Nervous Tissue: Structure, Function, and Physiology

Nervous Tissue

Introduction

Nervous tissue is a specialized tissue responsible for regulating body activities, maintaining homeostasis, and enabling perception, behavior, and voluntary movement. It achieves these functions through the rapid transmission of electrical signals known as action potentials.

Nervous System: Structure and Function

Main Functions

• Regulation and Homeostasis: The nervous system rapidly responds to internal and external stimuli to maintain the body's internal balance.

• Perception, Behavior, and Memory: It is responsible for sensory perception, cognitive processes, and memory storage.

• Initiation of Voluntary Movements: All voluntary muscle movements are initiated by the nervous system.

Structural Components

• Brain: Enclosed in the skull, containing approximately 100 billion neurons.

• Cranial Nerves: 12 pairs of nerves emerging from the brain, each consisting of hundreds to thousands of axons.

• Spinal Nerves: 31 pairs of nerves emerging from the spinal cord, supplying the body below the head.

• Spinal Cord: Enclosed in the spinal cavity, connecting to the brain at the foramen magnum; contains about 100 million neurons.

• Ganglia: Small masses of nervous tissue containing neuron cell bodies, located outside the brain and spinal cord.

Basic Functions

1. Sensory Function: Detects internal and external stimuli via sensory receptors (e.g., Merkel cells, Meissner's corpuscles, Pacinian corpuscles, free nerve endings, baroreceptors). Sensory (afferent) neurons carry this information to the CNS.

2. Integrative Function: The CNS processes and integrates sensory information, allowing for appropriate responses.

3. Motor Function: The CNS sends commands via motor (efferent) neurons to effectors (muscles and glands) to elicit responses such as contraction or secretion.

Organization of the Nervous System

Major Divisions

• Central Nervous System (CNS): Composed of the brain and spinal cord.

• Peripheral Nervous System (PNS): All nervous tissue outside the CNS.

Subdivisions of the PNS

• Somatic Nervous System (SNS): Sensory neurons from the head, body wall, limbs, and special sense organs; motor neurons to skeletal muscles. Voluntary control.

• Autonomic Nervous System (ANS): Sensory neurons from visceral organs; motor neurons to smooth muscle, cardiac muscle, and glands. Involuntary control.

• Enteric Nervous System (ENS): Nerves controlling the digestive system. Involuntary control.

ANS Motor Subdivisions

• Sympathetic Division: Controls "fight or flight" responses (e.g., increases heart rate during emergencies).

• Parasympathetic Division: Controls "rest and digest" responses (e.g., decreases heart rate during relaxation).

Histology of Nervous Tissue

Cell Types

• Neurons: Electrically excitable cells that respond to stimuli and generate action potentials.

• Neuroglia: Support, nourish, and protect neurons; capable of cell division.

Neurons: Structure and Function

Parts of a Neuron

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

• Dendrites: Multiple, short, branched structures that receive input.

• Axon: Long projection that propagates action potentials. The axon hillock is the initial segment where impulses are generated. Axon terminals form synaptic end bulbs containing neurotransmitters.

Structural Diversity

• Multipolar Neurons: Several dendrites, one axon; found in brain and spinal cord.

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

• Unipolar Neurons: Dendrite and axon fused; found in ganglia of spinal/cranial nerves.

Neuroglia

General Features

• Support and nourish neurons; make up about half of CNS volume.

• Smaller and more numerous than neurons (5-50 times).

• Capable of cell division; do not generate action potentials.

Neuroglia of the CNS

• Astrocytes: Star-shaped, most numerous; support neurons, maintain chemical environment, regulate ions, and uptake neurotransmitters.

• Oligodendrocytes: Form and maintain myelin sheath around CNS axons, increasing conduction speed.

• Microglia: Small, phagocytic cells that remove debris and pathogens.

• Ependymal Cells: Line brain ventricles and spinal cord central canal; produce and circulate cerebrospinal fluid (CSF).

Neuroglia of the PNS

• Schwann Cells: Form myelin sheaths around PNS axons; aid in axon regeneration.

• Satellite Cells: Surround neuron cell bodies in PNS ganglia; regulate material exchange.

Myelination

Process and Function

• Myelinated Neurons: Insulated by myelin sheath, conduct action potentials rapidly.

• Unmyelinated Neurons: Lack myelin, conduct impulses more slowly.

• PNS Myelination: Schwann cells wrap around axons, forming neurolemma (outer layer) and myelin sheath (inner layers). Gaps are called nodes of Ranvier.

• CNS Myelination: Oligodendrocytes myelinate multiple axons; no neurolemma, limiting regeneration after injury.

Gray and White Matter

• White Matter: Myelinated nervous tissue (white due to myelin).

• Gray Matter: Unmyelinated nervous tissue.

• Spinal Cord: White matter surrounds an inner H-shaped core of gray matter.

• Brain: Thin shell of gray matter surrounds the core of white matter.

Electrical Signals in Neurons

Action Potentials

• Action potentials are electrical signals generated by the movement of ions (Na+, K+) across the neuron's plasma membrane.

• Ions move along electrochemical gradients, creating changes in membrane potential.

Ion Channels

• Allow ions to cross the membrane; have gates that open/close in response to stimuli.

• Types of ion channels:

  • Leakage Channels: Randomly open/close, contribute to resting membrane potential (especially K+).

  • Voltage-Gated Channels: Open in response to changes in membrane potential; essential for action potentials.

  • Ligand-Gated Channels: Open in response to binding of neurotransmitters, hormones, or ions.

  • Mechanically-Gated Channels: Open in response to mechanical stimuli (vibration, pressure, stretch).

Resting Membrane Potential

• The voltage across the neuron's membrane at rest, typically -70 mV (inside negative relative to outside).

• Maintained by ion gradients and selective permeability of the membrane.

• Key equation: $V_{rest} \approx -70\ \text{mV}$

Generation of Action Potentials

• Occurs in two phases: depolarization and repolarization.

• All-or-None Principle: An action potential is generated only if the stimulus reaches threshold (about -55 mV); otherwise, no action potential occurs.

• Depolarization: Voltage-gated Na+ channels open, Na+ rushes in, membrane potential becomes positive (up to +30 mV).

• Repolarization: Voltage-gated K+ channels open (slower), K+ exits, membrane potential returns to negative.

Propagation of Action Potentials

• Continuous Conduction: Occurs in unmyelinated axons; action potential moves along every part of the membrane.

• Saltatory Conduction: Occurs in myelinated axons; action potential "jumps" from node to node (nodes of Ranvier), increasing speed.

• Axon Diameter: Larger diameter axons conduct impulses faster.

Transmission at the Synapse

Synapse Structure and Function

• Synapse: Junction between axon terminal of one neuron and another neuron's dendrite, cell body, or axon.

• Presynaptic Neuron: Sends the signal.

• Postsynaptic Neuron: Receives the signal.

Chemical Synapse Steps

1. Nerve impulse arrives at synaptic end bulb.

2. Depolarization opens voltage-gated Ca2+ channels; Ca2+ enters.

3. Ca2+ influx triggers exocytosis of synaptic vesicles, releasing neurotransmitter.

4. Neurotransmitter diffuses across synaptic cleft, binds to receptors on postsynaptic membrane.

5. Ligand-gated channels open, Na+ influx depolarizes postsynaptic membrane.

6. If threshold is reached, a new action potential is generated in the postsynaptic neuron.

Neurotransmitter Removal

• Diffusion: Neurotransmitter diffuses away from synaptic cleft.

• Enzymatic Degradation: Enzymes break down neurotransmitter (e.g., acetylcholinesterase).

• Reuptake: Neurotransmitter is reabsorbed by presynaptic neuron for recycling.

Clinical Application: SSRIs

• Selective Serotonin Reuptake Inhibitors (SSRIs): Drugs that block reuptake of serotonin, increasing its availability in the synaptic cleft; used to treat depression (e.g., Prozac, Zoloft).

Neurotransmitters

• Over 100 types identified; acetylcholine (ACh) is the most common, found in both CNS and PNS, and at neuromuscular junctions.

Regeneration and Repair of Nervous Tissue

Regeneration Capacity

• Neurons have limited ability to replicate or repair.

• PNS: Myelinated axons/dendrites can regenerate if cell body and Schwann cells remain intact.

• CNS: Little to no repair due to absence of Schwann cells and neurolemma.

Repair in the PNS

• Schwann cells form neurolemma, enabling axon/dendrite repair if cell body is intact.

• After injury: Nissl bodies break down (chromatolysis), distal axon/myelin degenerate (Wallerian degeneration), macrophages clear debris, Schwann cells form regeneration tube guiding regrowth.

Clinical Correlations

Multiple Sclerosis (MS)

• Autoimmune disease causing destruction of CNS myelin sheaths, leading to scar tissue (scleroses).

• Symptoms: Speech difficulties, double vision, muscle weakness; more common in women aged 20-40.

Epilepsy

• Characterized by recurrent seizures due to abnormal electrical discharges in the brain.

• Causes: Birth injury, metabolic disturbances, infections, toxins, trauma, tumors.

• Treatment: Antiepileptic drugs (e.g., carbamazepine, phenytoin).