The Nervous System: Structure, Function, and Cellular Organization

The Nervous System

Overview

The nervous system is the master controlling and communicating system of the body. It utilizes both electrical and chemical signals to coordinate rapid and specific responses, often resulting in immediate effects.

• Electrical and Chemical Communication: Neurons transmit information via action potentials and neurotransmitter release.

• Immediate Responses: The system is designed for speed and specificity in response to stimuli.

Functions of the Nervous System

Major Functions

• Sensory Input: Sensory receptors gather information about internal and external changes.

• Integration: The nervous system processes and interprets sensory input to determine an appropriate response.

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

Example: Seeing a glass of water (sensory input), the brain processes the information (integration), and the arm muscles move to pick up the glass (motor output).

Divisions of the Nervous System

Central Nervous System (CNS)

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

• Acts as the integration and control center, interpreting sensory input and dictating motor output.

Peripheral Nervous System (PNS)

• Consists of nerves outside the CNS, including cranial and spinal nerves.

• Links the CNS to the rest of the body.

Peripheral Nervous System (PNS)

Functional Divisions

• Sensory (Afferent) Division: Transmits impulses from sensory receptors to the CNS.

  • Somatic sensory fibers: From skin, skeletal muscles, and joints.

  • Visceral sensory fibers: From visceral organs.

• Motor (Efferent) Division: Transmits impulses from the CNS to effector organs (muscles and glands).

  • Somatic Nervous System: Voluntary control of skeletal muscles.

  • Autonomic Nervous System (ANS): Involuntary control of smooth muscle, cardiac muscle, and glands. Subdivided into sympathetic and parasympathetic divisions, which generally have opposing effects.

Histology of Nervous Tissue

Cell Types

• Neuroglia (Glial Cells): Support, protect, and insulate neurons.

• Neurons (Nerve Cells): Excitable cells responsible for transmitting electrical signals.

Types of Neuroglia

• Astrocytes: Most abundant glial cells in the CNS; support neurons, regulate the chemical environment, and participate in information processing.

• Microglial Cells: Small, defensive cells that monitor neuron health and can phagocytize debris and pathogens.

• Ependymal Cells: Line the central cavities of the brain and spinal cord; may be ciliated to help circulate cerebrospinal fluid (CSF).

• Oligodendrocytes: Form myelin sheaths around CNS nerve fibers, insulating them and increasing conduction speed.

• Satellite Cells (PNS): Surround neuron cell bodies in the PNS; similar function to astrocytes.

• Schwann Cells (PNS): Form myelin sheaths around peripheral nerve fibers and are vital for nerve regeneration.

Neurons

General Characteristics

• Structural units of the nervous system.

• Specialized for impulse conduction.

• Long-lived (often a lifetime), amitotic (do not divide), and have a high metabolic rate (require constant oxygen and glucose).

• Composed of a cell body and one or more processes (dendrites and axons).

Neuron Cell Body (Perikaryon or Soma)

• Biosynthetic center: synthesizes proteins, membranes, and other chemicals.

• Contains a spherical nucleus with nucleolus and rough ER (Nissl bodies).

• Most cell bodies are located in the CNS (nuclei); in the PNS, they are found in ganglia.

Dendrites

• Short, branched processes that receive input and convey it toward the cell body as graded potentials.

• Specialized dendritic spines increase surface area for synaptic input.

Axon

• Each neuron has a single axon arising from the axon hillock.

• Long axons are called nerve fibers; may branch (axon collaterals) and end in axon terminals (boutons).

• Conducts impulses away from the cell body and releases neurotransmitters at terminals.

• Relies on the cell body for protein and membrane renewal; lacks rough ER and Golgi apparatus.

Axonal Transport

• Anterograde: Movement away from the cell body (e.g., mitochondria, enzymes).

• Retrograde: Movement toward the cell body (e.g., degraded organelles, viruses).

Myelin Sheath

Structure and Function

• Composed of a whitish, protein-lipid substance called myelin.

• Insulates axons, increasing the speed of impulse transmission.

• Myelinated fibers conduct impulses rapidly; nonmyelinated fibers conduct more slowly.

Myelination in the PNS

• Schwann cells wrap around axons, forming myelin sheaths with gaps called nodes of Ranvier (sites for signal propagation and axon collateral emergence).

• Nonmyelinated fibers are thin and surrounded by Schwann cells without coiling.

Myelination in the CNS

• Oligodendrocytes form myelin sheaths by wrapping processes around multiple axons.

• White matter: regions with dense collections of myelinated fibers.

• Gray matter: mostly neuron cell bodies and nonmyelinated fibers.

Structural and Functional Classification of Neurons

Structural Classes

Type	Structure	Location
Multipolar	Many processes extend from cell body; all are dendrites except for a single axon.	Most abundant; major neuron type in CNS.
Bipolar	Two processes extend from cell body; one is a fused dendrite, the other an axon.	Rare; found in special sensory organs (e.g., retina, olfactory mucosa).
Unipolar (Pseudounipolar)	One process extends from cell body and forms central and peripheral processes, together comprising an axon.	Mainly in PNS; common in dorsal root ganglia of spinal cord and sensory ganglia of cranial nerves.

Functional Classes

• Sensory (Afferent) Neurons: Transmit impulses from sensory receptors toward the CNS; almost all are unipolar.

• Motor (Efferent) Neurons: Carry impulses from the CNS to effectors; multipolar.

• Interneurons (Association Neurons): Lie between sensory and motor neurons; shuttle signals through CNS pathways; most are confined to the CNS and constitute 99% of all neurons.

Membrane Potential and Neural Signaling

Membrane Potential Changes

• Changes in ion concentrations or membrane permeability alter membrane potential.

• Two main types of signals:

  • Graded Potentials: Short-distance, localized changes in membrane potential.

  • Action Potentials: Long-distance signals of axons; do not decay over distance.

Key Terms

• 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.

Equations

• Resting Membrane Potential:

$V_m = V_{inside} - V_{outside}$

• Nernst Equation (for a single ion):

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

Additional info: The Nernst equation calculates the equilibrium potential for a particular ion based on its concentration gradient across the membrane.