The Nervous System: Structure, Function, and Neural Communication

The Nervous System

Overview and Functions

The nervous system is the master controlling and communicating system of the body. It coordinates all body activities by transmitting signals between different parts of the body using electrical and chemical means.

• Communication: Nerve cells (neurons) communicate via electrical and chemical signals.

• Speed: Responses are rapid and specific, usually causing almost immediate effects.

Major Functions of the Nervous System

• Sensory Input: Gathering information from sensory receptors about internal and external changes.

• Integration: Processing and interpretation of sensory input to determine the appropriate response.

• Motor Output: Activation of effector organs (muscles and glands) to produce a response.

Divisions of the Nervous System

Central Nervous System (CNS)

• Consists of the brain and spinal cord located in the dorsal body cavity.

• Acts as the integration and control center of the body.

Peripheral Nervous System (PNS)

• Composed of nerves extending from the brain and spinal cord.

• Divided into two functional divisions:

  • Sensory (afferent) division: Transmits sensory information to the CNS.

  • Motor (efferent) division: Transmits impulses from the CNS to effector organs. Includes:

    • Somatic nervous system (voluntary control of skeletal muscles)

    • Autonomic nervous system (involuntary control of smooth muscle, cardiac muscle, and glands)

Histology of Nervous Tissue

Cell Types

• Neurons: Excitable cells that transmit electrical signals.

• Neuroglia (glial cells): Support, protect, and insulate neurons.

Types of Neuroglia

• Astrocytes (CNS): Most abundant, versatile, and highly branched. Functions include providing structural support, maintaining the blood-brain barrier, regulating ion balance, and repairing tissue after injury.

• Microglial Cells (CNS): Small, ovoid cells with thorny processes. Act as macrophages, monitoring neuron health and removing debris.

• Ependymal Cells (CNS): Line the central cavities of the brain and spinal cord. Produce and circulate cerebrospinal fluid (CSF).

• Oligodendrocytes (CNS): Form myelin sheaths around CNS nerve fibers, increasing the speed of impulse transmission.

• Satellite Cells (PNS): Surround neuron cell bodies in the PNS, providing support and regulating the environment.

• Schwann Cells (PNS): Form myelin sheaths around peripheral nerve fibers and assist in nerve regeneration.

Myelin Sheath

Structure and Function

• Composed of a whitish, protein-lipoid substance.

• Forms a segmented sheath around most long or large-diameter axons.

• Functions:

  • Protects and electrically insulates axons.

  • Increases the speed of nerve impulse transmission.

• Unmyelinated fibers conduct impulses more slowly.

Myelination in CNS vs. PNS

• CNS: Oligodendrocytes form myelin sheaths; regions with dense myelinated fibers are called white matter, while regions with mostly cell bodies and unmyelinated fibers are gray matter.

• PNS: Schwann cells form myelin sheaths; gaps between Schwann cells are called nodes of Ranvier, which facilitate rapid signal conduction.

Neurons

Structure and Properties

• Large, highly specialized cells for conduction of electrical impulses.

• Extreme longevity (can last a lifetime), generally amitotic (do not divide), and have a high metabolic rate.

• Consist of a cell body (soma), dendrites (input regions), and a single axon (output region).

Neuron Cell Body

• Contains the nucleus and is the biosynthetic center of the neuron.

• Most cell bodies are located in the CNS; clusters in the PNS are called ganglia.

Neuron Processes

• Dendrites: Short, branched processes that receive signals and convey them toward the cell body.

• Axon: Single, long process that transmits impulses away from the cell body to other neurons or effectors. Axons may branch (axon collaterals) and end in axon terminals, where neurotransmitters are released.

Classification of Neurons

Structural Classification

• Multipolar: Three or more processes (most common type in CNS).

• Bipolar: Two processes (rare, found in special sensory organs).

• Unipolar: Single short process (mainly in PNS, sensory neurons).

Functional Classification

• Sensory (afferent) neurons: Transmit impulses from sensory receptors toward the CNS; mostly unipolar.

• Motor (efferent) neurons: Carry impulses from the CNS to effectors; multipolar.

• Interneurons (association neurons): Lie between sensory and motor neurons; most are multipolar and confined to the CNS.

Neural Signaling and Membrane Potentials

Ion Channels and Membrane Potentials

• Neurons are highly irritable and respond to stimuli by generating action potentials (nerve impulses).

• Types of ion channels:

  • Chemically gated channels: Open in response to binding of a specific chemical (e.g., neurotransmitter).

  • Voltage-gated channels: Open and close in response to changes in membrane potential.

  • Mechanically gated channels: Open in response to physical deformation of the receptor.

Resting Membrane Potential

• Potential difference across the membrane of a resting cell, typically about -70 mV in neurons (inside is negative relative to outside).

• Maintained by differences in ionic composition of intracellular and extracellular fluids and selective permeability of the plasma membrane.

• The sodium-potassium pump ($3 \text{Na}^+$ out, $2 \text{K}^+$ in) helps maintain this potential.

Changes in Membrane Potential

• Depolarization: Decrease in membrane potential (inside becomes less negative); increases probability of producing a nerve impulse.

• Hyperpolarization: Increase in membrane potential (inside becomes more negative); reduces probability of producing a nerve impulse.

Action Potentials

• Brief, rapid, large changes in membrane potential that propagate along the axon.

• All-or-none events: once threshold is reached, an action potential is generated.

• Phases:

  • Depolarization: Na+ channels open, Na+ enters the cell.

  • Repolarization: K+ channels open, K+ leaves the cell.

  • Hyperpolarization: K+ continues to leave, membrane potential becomes more negative than resting.

• After repolarization, the sodium-potassium pump restores ionic conditions.

Refractory Periods

• Absolute refractory period: Time during which another action potential cannot be generated.

• Relative refractory period: Follows the absolute period; a stronger stimulus is required to initiate another action potential.

Conduction Velocity

• Depends on axon diameter (larger = faster) and degree of myelination (myelinated = faster).

• Saltatory conduction: In myelinated axons, action potentials jump from node to node, increasing speed.

Synapses

Types of Synapses

• Electrical synapses: Neurons are connected by gap junctions, allowing direct electrical communication; rare in adults, more common in embryonic tissue.

• Chemical synapses: Specialized for release and reception of neurotransmitters across a synaptic cleft; most common type.

Components of a Chemical Synapse

• Presynaptic neuron: Conducts impulses toward the synapse; releases neurotransmitter.

• Postsynaptic neuron: Receives the signal; may be another neuron or an effector cell.

Neurotransmitters

Classification by Chemical Structure

• Acetylcholine (ACh): First identified; released at neuromuscular junctions and by some CNS and ANS neurons.

• Biogenic amines: Dopamine, norepinephrine (NE), epinephrine.

• Amino acids: Glutamate, aspartate, GABA, glycine.

• Purines: ATP, adenosine.

• Peptides: Endorphins (natural opiates, reduce pain perception).

• Gases and lipids: Nitric oxide (NO), carbon monoxide (CO), hydrogen sulfide (H2S).

• Endocannabinoids: Act at the same receptors as THC (active ingredient in marijuana).

Classification by Function

• Effects: Excitatory (e.g., glutamate), inhibitory (e.g., GABA, glycine), or both (e.g., ACh, NE).

• Actions: Direct (bind to and open ion channels) or indirect (act through second messengers).

Neurotransmitter Receptors

• Channel-linked receptors: Mediate fast synaptic transmission; brief, localized changes.

• G protein-linked receptors: Mediate slow, prolonged, and widespread responses.

Neural Integration

Processing Patterns

• Serial processing: Input travels along one pathway to a specific destination; produces predictable responses (e.g., spinal reflexes).

• Parallel processing: Input travels along several pathways simultaneously; important for higher-level mental functioning and allows one stimulus to promote multiple responses.

Reflex Arcs

• Rapid, automatic responses to stimuli that occur over specific neural pathways called reflex arcs.

• Components: stimulus, sensory neuron, integration center, motor neuron, effector, and response.

Neuroglia Type	Location	Main Function
Astrocytes	CNS	Support, blood-brain barrier, repair, ion balance
Microglial cells	CNS	Immune defense, phagocytosis
Ependymal cells	CNS	Produce and circulate CSF
Oligodendrocytes	CNS	Form myelin sheaths
Satellite cells	PNS	Support neuron cell bodies
Schwann cells	PNS	Form myelin sheaths, aid regeneration

Additional info: Some details, such as the precise mechanisms of neurotransmitter action and the full range of neurotransmitter types, have been expanded for academic completeness.