35.1 Neurons and Glial Cells

Neuron Structure and Function

Neurons are the cells responsible for transmitting signals throughout the nervous system. They control everything from reflexes to complex thought by passing electrical and chemical signals to one another. Understanding their structure is the first step to understanding how the nervous system works.

Components and Functions of Neurons

Every neuron has the same basic parts, each with a specific job in signal transmission:

• Cell body (soma) contains the nucleus and organelles responsible for protein synthesis and cellular metabolism. This is the neuron's metabolic center.
• Dendrites are branched extensions that receive signals from other neurons. Their branching increases surface area, which means more connections with neighboring cells.
• Axon is a long, thin extension that carries electrical signals away from the cell body toward the next cell. Some axons are wrapped in a myelin sheath, a fatty insulating layer that speeds up signal transmission.
• Axon terminal is the endpoint of the axon, where it forms a synapse with another neuron or a target cell (like a muscle fiber). Axon terminals contain synaptic vesicles that store neurotransmitters such as dopamine and serotonin.
• Synapse is the junction between one neuron's axon terminal and the dendrite or cell body of the next neuron. Signals cross this gap through chemical signaling (neurotransmitters) or, less commonly, electrical signaling. A well-known example is the neuromuscular junction, where a motor neuron meets a muscle cell.

On the postsynaptic side of the synapse, neurotransmitter receptors sit on the membrane and bind to specific neurotransmitters. This binding triggers a response in the receiving cell.

Types of Neurons

Neurons are classified by their function and by their shape:

By function:

• Sensory neurons (afferent neurons) carry information from sensory receptors toward the central nervous system (CNS). They typically have long dendrites and short axons. Dorsal root ganglion neurons are a common example.
• Motor neurons (efferent neurons) carry signals from the CNS out to muscles or glands. They typically have short dendrites and long axons. Spinal motor neurons that control skeletal muscle are a classic example.
• Interneurons are found entirely within the CNS. They relay signals between sensory and motor neurons or between other interneurons. Their structure varies widely depending on their role. Purkinje cells in the cerebellum are one type.

By structure:

• Multipolar neurons are the most common type. They have one axon and multiple dendrites extending from the cell body. You'll find them throughout the brain, spinal cord, and autonomic nervous system. Pyramidal cells in the cerebral cortex are multipolar.

Glial Cells and Their Functions

Glial cells don't transmit signals the way neurons do, but the nervous system can't function without them. They provide structural support, insulation, immune defense, and help maintain the chemical environment neurons need to work properly. There are roughly as many glial cells as neurons in the brain.

Roles of Glial Cells

• Astrocytes are star-shaped cells that provide structural support and deliver nutrients to neurons. They also regulate ion and neurotransmitter concentrations in the synaptic cleft (for example, by taking up excess glutamate). Astrocytes are a major contributor to the blood-brain barrier, which controls what substances can pass from the bloodstream into brain tissue.
• Oligodendrocytes produce the myelin sheath around axons in the central nervous system. A single oligodendrocyte can myelinate segments of multiple axons, enabling the fast, efficient signal transmission that the CNS depends on.
• Schwann cells serve the same myelinating function but in the peripheral nervous system. Each Schwann cell wraps around a single segment of one axon. They also play a key role in nerve regeneration after injury, which is why peripheral nerves can sometimes recover while CNS damage is often permanent.
• Microglia are the immune cells of the CNS. They patrol brain tissue, phagocytosing (engulfing) debris, dead cells, and pathogens. When activated, they secrete signaling molecules called cytokines (such as IL-1 and TNF-α) to coordinate immune responses.
• Ependymal cells line the ventricles of the brain and the central canal of the spinal cord. Their cilia help produce and circulate cerebrospinal fluid (CSF), which cushions the brain and removes waste. They also contribute to the blood-cerebrospinal fluid barrier.

Neuronal Dynamics and Plasticity

Beyond basic structure, a few concepts about how neurons behave over time are worth knowing:

• Resting membrane potential is the voltage difference across a neuron's membrane when it is not sending a signal. In a typical neuron, this sits around $-70 \text{ mV}$. This charge difference is maintained by ion pumps and ion channels, and it's what makes the neuron ready to fire when stimulated.
• Neuroplasticity is the brain's ability to reorganize its structure and connections in response to experience, learning, or injury. This is why practice strengthens skills and why the brain can sometimes compensate after damage by rerouting functions to undamaged areas.
• Neurogenesis is the production of new neurons. It was once thought that adults couldn't generate new neurons, but research has shown it occurs in specific brain regions (notably the hippocampus, which is involved in learning and memory) throughout life.