Nervous Tissue Overview

Introduction

The nervous tissue is a fundamental component of the nervous system, responsible for communication, integration, and control throughout the body. It consists of specialized cells that transmit electrical and chemical signals, enabling rapid responses to internal and external stimuli.

Major Components of Nervous Tissue

• Neurons: The primary signaling cells, responsible for transmitting nerve impulses.

• Glial Cells (Neuroglia): Supportive cells that maintain the environment around neurons, provide insulation, and participate in immune defense.

Neuron Structure

Key Parts of a Neuron

• Soma (Cell Body): Contains the nucleus and organelles; site of metabolic activity.

• Axon: Long, slender projection that transmits electrical impulses away from the soma.

• Axon Hillock: Region where the axon originates; site of action potential initiation.

• Dendrites: Branching extensions that receive signals from other neurons.

• Axon Collateral: Branches off the main axon, allowing communication with multiple targets.

• Terminal Extensions: Endings of the axon that form synapses with other cells.

• Synaptic Knob: Bulbous end of the axon terminal, containing neurotransmitters.

• Myelin Sheath: Insulating layer around axons, increasing conduction speed.

• Neurofibril Nodes (Nodes of Ranvier): Gaps in the myelin sheath where action potentials are regenerated.

Types of Neurons

Classification Based on Structure

• Multipolar Neuron: Multiple dendrites and a single axon; most common type in CNS.

• Bipolar Neuron: One dendrite and one axon; found in sensory organs (e.g., retina).

• Unipolar Neuron: Single process that splits into two branches; common in sensory neurons of PNS.

• Anaxonic Neuron: No distinct axon; found in the brain and sense organs.

Types of Glial Cells

Functions and Locations

• Astrocyte: Maintains blood-brain barrier, regulates ion balance, and supports neurons.

• Ependymal Cell: Lines ventricles of the brain and central canal of spinal cord; produces cerebrospinal fluid.

• Microglial Cell: Acts as immune defense in the CNS; phagocytizes debris and pathogens.

• Oligodendrocyte: Forms myelin sheath in the CNS.

• Satellite Cell: Supports neurons in the PNS.

• Neurolemmocyte (Schwann Cell): Forms myelin sheath in the PNS.

Nerve Structure

Connective Tissue Layers

• Epineurium: Outermost layer; surrounds the entire nerve.

• Perineurium: Surrounds bundles (fascicles) of axons.

• Endoneurium: Surrounds individual axons.

Functional Organization of the Nervous System

Major Divisions

• Central Nervous System (CNS): Brain and spinal cord; processes and integrates information.

• Peripheral Nervous System (PNS): All neural tissue outside the CNS; transmits signals to and from the CNS.

General Functions of the Nervous System

• Sensory Input: Detects changes in the environment.

• Integration: Processes and interprets sensory input.

• Motor Output: Initiates responses by activating effectors (muscles or glands).

Synapses

Structure and Function

• Presynaptic Neuron: Sends the signal.

• Postsynaptic Neuron: Receives the signal.

• Synaptic Cleft: Gap between neurons where neurotransmitters are released.

Action Potentials and Nerve Impulses

Key Concepts

• Resting Membrane Potential: The electrical potential difference across the membrane of a resting neuron, typically around -70 mV.

• Depolarization: Reduction in membrane potential, making the inside less negative.

• Repolarization: Restoration of the resting potential after depolarization.

• Hyperpolarization: Increase in membrane potential, making the inside more negative than resting.

Equation for Resting Membrane Potential:

Additional info: The Goldman-Hodgkin-Katz equation is often used to calculate the resting membrane potential based on the permeability and concentration of multiple ions.

Myelination and Conduction Velocity

Role of Myelin

• Myelinated Axons: Conduct impulses rapidly via saltatory conduction (jumping between nodes of Ranvier).

• Unmyelinated Axons: Conduct impulses more slowly via continuous conduction.

Wallerian Degeneration and Axon Regeneration

Process Overview

• Wallerian Degeneration: Breakdown of the distal part of an axon after injury.

• Axon Regeneration: Limited in the CNS, more successful in the PNS due to Schwann cell activity.

Voltage-Gated Sodium Channels

States and Function

• Activation State: Channel opens in response to depolarization, allowing Na+ influx.

• Inactivation State: Channel closes shortly after opening, stopping Na+ influx.

• Resting State: Channel is closed but ready to open if stimulated.

Neurotransmitters

Role and Removal

• Function: Chemical messengers that transmit signals across synapses.

• Removal Mechanisms: Reuptake into presynaptic cell, enzymatic degradation, or diffusion away from synaptic cleft.

Table: Types of Neurons and Glial Cells

Summary

Nervous tissue is essential for rapid communication and control in the body. Understanding the structure and function of neurons and glial cells, the organization of nerves, and the mechanisms of signal transmission is fundamental for students of anatomy and physiology.