The Nervous System: Nervous Tissue – Structure, Function, and Physiology

The Nervous System: Nervous Tissue

Introduction

The nervous system consists of all neural tissue in the body and is responsible for carrying information and instructions from one region to another. It is the master controlling and communicating system of the body.

Functions of the Nervous System

Integration of Body Processes

• Integration: The nervous system links all body systems, allowing them to work together in unity.

• Control of Voluntary Effectors: Effectors are cells that respond to neural impulses, such as skeletal muscle fibers.

• Control of Involuntary Effectors: The nervous system also controls smooth muscle, cardiac muscle, and glands, which are effectors in involuntary actions.

• Regulation of Homeostasis: Maintains stability of internal environment.

• Regulation of Sensory Input: Processes information from senses (vision, hearing, touch, taste, smell).

Responsible for Conscious Perception, Emotion, Personality, etc.

• The nervous system is essential for conscious perception, emotions, and personality.

Structural Organization of the Nervous System

Central Nervous System (CNS)

• Consists of the brain and spinal cord.

• Responsible for integrating and processing information.

Peripheral Nervous System (PNS)

• Consists of nerves and ganglia outside the CNS.

• Connects the CNS to limbs and organs.

Functional Divisions

• Somatic Nervous System: Controls voluntary movements.

• Autonomic Nervous System: Controls involuntary functions (e.g., heart rate, digestion).

Histology of Nervous Tissue

Neurons

• Neurons are the primary cells of the nervous system, specialized for communication.

• They consist of a cell body (soma), dendrites, and an axon.

• Neurons are highly specialized and do not divide after development.

Neuronal Structure

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

• Axon: Long, slender process that transmits impulses away from the cell body.

• Myelin Sheath: Insulating layer around the axon, produced by glial cells, increases speed of impulse transmission.

Representative Neuron Diagram

Key structures: Dendrites, cell body, axon, myelin sheath, axon terminals.

Neuroglia (Glial Cells)

Functions

• Support and protect neurons.

• Maintain homeostasis, form myelin, and provide support and protection for neurons.

Types of Glial Cells

Type	Location	Function
Astrocytes	CNS	Support neurons, maintain blood-brain barrier, regulate ion/nutrient transport.
Oligodendrocytes	CNS	Produce myelin sheaths in CNS.
Microglia	CNS	Phagocytic cells, remove debris and pathogens.
Ependymal cells	CNS	Line ventricles, produce and circulate cerebrospinal fluid (CSF).
Schwann cells	PNS	Produce myelin sheaths in PNS.
Satellite cells	PNS	Support neuron cell bodies in ganglia.

Classification of Neurons

By Structure

• Multipolar Neurons: Many dendrites, one axon (most common type).

• Bipolar Neurons: One dendrite, one axon (found in sensory organs).

• Unipolar Neurons: Single process that splits into two branches (sensory neurons in PNS).

By Function

• Sensory (Afferent) Neurons: Transmit impulses from receptors to CNS.

• Motor (Efferent) Neurons: Transmit impulses from CNS to effectors (muscles/glands).

• Interneurons: Connect sensory and motor neurons within CNS.

Myelination

Functions of Myelin Sheath

• Insulates axons to prevent "short-circuits" between fibers.

• Enables faster conduction of nerve impulses.

• Produced by oligodendrocytes in CNS and Schwann cells in PNS.

Gray Matter vs. White Matter

Type	Location	Composition
Gray Matter	Brain and spinal cord	Neuron cell bodies, dendrites, unmyelinated axons
White Matter	Brain and spinal cord	Myelinated axons

Neurophysiology: Transmission of Nerve Impulses

Resting Membrane Potential

• Describes the electrical potential across the membrane of a resting neuron.

• Typical value: -70 mV (inside negative relative to outside).

Equation:

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

Action Potential

• Rapid change in membrane potential that travels along the axon.

• Triggered when local potentials reach threshold.

• Phases: depolarization, repolarization, hyperpolarization.

Key Steps:

1. Stimulus causes Na+ channels to open, Na+ enters cell (depolarization).

2. K+ channels open, K+ exits cell (repolarization).

3. Temporary hyperpolarization before return to resting potential.

Refractory Periods

• Absolute Refractory Period: No new action potential can be initiated.

• Relative Refractory Period: Action potential possible with strong stimulus.

Saltatory Conduction

• Action potentials "jump" from node to node in myelinated axons, increasing speed.

The Synapse

Structure and Function

• Synapse: Junction between two neurons or a neuron and another cell (e.g., muscle).

• Consists of presynaptic terminal, synaptic cleft, and postsynaptic membrane.

Types of Synapses

• Axodendritic: Between axon and dendrite.

• Axosomatic: Between axon and cell body.

• Axoaxonic: Between axon and axon.

Neurotransmitters and Their Receptors

• Neurotransmitters are chemicals released at synapses to transmit signals.

• Can be excitatory or inhibitory.

• Examples: acetylcholine, norepinephrine, dopamine, serotonin, GABA.

Basic Concepts of Neural Integration

Organization of Neurons

• Neuronal pools: Groups of neurons that integrate incoming information and relay it.

Types of Circuits

• Diverging Circuits: One input, many outputs (e.g., sensory pathways).

• Converging Circuits: Many inputs, one output (e.g., motor pathways).

• Reverberating Circuits: Feedback loops for rhythmic activities.

• Parallel After-Discharge Circuits: Multiple pathways for complex processing.

Developmental Aspects of Neurons

• Neurons originate from the neural tube and neural crest during embryonic development.

• Neurogenesis and synaptogenesis occur during fetal development.

• Most neurons do not divide after birth; apoptosis shapes final neuron population.

Example: Action Potential Graph

The action potential graph shows the rapid depolarization and repolarization of the neuron membrane during impulse transmission.

Additional info: The notes provide a comprehensive overview of nervous tissue, including structure, function, and physiology, suitable for college-level Anatomy & Physiology students.