Chapter 12: Nervous Tissue – Structured Study Notes

Nervous Tissue: Structure and Function

Overview of the Nervous System

The nervous system is a complex network responsible for communication, coordination, and control throughout the body. It consists of various organs, including the brain, spinal cord, sensory receptors, and nerves that connect to other systems.

• Central Nervous System (CNS): Composed of the brain and spinal cord; processes and coordinates sensory data, motor commands, and higher functions such as intelligence, memory, learning, and emotion.

• Peripheral Nervous System (PNS): Includes all nervous tissue outside the CNS; delivers sensory information to the CNS and carries motor commands to peripheral tissues.

• Enteric Nervous System (ENS): Located in the walls of the digestive tract; initiates and coordinates visceral reflexes locally.

Cell Types in Nervous Tissue

Nervous tissue contains two main types of cells:

• Neurons: Specialized for intercellular communication, information processing, and control.

• Neuroglia (Glial Cells): Support and protect neurons, preserve nervous tissue structure, and are essential for neuron survival and function.

Anatomical and Functional Divisions

Anatomical Divisions

• CNS: Brain and spinal cord; contains nervous tissue, neuroglia, and blood vessels.

• PNS: Cranial nerves, spinal nerves, and peripheral nerves; connects CNS to the rest of the body.

Functional Divisions of the PNS

The PNS is divided into afferent and efferent divisions:

• Afferent Division: Carries sensory information from receptors to the CNS.

• Efferent Division: Carries motor commands from the CNS to effectors (muscles, glands, adipose tissue).

Efferent Division Subdivisions

• Somatic Nervous System (SNS): Controls voluntary and involuntary skeletal muscle contractions.

• Autonomic Nervous System (ANS): Controls subconscious actions, smooth and cardiac muscle contractions, and glandular secretions.

  • Sympathetic Division: Stimulating effect.

  • Parasympathetic Division: Relaxing effect.

• Enteric Nervous System (ENS): Controls digestive tract functions independently of CNS, but can be influenced by ANS.

Neurons: Structure and Classification

Neuron Structure

Neurons are the basic functional units of the nervous system.

• Cell Body (Soma): Contains nucleus, nucleolus, perikaryon (cytoplasm), mitochondria, rough endoplasmic reticulum (RER), and ribosomes.

• Dendrites: Short, highly branched processes that receive information from other neurons.

• Axon: Single, long process that propagates electrical signals (action potentials).

• Axon Hillock: Thick region connecting the axon to the cell body.

• Axon Terminals: Tips of telodendria where synaptic transmission occurs.

Axonal Transport

Axonal (axoplasmic) transport is the movement of materials between the cell body and axon terminals, powered by mitochondria, kinesin, and dynein.

Structural Classification of Neurons

Neurons are classified based on their structure:

Type	Structure	Location
Anaxonic	Small, processes look similar	Brain, special sense organs
Bipolar	One dendrite, one axon	Special sense organs (sight, smell, hearing)
Unipolar	Axon and dendrites fused, cell body to one side	Most sensory neurons of PNS
Multipolar	One long axon, two or more dendrites	Common in CNS, all motor neurons controlling skeletal muscles

Functional Classification of Neurons

• Sensory Neurons (Afferent): Unipolar, cell bodies in sensory ganglia, transmit sensory information to CNS.

• Motor Neurons (Efferent): Carry instructions from CNS to effectors; somatic motor neurons innervate skeletal muscles, visceral motor neurons innervate smooth/cardiac muscle, glands, adipose tissue.

• Interneurons: Located between sensory and motor neurons; responsible for distribution of sensory information and coordination of motor activity; involved in higher functions (memory, planning, learning).

Neuroglia: Types and Functions

Neuroglia in the CNS

• Astrocytes: Maintain blood-brain barrier, create CNS framework, guide neuron development, control interstitial environment.

• Ependymal Cells: Line central canal and ventricles, produce and monitor cerebrospinal fluid (CSF), have cilia to circulate CSF.

• Oligodendrocytes: Form myelin sheath around axons, increase speed of action potentials, create white matter.

• Microglia: Migrate through nervous tissue, clean up debris, wastes, and pathogens.

Neuroglia in the PNS

• Satellite Cells: Surround ganglia, regulate interstitial fluid around neurons.

• Schwann Cells: Form myelin sheath or indented folds around axons, insulate axons, aid in regeneration after injury.

Membrane Potentials and Electrical Activity

Resting Membrane Potential

The resting membrane potential is the electrical potential across the plasma membrane of a resting cell, typically −70 mV.

• ECF contains high Na+ and Cl−; cytosol contains high K+ and negatively charged proteins.

• Selective permeability of the membrane allows for differences in ion concentration.

• Passive and active processes (current, resistance, chemical/electrical gradients) maintain the potential.

Active Processes: Sodium–Potassium Pump

The sodium–potassium exchange pump uses ATP to eject 3 Na+ for every 2 K+ brought in, stabilizing the resting membrane potential.

• Equation: $\text{Na}^+/\text{K}^+ \text{ pump:} \ 3\text{Na}^+_{\text{out}} : 2\text{K}^+_{\text{in}}$

Types of Ion Channels

• Passive (Leak) Channels: Always open, permeability changes with conditions.

• Active (Gated) Channels: Open/close in response to stimuli; include chemically gated, voltage-gated, and mechanically gated channels.

Graded and Action Potentials

Graded Potentials

Graded potentials are localized changes in membrane potential that cannot spread far from the site of stimulation.

• Depolarization: Opening of Na+ channels causes membrane potential to rise.

• Repolarization: Removal of stimulus returns membrane potential to normal.

• Hyperpolarization: Opening of K+ channels increases negativity.

Action Potentials

Action potentials are propagated changes in membrane potential that affect the entire excitable membrane.

• All-or-none principle: Any stimulus reaching threshold triggers an action potential.

• Steps: Depolarization to threshold, activation of voltage-gated Na+ channels, inactivation of Na+ channels and activation of K+ channels, return to resting potential.

Propagation of Action Potentials

• Continuous Propagation: Occurs in unmyelinated axons; action potential moves segment by segment.

• Saltatory Propagation: Occurs in myelinated axons; action potential jumps from node to node, faster and more energy-efficient.

Types of Axons

Type	Myelination	Diameter	Speed	Function
A	Myelinated	Large	120 m/sec	Rapid sensory/motor info
B	Myelinated	Medium	18 m/sec	Intermediate speed
C	Unmyelinated	Small	1 m/sec	Slow sensory info

Synapses and Neurotransmitters

Types of Synapses

• Electrical Synapses: Direct physical contact via gap junctions; rapid transmission.

• Chemical Synapses: Signal transmitted across a gap by neurotransmitters; most common.

Neurotransmitters

• Acetylcholine (ACh): Released at neuromuscular junctions, CNS, PNS, and parasympathetic ANS.

• Biogenic Amines: Norepinephrine (NE), dopamine, serotonin.

• Amino Acids: Glutamate (excitatory), glycine (inhibitory), GABA (inhibitory).

• Neuropeptides: Substance P, opioids (enkephalins, endorphins, dynorphins).

• Dissolved Gases: Nitric oxide (NO), carbon monoxide (CO).

Synaptic Transmission

• Action potential arrives at axon terminal, triggers exocytosis of neurotransmitter.

• Neurotransmitter binds to postsynaptic membrane, causing graded potential.

• Enzymes break down neurotransmitter; axon terminal reabsorbs and reassembles components.

Information Processing and Integration

Postsynaptic Potentials

• Excitatory Postsynaptic Potential (EPSP): Graded depolarization.

• Inhibitory Postsynaptic Potential (IPSP): Graded hyperpolarization.

Summation

• Temporal Summation: Rapid, repeated stimuli at a single synapse.

• Spatial Summation: Simultaneous stimuli at multiple synapses.

Axoaxonic Synapses

• Presynaptic Inhibition: Decreases neurotransmitter release.

• Presynaptic Facilitation: Increases neurotransmitter release.

Summary Table: Nervous Tissue Components

Component	Function
Neuron	Communication, processing, control
Neuroglia	Support, protection, maintenance
CNS	Integration, coordination, higher functions
PNS	Sensory input, motor output
ENS	Digestive tract control