Nervous System: Structure, Function, and Signal Transmission

Nervous System Overview

General Functions

The nervous system is responsible for monitoring changes inside and outside the body, processing sensory information, and coordinating responses. It works closely with the endocrine system to maintain homeostasis and control rapid responses.

• Sensory Input: Millions of sensory receptors detect changes (stimuli) and send information to the brain and spinal cord.

• Integration: The nervous system processes and interprets sensory input, deciding what action to take.

• Motor Output: Activates muscles or glands (effectors) to produce a response.

Organization of the Nervous System

Central Nervous System (CNS)

The CNS consists of the brain and spinal cord, located in the dorsal body cavity. It is the main control center for processing information and directing responses.

• Brain: Located in the cranial cavity.

• Spinal Cord: Located in the spinal cavity.

Peripheral Nervous System (PNS)

The PNS includes all neural structures outside the CNS. It connects the CNS to limbs and organs, serving as a communication line.

• Cranial Nerves: 12 pairs

• Spinal Nerves: 31 pairs

• Main Functions: Carry sensory input to the CNS and motor output from the CNS to muscles and glands.

Functional Classification of the Nervous System

Sensory (Afferent) Division

The sensory division transmits information from sensory receptors to the CNS. It detects changes inside and outside the body.

• Somatic Sensory Fibers: Carry impulses from skin, skeletal muscles, and joints.

• Visceral Sensory Fibers: Carry impulses from organs.

Motor (Efferent) Division

The motor division transmits signals from the CNS to muscles and glands, causing a response.

• Somatic Nervous System: Voluntary control of skeletal muscles.

• Autonomic Nervous System: Involuntary control of smooth muscle, cardiac muscle, and glands.

  • Sympathetic Division: "Fight or flight" responses; increases heart rate, dilates pupils, etc.

  • Parasympathetic Division: "Rest and digest" responses; slows heart rate, stimulates digestion, etc.

Nervous Tissue: Structure and Function

Cell Types

• Neurons: Transmit electrical impulses; basic functional units of the nervous system.

• Neuroglia (Glial Cells): Support, protect, and nourish neurons; do not transmit impulses.

Types of Neuroglia

• Astrocytes: Star-shaped; anchor neurons to blood capillaries; control chemical environment.

• Microglia: Phagocytes; remove debris and pathogens.

• Ependymal Cells: Line cavities of brain and spinal cord; circulate cerebrospinal fluid (CSF).

• Oligodendrocytes: Form myelin sheaths in CNS.

• Schwann Cells: Form myelin sheaths in PNS.

• Satellite Cells: Protect and cushion neurons in PNS.

Neuron Structure

• Cell Body (Soma): Contains nucleus and organelles.

• Processes: Extensions from the cell body.

  • Dendrites: Receive signals and carry them toward the cell body.

  • Axon: Carries signals away from the cell body; ends in axon terminals.

• Synapse: Junction between axon terminal and next cell; neurotransmitters cross the synaptic cleft.

Myelin Sheaths

Function and Formation

Myelin is a fatty substance that insulates axons, speeding up nerve impulse transmission.

• CNS: Myelin formed by oligodendrocytes.

• PNS: Myelin formed by Schwann cells; gaps between segments are called nodes of Ranvier.

Myelinated fibers conduct impulses faster than unmyelinated fibers.

Terminology

• Nucleus: Cluster of neuron cell bodies in CNS.

• Ganglion: Cluster of neuron cell bodies in PNS.

• Tract: Bundle of nerve fibers in CNS.

• Nerve: Bundle of nerve fibers in PNS.

Functional Classification of Neurons

• Sensory (Afferent) Neurons: Carry impulses toward CNS.

• Motor (Efferent) Neurons: Carry impulses away from CNS to effectors.

• Interneurons: Connect sensory and motor neurons within CNS.

Structural Classification of Neurons

• Multipolar Neurons: Many processes; most common type.

• Bipolar Neurons: Two processes; found in special sense organs (eye, nose).

• Unipolar Neurons: One process that splits into two branches; found in sensory neurons of PNS.

Nerve Signal Conduction

Steps of Action Potential Transmission

1. Resting Membrane Potential: Inside of neuron is negative; maintained by Na+/K+ pump. $\text{Na}^+/\text{K}^+ \text{ pump: 3 Na}^+ \text{ out, 2 K}^+ \text{ in}$

2. Stimulus Initiates Local Depolarization: Stimulus opens sodium channels; Na+ enters cell, making it more positive.

3. Depolarization & Action Potential Generation: If threshold is reached, action potential is generated.

4. Propagation of Action Potential: Depolarization spreads along axon; in myelinated axons, impulse jumps between nodes (saltatory conduction).

5. Repolarization: K+ channels open; K+ exits cell, restoring negative charge.

6. Restoring Ionic Conditions: Na+/K+ pump resets ion positions using ATP.

Summary Table: Nerve Signal Stages

Stage	Ion Movement	Inside Charge	Description
Resting	Na+ out, K+ in	Negative	Neuron inactive, ready to fire
Depolarization	Na+ in	Positive	Stimulus causes Na+ to enter, action potential starts
Repolarization	K+ out	Negative	K+ exits, restoring negative charge
Restoration	Na+/K+ pump	Negative	ATP resets ions to resting potential

Types of Sensory Receptors

• Cutaneous Receptors: In skin; detect touch, pressure, temperature, pain.

• Proprioceptors: In muscles, tendons, joints; detect stretch, position.

• Pain Receptors: Free nerve endings; warn of possible damage.

Quick Summary Chart: Neuron Types

Classification Type	Description / Direction	Location of Cell Body	Example / Function
Sensory (Afferent)	To CNS	In PNS (ganglia)	Detects touch, pain, etc.
Motor (Efferent)	From CNS	In CNS	Sends signals to muscles/glands
Interneuron	Connects sensory & motor	In CNS	Communication bridge

Additional info:

• Neuroglia are essential for neuron health and function, even though they do not transmit impulses.

• Myelin sheaths are critical for rapid signal transmission and regeneration in the PNS.

• Saltatory conduction allows for faster nerve impulse transmission in myelinated axons.