Nervous Tissue: Structure, Function, and Physiology

Chapter 12: Nervous Tissue

Introduction to the Nervous System

The nervous system is a complex network responsible for communication, integration, and coordination throughout the body. It includes the brain, spinal cord, sensory receptors, and nerves that connect to other systems.

• Functions:

  • Receive information from internal and external stimuli

  • Process the information

  • Initiate responses

• Major Components:

  • Neurons: Specialized for intercellular communication

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

Divisions of the Nervous System

Anatomical Divisions

• Central Nervous System (CNS): Consists of the 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 the CNS

  • Delivers sensory information to the CNS

  • Carries motor commands from the CNS to peripheral tissues

  • Nerves (peripheral nerves): Bundles of axons with connective tissue and blood vessels

    • Cranial nerves: Connect to the brain

    • Spinal nerves: Connect to the spinal cord

Functional Divisions of the PNS

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

  • Receptors: Detect stimuli (can be single cells or complex organs)

  • Types: Visceral (internal organs), Somatic (muscles, joints, skin), Special senses (e.g., vision, hearing)

• Efferent Division: Carries motor commands from the CNS to effectors

  • Effectors: Muscles, glands, adipose tissue

Subdivisions of the Efferent Division

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

• Autonomic Nervous System (ANS): Controls smooth muscle, cardiac muscle, adipose tissue, and glands (involuntary)

  • Sympathetic Division: Prepares body for 'fight or flight'

  • Parasympathetic Division: Promotes 'rest and digest' activities

• Enteric Nervous System (ENS): Neurons in the digestive tract that coordinate local reflexes independently of the CNS, but can be influenced by the ANS

Neurons: Structure and Classification

Structure of a Typical Neuron

• Cell Body (Soma):

  • Contains nucleus and nucleolus

  • Perikaryon: Cytoplasm surrounding the nucleus

  • Cytoskeleton: Neurofilaments, neurotubules, neurofibrils (support and transport)

  • No centrioles (neurons do not divide)

  • Nissl bodies: Dense areas of rough ER and ribosomes (gray color)

• Dendrites: Short, branched processes that receive information from other neurons; contain dendritic spines for increased surface area

• Axon: Long process that propagates action potentials

  • Axoplasm: Cytoplasm of the axon

  • Axolemma: Plasma membrane of the axon

  • Initial segment: First part of the axon

  • Axon hillock: Connects initial segment to cell body

  • Collaterals: Branches of the axon

  • Telodendria: Fine branches at the end of the axon

  • Axon terminals: Expanded tips of telodendria (synaptic terminals)

Axonal Transport

• Movement of materials between cell body and axon terminals along neurotubules

• Kinesin: Anterograde transport (cell body to axon terminal)

• Dynein: Retrograde transport (axon terminal to cell body)

• Requires ATP

• Clinical relevance: Rabies virus uses retrograde transport

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 (rare)
Unipolar (Pseudounipolar)	Continuous dendrite and axon, cell body/soma off to side	Most sensory neurons of PNS
Multipolar	One long axon, two or more dendrites	Most common in CNS, all motor neurons in PNS

Functional Classification of Neurons

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

  • Somatic sensory: Monitor external environment and position

  • Visceral sensory: Monitor internal environment

  • Receptors:

    • Interoceptors: Internal systems (e.g., digestive, urinary), stretch, pain

    • Exteroceptors: Touch, temperature, pressure, special senses

    • Proprioceptors: Position and movement of muscles/joints

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

  • Somatic motor: Innervate skeletal muscles

  • Visceral motor: Innervate smooth/cardiac muscle, glands, adipose tissue

  • Preganglionic and postganglionic neurons in ANS

• Interneurons: Located between sensory and motor neurons; integrate sensory input and coordinate motor output; involved in higher functions (memory, learning, planning)

Neuroglia: Support Cells of the Nervous System

Neuroglia in the CNS

Type	Main Functions
Astrocytes	Maintain blood-brain barrier, structural support, repair, guide development, regulate interstitial environment
Oligodendrocytes	Form myelin sheath in CNS, increase action potential speed
Ependymal cells	Line ventricles and central canal, produce and circulate cerebrospinal fluid (CSF)
Microglia	Phagocytize debris, waste, and pathogens

Neuroglia in the PNS

Type	Main Functions
Satellite cells	Surround neuron cell bodies in ganglia, regulate interstitial fluid
Schwann cells	Form myelin sheath around axons, aid in regeneration after injury

Myelination and Demyelination

• Myelin: Lipid-rich insulation that increases speed of action potential propagation

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

• White matter: Regions with many myelinated axons

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

• Demyelination: Loss of myelin due to toxins, diseases (e.g., multiple sclerosis, diphtheria)

Neural Response to Injury

• PNS: Wallerian degeneration (distal axon degenerates), Schwann cells guide regrowth (may not restore full function)

• CNS: Limited regeneration due to astrocyte scar tissue and inhibitory chemicals

Membrane Potential and Neural Activity

Resting Membrane Potential

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

• ECF: High Na+ and Cl-

• Cytosol: High K+ and negatively charged proteins

• Selective permeability: More K+ leaks out than Na+ leaks in

• Maintained by sodium-potassium pump (3 Na+ out, 2 K+ in per ATP)

Electrochemical Gradients

• Chemical gradient: Ions move from high to low concentration

• Electrical gradient: Ions move toward opposite charge

• Electrochemical gradient: Sum of chemical and electrical forces

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

  • For K+: $E_K = -90\,\text{mV}$

  • For Na+: $E_{Na} = +66\,\text{mV}$

Membrane Channels

• Leak channels: Always open, allow passive ion movement

• Gated channels: Open/close in response to stimuli

  • Chemically gated: Open when bound to specific chemicals (e.g., ACh)

  • Voltage-gated: Open/close in response to membrane potential changes

  • Mechanically gated: Open/close in response to physical distortion

Graded Potentials

• Local, temporary changes in membrane potential

• Produced by opening of gated channels

• Can cause depolarization (less negative) or hyperpolarization (more negative)

• Repolarization: Return to resting potential after depolarization

• Occur in many cell types; can trigger action potentials in neurons and muscle cells

Action Potentials

Generation and Propagation

• Large, rapid depolarization that propagates along the axon without diminishing

• Triggered when graded potential reaches threshold (typically -60 to -55 mV)

• All-or-none principle: Action potential either occurs fully or not at all

Phases of Action Potential

1. Depolarization to threshold: Graded potential brings membrane to threshold

2. Activation of voltage-gated Na+ channels: Na+ influx causes rapid depolarization

3. Inactivation of Na+ channels, activation of K+ channels: K+ efflux causes repolarization

4. Hyperpolarization: K+ channels close slowly, causing brief overshoot

5. Return to resting potential: Sodium-potassium pump restores balance

Refractory Periods

• Absolute refractory period: No action potential possible (Na+ channels inactivated)

• Relative refractory period: Action potential possible with stronger stimulus (membrane hyperpolarized)

Propagation Types

• Continuous propagation: Unmyelinated axons; action potential moves stepwise along axon

• Saltatory propagation: Myelinated axons; action potential jumps from node to node (Nodes of Ranvier); faster and more energy-efficient

Axon Types and Conduction Speed

Type	Myelination	Diameter	Speed	Function
A fibers	Myelinated	Large	120 m/s	Fastest; sensory (position, balance), motor to skeletal muscle
B fibers	Myelinated	Medium	18 m/s	Intermediate speed; sensory/motor to smooth/cardiac muscle, glands
C fibers	Unmyelinated	Small	1 m/s	Slowest; sensory/motor to smooth/cardiac muscle, glands

Synapses

Structure and Types

• Synapse: Site where a neuron communicates with another cell

• Presynaptic cell: Sends the message

• Postsynaptic cell: Receives the message

• Types:

  • Electrical synapses: Direct physical contact (gap junctions); rapid, bidirectional

  • Chemical synapses: Use neurotransmitters across a synaptic cleft; most common

Chemical Synapse Function (Cholinergic Example)

1. Action potential arrives at axon terminal, depolarizing membrane

2. Ca2+ influx triggers exocytosis of acetylcholine (ACh)

3. ACh binds to receptors on postsynaptic membrane, causing Na+ influx and graded potential

4. ACh is broken down by acetylcholinesterase (AChE); byproducts removed

Synaptic Delay and Fatigue

• Synaptic delay: 0.2–0.5 ms delay due to neurotransmitter release

• Synaptic fatigue: Occurs when neurotransmitter supply cannot keep up with demand

Neurotransmitters and Neuromodulators

Classes and Effects

• Excitatory: Cause depolarization, promote action potentials

• Inhibitory: Cause hyperpolarization, suppress action potentials

• Effect depends on receptor type, not just neurotransmitter

Major Neurotransmitters

Class	Examples	Effect/Notes
Acetylcholine (ACh)	NMJ, CNS, PNS	Excitatory
Biogenic amines	Norepinephrine (NE), Dopamine, Serotonin	NE: Excitatory; Dopamine: Excitatory/inhibitory; Serotonin: Mood, depression
Amino acids	Glutamate, Glycine, GABA	Glutamate: Excitatory; Glycine, GABA: Inhibitory
Neuropeptides	Substance P, Opioids (enkephalins, endorphins, dynorphins)	Pain modulation, neuromodulation
Dissolved gases	Nitric oxide (NO), Carbon monoxide (CO)	Modulate smooth muscle, CNS function

Mechanisms of Action

• Ionotropic: Direct effect via chemically gated ion channels (e.g., ACh, glutamate)

• Metabotropic: Indirect effect via G protein-coupled receptors and second messengers (e.g., NE, dopamine, serotonin, GABA)

• Intracellular enzyme activation: Lipid-soluble gases (NO, CO) enter cell and activate enzymes

Information Processing in Nervous Tissue

Postsynaptic Potentials

• Excitatory postsynaptic potential (EPSP): Graded depolarization, increases likelihood of action potential

• Inhibitory postsynaptic potential (IPSP): Graded hyperpolarization, decreases likelihood of action potential

Summation

• Temporal summation: Rapid, repeated stimuli at a single synapse

• Spatial summation: Simultaneous stimuli at multiple synapses

Facilitation and Presynaptic Modulation

• Facilitation: Membrane potential brought closer to threshold by EPSPs

• Presynaptic inhibition: Decreases neurotransmitter release (axoaxonic synapse)

• Presynaptic facilitation: Increases neurotransmitter release (axoaxonic synapse)

Rate of Action Potential Generation

• Frequency of action potentials encodes stimulus strength

• Maximum rate reached when relative refractory period is eliminated by strong depolarization