Nervous Tissue: Structure, Function, and Information Processing

Chapter 12: Nervous Tissue

An Introduction to the Nervous System

The nervous system is a complex network responsible for communication, coordination, and control throughout the body. It consists of specialized organs and tissues that detect stimuli, process information, and initiate responses.

• Main Components: Brain, spinal cord, sensory receptors (e.g., eyes, ears), and nerves connecting to other systems.

• Cell Types:

  • Neurons: Specialized for intercellular communication.

  • Neuroglia (Glial Cells): Support, protect, and maintain the environment for neurons.

Divisions of the Nervous System

Anatomical Divisions

• Central Nervous System (CNS): Brain and spinal cord; processes sensory data, coordinates motor commands, and is responsible for higher functions (intelligence, memory, learning, emotion).

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

• Enteric Nervous System (ENS): Network of neurons in the digestive tract; can function independently but is influenced by the ANS.

Functional Divisions of the 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 voluntary and involuntary (reflex) skeletal muscle contractions.

• Autonomic Nervous System (ANS): Controls subconscious actions (smooth/cardiac muscle, glands); includes sympathetic (stimulating) and parasympathetic (relaxing) divisions.

Neurons

Structure and Function

• Cell Body (Soma): Contains nucleus, nucleolus, perikaryon (cytoplasm), mitochondria, rough ER, ribosomes, and cytoskeleton (neurofilaments, neurotubules, neurofibrils).

• Nissl Bodies: Dense areas of RER and ribosomes; responsible for gray appearance of gray matter.

• Dendrites: Short, branched processes; receive information from other neurons via dendritic spines.

• Axon: Long process that propagates action potentials; includes axoplasm, axolemma, initial segment, axon hillock, collaterals, telodendria, and axon terminals.

• Axonal Transport: Movement of materials between soma and axon terminals via neurotubules, powered by mitochondria, kinesin, and dynein.

Structural Classification of Neurons

• Anaxonic: Small, indistinguishable processes; found in brain and special sense organs.

• Bipolar: One dendrite, one axon; rare, found in special senses (sight, smell, hearing).

• Unipolar (Pseudounipolar): Fused axon and dendrite; most sensory neurons in PNS.

• Multipolar: One axon, multiple dendrites; common in CNS and all motor neurons controlling skeletal muscle.

Functional Classification of Neurons

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

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

• Interneurons: Located in CNS and autonomic ganglia; distribute sensory information and coordinate motor activity; involved in higher functions (memory, planning, learning).

Types of Sensory Receptors

• Interoceptors: Monitor internal systems (digestive, urinary) and internal senses (stretch, deep pressure, pain).

• Exteroceptors: Monitor external environment (temperature, sight, smell, hearing).

• Proprioceptors: Monitor position and movement of skeletal muscles and joints.

Neuroglia

Types and Functions

• Astrocytes (CNS): Maintain blood-brain barrier, provide structural support, repair tissue, guide development, control interstitial environment.

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

• Oligodendrocytes (CNS): Form myelin sheaths around axons; increase speed of action potentials; create white matter (myelinated axons) and gray matter (unmyelinated axons, cell bodies, dendrites).

• Microglia (CNS): Phagocytic cells; remove debris, waste, and pathogens.

• Satellite Cells (PNS): Surround neuron cell bodies in ganglia; regulate interstitial fluid.

• Schwann Cells (PNS): Form myelin sheaths around axons; assist in regeneration after injury.

Neural Responses to Injury

• Wallerian Degeneration: Axon distal to injury degenerates; Schwann cells form path for new growth.

• CNS Regeneration: Limited by astrocyte scar tissue and inhibitory chemicals.

Membrane Potential

Resting Membrane Potential

All cell membranes produce electrical signals due to ion movement. Neurons are especially dependent on membrane potential for function.

• Key Concepts:

  • ECF: High Na+, Cl–

  • Cytosol: High K+, negatively charged proteins

  • Selective permeability of membrane; permeability varies by ion

• Passive Processes:

  • Chemical Gradients: Ion concentration differences

  • Electrical Gradients: Charge separation across membrane

  • Electrochemical Gradient: Combined chemical and electrical forces

• Equilibrium Potentials:

  • K+: –90 mV

  • Na+: +66 mV

  • Resting membrane potential: –70 mV (closer to K+ due to higher permeability)

• Active Processes: Sodium–potassium exchange pump maintains gradients and resting potential.

Equation (Nernst Equation for Equilibrium Potential):

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{out}}{[ion]_{in}} \right)$

Additional info: The Nernst equation calculates the equilibrium potential for a specific ion based on its concentration gradient across the membrane.

Ion Channels

• Passive (Leak) Channels: Always open; permeability can change.

• Active (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 in response to physical distortion.

Graded Potentials

• Local changes in membrane potential; do not spread far from stimulus site.

• Depolarization: Opening Na+ channels; membrane potential becomes less negative.

• Repolarization: Return to resting potential after stimulus removal.

• Hyperpolarization: Opening K+ channels; membrane potential becomes more negative.

• Stronger stimuli produce larger, more widespread graded potentials.

Action Potentials

Generation and Propagation

• All-or-none events; triggered when membrane depolarizes to threshold (–60 to –55 mV).

• Four Steps:

  1. Depolarization to threshold

  2. Activation of voltage-gated Na+ channels (Na+ influx)

  3. Inactivation of Na+ channels, activation of K+ channels (K+ efflux, repolarization)

  4. Return to resting potential (hyperpolarization, then stabilization)

• Refractory Periods:

  • Absolute: No action potential possible.

  • Relative: Only strong stimulus can trigger action potential.

• Sodium–Potassium Pump: Restores ion concentrations post-action potential.

Propagation Types

• Continuous Propagation: Unmyelinated axons; action potential moves segment by segment (slow).

• Saltatory Propagation: Myelinated axons; action potential jumps from node to node (fast, energy-efficient).

Axon Types and Conduction Speed

Type	Myelination	Diameter	Speed	Function
Type A	Myelinated	Large	~120 m/s	Critical sensory/motor (e.g., balance, skeletal muscle)
Type B	Myelinated	Medium	~18 m/s	Intermediate signals
Type C	Unmyelinated	Small	~1 m/s	Slow sensory/motor (e.g., pain, temperature)

Synapses

Types of Synapses

• Electrical Synapses: Direct physical contact via gap junctions; rapid transmission; found in some brain regions, eye, ciliary ganglia.

• Chemical Synapses: Most common; use neurotransmitters to transmit signals across synaptic cleft.

Chemical Synapse Events

1. Action potential arrives at axon terminal.

2. Ca2+ influx triggers neurotransmitter release (e.g., ACh).

3. Neurotransmitter binds to postsynaptic receptors, causing graded potential.

4. Neurotransmitter is removed (e.g., by acetylcholinesterase).

• Synaptic Delay: 0.2–0.5 ms due to neurotransmitter release and binding.

• Synaptic Fatigue: Occurs when neurotransmitter supply cannot meet demand.

Neurotransmitters and Neuromodulators

Classes and Effects

• Excitatory: Cause depolarization (promote action potentials).

• Inhibitory: Cause hyperpolarization (suppress action potentials).

• Effect depends on receptor, not neurotransmitter itself.

Major Neurotransmitter Classes

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

• Amino Acids: Glutamate, GABA (inhibitory in CNS).

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

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

Neuromodulators

• Alter rate of neurotransmitter release or postsynaptic response.

• Effects are long-term, slow, and involve multiple steps.

• May be released alone or with neurotransmitters.

Mechanisms of Action

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

• Indirect via G Proteins: Activate second messengers (e.g., cAMP via adenylate cyclase).

• Indirect via Intracellular Enzymes: Lipid-soluble gases diffuse and activate enzymes inside cells.

Equation (cAMP Synthesis):

$ATP \xrightarrow{adenylate\ cyclase} cAMP + PP_i$

Information Processing

Postsynaptic Potentials

• Excitatory Postsynaptic Potential (EPSP): Graded depolarization.

• Inhibitory Postsynaptic Potential (IPSP): Graded hyperpolarization.

• Net effect at axon hillock determines action potential generation.

Summation

• Temporal Summation: Rapid, repeated stimuli at one synapse.

• Spatial Summation: Simultaneous stimuli at multiple synapses.

• Facilitation: EPSPs accumulate, bringing neuron closer to threshold.

Presynaptic Modulation

• Axoaxonic Synapses: Synapses between axons; can inhibit (presynaptic inhibition) or facilitate (presynaptic facilitation) neurotransmitter release.

Information Coding

• Information is encoded by the frequency of action potentials.

• Maximum firing rate is limited by the refractory period.

Summary Table: Synaptic Effects

Effect	Mechanism	Result
EPSP	Na+ influx	Depolarization
IPSP	K+ efflux or Cl– influx	Hyperpolarization
Presynaptic Inhibition	Reduced neurotransmitter release	Decreased postsynaptic response
Presynaptic Facilitation	Increased neurotransmitter release	Enhanced postsynaptic response

Example: At the neuromuscular junction, acetylcholine (ACh) release causes a graded potential that, if strong enough, triggers an action potential in the muscle fiber, leading to contraction.

Additional info: Neuromodulators and hormones can shift the balance between excitation and inhibition, altering neuronal responsiveness and network activity.