurium: Loose areolar tissue wrapping each individual axon (nerve fiber).
• Perineurium: Dense irregular connective tissue wrapping a fascicle (a bundle of nerve fibers).
• Epineurium: Dense irregular connective tissue wrapping the entire nerve.

The Synapse: How Neurons Communicate

Communication between neurons occurs at a junction called a synapse. A synapse consists of three parts:

1. Presynaptic Neuron: The neuron sending the signal, which features synaptic knobs at its axon endings.
2. Synaptic Cleft: The narrow, fluid-filled space between the two neurons.
3. Postsynaptic Neuron: The neuron receiving the signal.

When an impulse reaches the terminal bouton, neurotransmitters (such as acetylcholine or dopamine) are released from synaptic vesicles into the cleft. These chemicals cross the gap and bind to receptors on the postsynaptic neuron to continue the signal.