Nervous Tissue

Introduction to Nervous Tissue

The nervous tissue is a specialized tissue that integrates and coordinates all body activities. It works closely with the endocrine system to maintain homeostasis and respond to internal and external stimuli.

• Basic Functions: Sensory input, integration, and motor output.

• Key Components: Neurons (nerve cells) and neuroglia (supporting cells).

Basic Functions of the Nervous System

Overview of Functions

The nervous system performs three essential functions:

• Sensory Function: Sensory receptors detect changes in the internal and external environments.

• Integrative Function: The nervous system processes, interprets, and stores sensory information, making decisions about appropriate responses.

• Motor Function: The nervous system elicits responses by activating effectors such as muscles (causing contraction) and glands (causing secretion).

Main Divisions of the Nervous System

Central and Peripheral Nervous Systems

The nervous system is divided into two main parts, each with specific roles and subdivisions.

• Central Nervous System (CNS): Consists of the brain and spinal cord. Responsible for processing and integrating information.

• Peripheral Nervous System (PNS): Includes all nervous tissue outside the CNS, such as cranial nerves and spinal nerves.

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

• Efferent (Motor) Fibers: Transmit impulses from the CNS to effectors (muscles and glands).

Subdivisions of the Peripheral Nervous System

• Somatic Nervous System (SNS): Controls voluntary movements by transmitting messages from the CNS to skeletal muscles. Sensory neurons convey information from external receptors to the CNS.

• Autonomic Nervous System (ANS): Regulates involuntary functions by transmitting messages from the CNS to smooth muscle, cardiac muscle, and glands. Divided into:

  • Sympathetic Division: Prepares the body for 'fight or flight' responses.

  • Parasympathetic Division: Promotes 'rest and digest' activities.

• Enteric Nervous System (ENS): Sometimes called the 'brain of the gut,' it controls gastrointestinal functions independently but can communicate with the CNS.

Cells of Nervous Tissue

Neurons vs. Neuroglia

Nervous tissue contains two main types of cells:

• Neurons: Specialized for conducting electrical impulses. They are the functional units of the nervous system.

• Neuroglia (Glial Cells): Support, protect, and nourish neurons. They are more numerous than neurons and do not conduct impulses.

Types of Neuroglial Cells

Neuroglial cells differ between the CNS and PNS:

Structure of a Neuron

Parts of a Neuron

• Cell Body (Soma): Contains the nucleus, Nissl bodies (rough ER), and organelles. Responsible for metabolic activities.

• Dendrites: Highly branched processes that receive impulses and conduct them toward the cell body.

• Axon: Long, cylindrical process that conducts impulses away from the cell body. Begins at the axon hillock and ends in axon terminals (synaptic end bulbs containing neurotransmitters).

Myelinated vs. Unmyelinated Neurons

• Myelinated Neurons: Axons are covered by a myelin sheath (lipid and protein), which acts as an electrical insulator and increases the speed of impulse conduction (saltatory conduction).

• Unmyelinated Neurons: Axons lack a myelin sheath or have only a thin covering, resulting in slower, continuous conduction.

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

Types of Neurons

Structural Classification

• Multipolar Neurons: Many dendrites and one axon; most common in CNS and all motor neurons.

• Bipolar Neurons: One dendrite and one axon; found in the retina, inner ear, and olfactory epithelium.

• Unipolar (Pseudounipolar) Neurons: Single process that splits into two branches; found in sensory ganglia of PNS.

Functional Classification

• Sensory (Afferent) Neurons: Transmit sensory information from receptors to the CNS.

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

• Interneurons (Association Neurons): Connect neurons within the CNS; involved in processing and integration.

Neural Circuits

Types and Functions

Neural circuits are patterns of synaptic connections that process specific types of information.

• Diverging Circuit: One neuron stimulates many others, amplifying the signal (e.g., motor pathways).

• Converging Circuit: Many neurons synapse on a single neuron, integrating information (e.g., sensory pathways).

• Reverberating Circuit: Neurons stimulate each other in a loop, producing rhythmic activities (e.g., breathing).

• Parallel After-Discharge Circuit: Single input diverges to stimulate a group of neurons, which then converge on a single output (e.g., complex calculations).

Regeneration and Repair of Nervous Tissue

Regeneration Process

• In the CNS: Limited ability to repair due to lack of neurolemma and inhibitory factors from neuroglia.

• In the PNS: Axons and dendrites may regenerate if the cell body is intact and Schwann cells remain active to form a regeneration tube.

• Wallerian Degeneration: Breakdown of axon and myelin distal to injury; regeneration occurs as axons grow along the Schwann cell tube.

Example: After peripheral nerve injury, Schwann cells proliferate and guide regrowth of axons at a rate of about 1.5 mm/day.

Key Terms and Concepts

• Action Potential: Rapid change in membrane potential that propagates along the axon.

• Synapse: Junction between two neurons or a neuron and an effector cell, where neurotransmitters mediate communication.

• Neurotransmitter: Chemical messenger released from synaptic vesicles to transmit signals across a synapse.

Summary Table: Neuroglial Cells

Key Equations

• Resting Membrane Potential:

• Ohm's Law (for membrane potential):

• Nernst Equation (for equilibrium potential):

Additional info: Some details, such as the Nernst equation and Ohm's law, are standard academic context for understanding membrane potentials and are included for completeness.