12.2 Nervous Tissue

Neurons and Neuroglia

Structure and Components of Neurons

The neuron is the functional unit of the nervous system, built to receive, process, and transmit electrical signals. Each part of the neuron has a specific job in that process.

• Cell body (soma) contains the nucleus and most organelles. Nissl bodies (clusters of rough endoplasmic reticulum) are especially prominent here because the neuron needs constant protein synthesis to maintain its long extensions and produce neurotransmitters.
• Dendrites are branching extensions covered in tiny projections called dendritic spines. They receive incoming signals from other neurons and carry those signals toward the cell body. More branching means more connections with other neurons.
• Axon is a single long projection that carries electrical signals away from the cell body toward target cells.
  • The axon hillock sits where the axon meets the soma. This is the trigger zone where the neuron "decides" whether to fire an action potential based on the sum of all incoming signals.
  • Axon terminals (synaptic knobs) are the bulb-shaped endings where neurotransmitters are stored in vesicles and released into the synapse.
• Myelin sheath is a fatty insulating layer wrapped around the axon. In the PNS, Schwann cells form myelin; in the CNS, oligodendrocytes do the job. Myelin dramatically increases signal speed.
  • Nodes of Ranvier are the small gaps between myelin segments. The action potential "jumps" from node to node in a process called saltatory conduction, which is much faster than continuous conduction along an unmyelinated axon.

Types of Neurons by Polarity and Function

Neurons are classified two ways: by their physical structure (how many processes extend from the soma) and by their functional role (which direction they carry information).

Structural classification (by polarity):

• Unipolar (pseudounipolar) neurons have a single process that splits into two branches. These are the sensory neurons found in dorsal root ganglia. One branch goes to a receptor in the body, the other enters the spinal cord.
• Bipolar neurons have two processes, one dendrite and one axon, extending from opposite ends of the soma. These are specialized sensory neurons found in the retina, olfactory epithelium, and inner ear.
• Multipolar neurons have multiple dendrites and a single axon. This is the most common structural type and includes motor neurons and interneurons.

Functional classification:

• Sensory (afferent) neurons carry information from receptors (for touch, pain, temperature, etc.) to the CNS for processing.
• Motor (efferent) neurons carry signals from the CNS to effectors like muscles and glands to produce a response.
• Interneurons are found entirely within the CNS. They connect sensory and motor pathways and handle the integration and processing of information. The vast majority of neurons in your body are interneurons.

Roles of Glial Cells

Glial cells (neuroglia) outnumber neurons and handle everything except transmitting electrical signals. They support, protect, insulate, and maintain the environment neurons need to function.

CNS Neuroglia:

• Astrocytes are the most abundant and versatile glial cells. They provide structural support, help form the blood-brain barrier (which controls what substances can pass from blood into brain tissue), regulate ion and neurotransmitter concentrations around synapses, and help supply neurons with nutrients.
• Oligodendrocytes form the myelin sheath in the CNS. A single oligodendrocyte can myelinate portions of several axons at once.
• Microglia are the resident immune cells of the CNS. They constantly monitor for pathogens, dead cells, and debris, then phagocytose (engulf and digest) anything harmful.
• Ependymal cells are ciliated cells that line the ventricles of the brain and the central canal of the spinal cord. They produce and help circulate cerebrospinal fluid (CSF).

PNS Neuroglia:

• Schwann cells form the myelin sheath in the PNS. Unlike oligodendrocytes, each Schwann cell wraps around just one segment of a single axon.
• Satellite cells surround neuron cell bodies in ganglia (clusters of cell bodies in the PNS). They regulate the local chemical environment around those cell bodies, similar to what astrocytes do in the CNS.

Neuroglia in CNS vs. PNS

A common exam question asks you to compare glial cells across the two divisions. Here's a quick reference:

The key parallel is that oligodendrocytes and Schwann cells both produce myelin, and astrocytes and satellite cells both support neuron cell bodies. The key difference is that microglia and ependymal cells exist only in the CNS, while satellite cells exist only in the PNS.

Neuronal Communication

Neurons communicate at junctions called synapses. Most synapses in the body are chemical synapses, where a signal is passed using neurotransmitters rather than direct electrical coupling.

Here's how a chemical synapse works, step by step:

1. An action potential travels down the axon and arrives at the axon terminal.
2. The electrical signal triggers voltage-gated calcium channels to open, allowing calcium ions into the terminal.
3. Calcium causes synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft (the tiny gap between neurons).
4. Neurotransmitters cross the cleft and bind to specific receptors on the postsynaptic cell (which could be another neuron, a muscle cell, or a gland cell).
5. Receptor binding causes ion channels to open or close on the postsynaptic cell, changing its membrane potential and either exciting or inhibiting it.

Membrane potential is the voltage difference between the inside and outside of the cell. At rest, a neuron sits at about $-70 \text{ mV}$ (the resting membrane potential), with the inside more negative than the outside. Changes to this voltage are what drive signal transmission.

Neurogenesis, the formation of new neurons, does occur in a few specific regions of the adult brain (notably the hippocampus, which is involved in learning and memory). This is limited compared to the extensive neurogenesis that happens during development.