Chapter 12: Nervous Tissue – Structured Study Notes for Anatomy & Physiology

Nervous Tissue

An Introduction to the Nervous System

The nervous system is a complex network responsible for receiving, processing, and responding to internal and external stimuli. It consists of the brain, spinal cord, sensory receptors, and nerves that connect to other body systems.

• Functions:

  • Receives information from internal and external stimuli

  • Processes information and initiates responses

• Cell Types:

  • Neurons: Specialized for intercellular communication

  • Neuroglia (glial cells): Support, protect, and preserve nervous tissue structure

Divisions of the Nervous System

The nervous system is divided anatomically and functionally into the central and peripheral nervous systems.

• Central Nervous System (CNS):

  • Includes brain and spinal cord

  • Integrates, processes, and coordinates sensory information and motor commands

  • Responsible for higher functions: intelligence, memory, learning, emotion

• Peripheral Nervous System (PNS):

  • All nervous tissue outside CNS

  • Delivers sensory information to CNS and carries motor commands to peripheral tissues

  • Nerves: Bundles of axons with connective tissues and blood vessels

  • Cranial nerves connect to brain; spinal nerves connect to spinal cord

• Functional Divisions of PNS:

  • Afferent Division: Carries sensory information from receptors to CNS

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

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

  • Autonomic Nervous System (ANS): Controls smooth muscle, cardiac muscle, adipose tissue, glands (involuntary); includes sympathetic and parasympathetic divisions

  • Enteric Nervous System (ENS): Neurons in digestive tract walls, coordinates local visceral reflexes

Neurons

Structure and Function of Neurons

Neurons are the basic functional units of the nervous system, specialized for communication, information processing, and control.

• Cell Body (Soma): Contains nucleus, perikaryon (cytoplasm), neurofilaments, neurotubules, and neurofibrils for structural support

• Dendrites: Highly branched processes that receive information from other neurons

• Axon: Single, long process that propagates electrical signals (action potentials); includes axoplasm, axolemma, initial segment, axon hillock, collaterals, telodendria, and axon terminals

Axonal Transport

• Anterograde: Movement from cell body to axon terminals

• Retrograde: Movement from axon terminals to cell body (e.g., rabies virus infection)

Classification of Neurons

• Structural Classification:

  • Anaxonic: Many dendrites, no obvious axon; found in brain and special sense organs

  • Bipolar: One dendrite, one axon; rare, found in special sense organs

  • Unipolar: Axon and dendrites continuous, soma off to side; most sensory neurons in PNS

  • Multipolar: One long axon, two or more dendrites; common in CNS and all PNS motor neurons

• Functional Classification:

  • Sensory (Afferent) Neurons: Carry information from receptors to CNS

  • Motor (Efferent) Neurons: Carry instructions from CNS to effectors

  • Interneurons: Integrate sensory information and coordinate motor commands; involved in higher functions

Neuroglia

Types and Functions of Neuroglia

Neuroglia are supporting cells that protect and maintain neurons, making up half the volume of the nervous system.

• CNS Neuroglia:

  • Astrocytes: Maintain blood-brain barrier, repair tissue, guide development, regulate environment

  • Oligodendrocytes: Form myelin sheath, increase speed of action potentials

  • Ependymal Cells: Line ventricles and central canal, produce and monitor cerebrospinal fluid

  • Microglia: Phagocytic cells that clean up debris and pathogens

• PNS Neuroglia:

  • Satellite Cells: Surround neuron cell bodies in ganglia, regulate interstitial fluid

  • Schwann Cells: Form myelin sheath or folds around axons, aid in regeneration

Myelination and Peripheral Nerve Regeneration

• Myelin: Lipid insulation that increases speed of action potentials

• Internodes: Myelinated segments

• Nodes of Ranvier: Gaps in myelin sheath

• White Matter: Regions with myelinated axons

• Gray Matter: Regions with unmyelinated axons, cell bodies, dendrites

Membrane Potential

Resting Membrane Potential

The resting membrane potential is the voltage difference across the cell membrane of an unstimulated neuron, typically around -70 mV.

• Ion Distribution:

  • ECF: High Na+ and Cl-

  • Cytosol: High K+ and negatively charged proteins

• Selective Permeability: More K+ leaks than Na+; proteins cannot cross membrane

• Sodium-Potassium Pump: Maintains gradient by moving 3 Na+ out and 2 K+ in per ATP

Electrochemical Gradients and Equilibrium Potential

• Electrochemical Gradient: Sum of chemical and electrical forces acting on an ion

• Equilibrium Potential: Membrane potential at which there is no net movement of a particular ion

Membrane Channels

• Gated Ion Channels: Open/close in response to stimuli; types include chemically gated, voltage-gated, and mechanically gated

Graded Potentials

• Definition: Temporary, localized changes in membrane potential

• Depolarization: Shift toward less negative potential (Na+ influx)

• Hyperpolarization: Shift toward more negative potential (K+ efflux)

• Repolarization: Return to resting potential

Action Potential

Generation and Propagation of Action Potentials

An action potential is a large depolarization that propagates along the axon, following the all-or-none principle.

• Steps:

  1. Depolarization to threshold

  2. Rapid depolarization (Na+ influx)

  3. Repolarization (K+ efflux)

  4. Hyperpolarization (brief, due to slow K+ channel closure)

• Refractory Period:

  • Absolute: No response possible

  • Relative: Larger stimulus required

Propagation Types

• Continuous Propagation: Unmyelinated axons; action potential moves stepwise

• Saltatory Propagation: Myelinated axons; action potential jumps from node to node, faster and more energy-efficient

Axon Types and Propagation Speed

• Type A: Myelinated, large diameter, rapid transmission (sensory info, motor impulses)

• Type B: Myelinated, medium diameter, intermediate speed

• Type C: Unmyelinated, small diameter, slow transmission (sensory info from skin, motor instructions to smooth/cardiac muscle and glands)

Synapses

Structure and Function of Synapses

A synapse is a specialized site where a neuron communicates with another cell, either electrically or chemically.

• Electrical Synapses: Direct physical contact, rapid transmission

• Chemical Synapses: Use neurotransmitters to transmit signals across a synaptic cleft

• Types: Axoaxonic, axosomatic, axodendritic, neuromuscular, neuroglandular

Cholinergic Synapses

• Release acetylcholine (ACh)

• Events: Action potential arrives, Ca2+ influx triggers ACh release, ACh binds to postsynaptic receptors, ACh broken down by acetylcholinesterase

Neurotransmitters and Neuromodulators

Types and Effects

• Excitatory: Cause depolarization, promote action potentials

• Inhibitory: Cause hyperpolarization, suppress action potentials

• Major Classes:

  • Acetylcholine: Excitatory, CNS and PNS

  • Biogenic Amines: Norepinephrine (excitatory), dopamine (excitatory/inhibitory), serotonin (affects mood)

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

  • Neuropeptides: Opioids (pain relief), enkephalins, endorphins, dynorphins

  • Dissolved Gases: Nitric oxide, carbon monoxide

Mechanisms of Action

• Ionotropic (Direct): Open/close ion channels directly (e.g., ACh, glutamate)

• Metabotropic (Indirect): Bind to G protein-coupled receptors, activate second messengers (e.g., NE, dopamine, serotonin, GABA)

• Intracellular Enzyme Activation: Lipid-soluble gases enter cell and activate enzymes (e.g., NO, CO)

Information Processing in Nervous Tissue

Postsynaptic Potentials and Summation

• Excitatory Postsynaptic Potential (EPSP): Graded depolarization

• Inhibitory Postsynaptic Potential (IPSP): Graded hyperpolarization

• Summation:

  • Temporal: Rapid, repeated stimuli at a single synapse

  • Spatial: Simultaneous stimuli at multiple synapses

Facilitation and Presynaptic Regulation

• Facilitation: Brings membrane potential closer to threshold

• Presynaptic Inhibition: Decreases neurotransmitter release

• Presynaptic Facilitation: Increases neurotransmitter release

Rate of Action Potential Generation

• Strength of response is proportional to frequency of stimulation

• Maximum rate reached when relative refractory period is eliminated

Summary

• Information is relayed as action potentials

• Neurotransmitters and neuromodulators can have excitatory or inhibitory effects

• Neurons may be facilitated or inhibited by chemicals

• Response of postsynaptic neuron can be altered by neuromodulators, activity at other synapses, or modification of neurotransmitter release

Key Equations

Nernst Equation for Equilibrium Potential

The equilibrium potential for an ion can be calculated using the Nernst equation:

• For potassium (K+): $E_{K^+} = \frac{RT}{zF} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$

• For sodium (Na+): $E_{Na^+} = \frac{RT}{zF} \ln \left( \frac{[Na^+]_{out}}{[Na^+]_{in}} \right)$

Summary Table: Types of Neuroglia

Neuroglia Type	Location	Main Function
Astrocytes	CNS	Maintain BBB, repair, guide development, regulate environment
Oligodendrocytes	CNS	Form myelin sheath
Ependymal Cells	CNS	Produce/monitor CSF
Microglia	CNS	Phagocytosis
Satellite Cells	PNS	Regulate interstitial fluid
Schwann Cells	PNS	Myelinate axons, aid regeneration

Summary Table: Structural Classification of Neurons

Type	Structure	Location
Anaxonic	Many dendrites, no obvious axon	Brain, special sense organs
Bipolar	One dendrite, one axon	Special sense organs
Unipolar	Axon and dendrites continuous, soma off to side	Sensory neurons in PNS
Multipolar	One long axon, two or more dendrites	CNS, all PNS motor neurons