Cells of the Nervous System: Neurons and Glia

Cells of the Nervous System

Overview

The nervous system is composed of specialized cells that transmit and process information throughout the body. The two main cell types are neurons, which carry electrical signals, and glial cells, which provide support and protection.

Neurons

Structure and Function

Neurons are the fundamental units of the nervous system responsible for receiving, integrating, and transmitting information. Each neuron consists of a cell body (soma), dendrites, and an axon.

• Cell body (soma): Contains the nucleus and is the metabolic center of the neuron.

• Dendrites: Branch-like extensions that receive incoming signals from other neurons.

• Axon: Long projection that carries outgoing signals from the cell body to target cells.

• Axon terminals: Specialized endings that transmit signals to other cells via synapses.

Categories of Neurons

Neurons are classified based on their structure and function:

• Afferent (Sensory) Neurons: Carry information about temperature, pressure, light, and other stimuli to the central nervous system (CNS). Specialized receptors convert stimuli into electrical energy.

• Interneurons: Highly branched neurons within the CNS that facilitate communication between other neurons.

• Efferent (Motor and Autonomic) Neurons:

  • Motor neurons: Control skeletal muscles.

  • Autonomic neurons: Influence internal organs; subdivided into sympathetic and parasympathetic branches.

Structural Classification of Neurons

Neurons can also be classified by their shape:

Type	Description	Example
Unipolar	Single process extending from the cell body	Sensory neurons for smell and vision
Bipolar	Two relatively equal fibers extending off the central cell body	Retinal neurons
Anaxonic	No apparent axon	CNS interneurons
Multipolar	Highly branched, lack long extensions	Typical CNS interneurons

Nerves

Nerves are bundles of peripheral neurons, containing both sensory (afferent) and motor (efferent) fibers. They are organized into fascicles and surrounded by connective tissue layers (endoneurium, perineurium, epineurium).

Axonal Transport

Mechanisms and Types

Axonal transport is the process by which materials are moved between the neuron’s cell body and its axon terminals. This is essential because the axon lacks the machinery for protein synthesis.

• Fast Axonal Transport: Moves membrane-bound proteins and organelles (e.g., vesicles, mitochondria).

  • Anterograde transport: From cell body to axon terminal, up to 400 mm/day.

  • Retrograde transport: From axon terminal to cell body, up to 200 mm/day.

• Slow Axonal Transport: Moves cytoplasmic proteins (e.g., enzymes, cytoskeletal proteins) at up to 8 mm/day. May involve frequent pauses.

Motor proteins such as kinesins (anterograde) and dyneins (retrograde) use ATP hydrolysis to drive movement along microtubules.

Synapses

Structure and Function

A synapse is the junction between two neurons where information is transmitted. Most synapses are chemical, involving neurotransmitter release across a synaptic cleft.

• Presynaptic terminal: Releases neurotransmitters.

• Synaptic cleft: Extracellular space containing proteins and carbohydrates that hold the pre- and postsynaptic membranes in close proximity.

• Postsynaptic membrane: Receives the signal via receptor proteins.

Establishing Synapses

During development, billions of neurons must find correct targets, guided by chemical signals and growth cones. Synapses are maintained through repeated use (“use it or lose it” principle).

Glial Cells

Overview

Glia provide support, protection, and nutrition for neurons. They are believed to outnumber neurons by 10-50 to 1 and play roles beyond simple physical support, including chemical communication and homeostasis.

Types of Glial Cells

• Myelin-forming Glia:

  • Oligodendrocytes (CNS): Each cell wraps segments of multiple axons.

  • Schwann cells (PNS): Each cell wraps a segment of a single axon.

  Myelin is a phospholipid-rich substance that insulates axons, enabling rapid (saltatory) conduction of electrical signals and supplying trophic factors.

• Non-myelinating Schwann cells: Provide structural stability and insulation.

• Satellite glial cells: Found within ganglia in the PNS, forming a supportive capsule around neuron cell bodies. They supply nutrients and provide structural support.

• Astrocytes: Highly branched CNS glia that make up about half of all brain cells. Functions include:

  • Releasing chemicals at synapses

  • Supplying neurons with substrates for ATP production

  • Maintaining homeostasis in extracellular fluid (e.g., uptake of K+ and water)

  • Forming part of the blood-brain barrier

  • Influencing vascular dynamics

• Microglia: Specialized immune cells in the CNS that protect neurons from pathogens and facilitate recovery from metabolic insults. Overactivation is implicated in neurodegenerative diseases and neuropathic pain.

• Ependymal cells: Line fluid-filled cavities in the brain and spinal cord, helping to circulate and regulate cerebrospinal fluid (CSF), which protects and nourishes the CNS.

Demyelination Disorders

Multiple Sclerosis (MS) is a disorder resulting from demyelination in the brain and spinal cord, leading to sensory, motor, and cognitive symptoms. The mechanisms are thought to involve immune-mediated destruction of myelin and genetic/environmental factors.

Summary Table: Glial Cell Types and Functions

Glial Cell Type	Location	Main Function
Oligodendrocyte	CNS	Myelin formation (multiple axons)
Schwann cell	PNS	Myelin formation (single axon)
Satellite cell	PNS (ganglia)	Support and nutrition for neuron cell bodies
Astrocyte	CNS	Homeostasis, blood-brain barrier, synaptic support
Microglia	CNS	Immune defense, phagocytosis
Ependymal cell	CNS (ventricles, spinal canal)	CSF production and circulation

Key Terms and Concepts

• Neuron: Electrically excitable cell that processes and transmits information.

• Glia: Non-neuronal cells that support and protect neurons.

• Axonal transport: Movement of materials along the axon via motor proteins.

• Synapse: Junction between two neurons for signal transmission.

• Myelin: Insulating layer around axons, increasing signal speed.

• Multiple Sclerosis: Disease caused by loss of myelin in CNS.

Relevant Equations

While this section does not include specific equations, the following is relevant for understanding nerve conduction:

Nerve conduction velocity (NCV):

NCV is influenced by myelination and axon diameter. A simplified relationship:

$v \propto \sqrt{d}$

where $v$ is conduction velocity and $d$ is axon diameter.

Additional info: Some content was expanded for clarity and completeness, including definitions, examples, and summary tables.