Module 11 Notes – Nervous Tissue & Neural Physiology

Module 11.1 Nervous System Divisions

• Two anatomical divisions
  • Central Nervous System (CNS)
  • Brain + spinal cord
  • Integrates, processes, coordinates sensory & motor commands
  • Peripheral Nervous System (PNS)
  • All neural tissue outside CNS
  • Conveys information between body & CNS

Information Flow (Fig. 11.1 logic pathway)

1. Receptors (somatic, special, visceral)
2. Afferent (sensory) division of PNS → CNS
3. Information processing (CNS)
4. Efferent (motor) division of PNS
5. Effectors
   • Somatic nervous system (SNS) → skeletal muscle
   • Autonomic nervous system (ANS) → smooth m., cardiac m., glands, adipose tissue

Sub-divisions of PNS

• Sensory (afferent) division brings input to CNS from…
  • Somatic sensory receptors → position, touch, pressure, pain, temperature
  • Special sensory receptors → smell, taste, vision, balance, hearing
  • Visceral sensory receptors → internal organs
• Motor (efferent) division carries commands from CNS to…
  • SNS → skeletal muscles (voluntary)
  • ANS → smooth m., cardiac m., glands, adipose tissue (involuntary)

Module 11.2 Neurons

• Neurons: specialized cells for intercellular communication
• Three fundamental regions
  • Dendrites → receive stimuli
  • Cell body (soma) → nucleus + organelles
  • Axon → conducts information to other cells

Detailed Axon Anatomy

• Axon hillock → origin from soma
• Initial segment → action potential (AP) initiation site
• Axolemma → specialized plasma membrane
• Axoplasm → cytoplasm (neurofibrils, neurotubules, vesicles, lysosomes, mitochondria, enzymes)
• Axon terminals / telodendria → communication sites

Synapse Components (Fig. 11.2-2)

• Presynaptic membrane (axon term.)
• Synaptic vesicles with neurotransmitter (NT)
• Synaptic cleft
• Postsynaptic membrane + receptors

Three Synapse Types

1. Neuron-to-neuron
2. Neuromuscular junctions (axon → skeletal muscle fiber)
3. Neuroglandular synapses (axon → gland cell)

Neuron Replacement

• Most CNS neurons lack centrioles ⇒ no mitosis ⇒ loss is usually permanent
• Limited neural stem cells (generally inactive)
  • Exceptions: olfactory epithelium, retina, hippocampus

Module 11.3 Neuron Classification

Structural Classes (4)

1. Anaxonic
   • Small, no obvious axon–dendrite distinction
   • In brain + special senses; function unclear
2. Bipolar
   • One dendritic process + one axon
   • Rare; in special sense organs
3. Unipolar (pseudounipolar)
   • Dendrites + axon continuous, soma off to side
   • Most PNS sensory neurons; axons can exceed 1 m (e.g., toes → spinal cord)
4. Multipolar
   • ≥2 dendrites + one axon
   • Most common CNS neuron; all somatic motor neurons; can be long (spinal cord → toe muscles)

Functional Classes (3)

1. Sensory neurons ≈ 10^7 (10 million)
2. Interneurons ≈ 2\times10^{10} (20 billion)
3. Motor neurons ≈ 5\times10^{5} (500 000)

Sensory Receptors & Fibers

• Interoceptors → distension, deep pressure, pain
• Proprioceptors → body position; joint & muscle movement
• Exteroceptors → external environment sensations
• Afferent fibers → axons carrying sensory input to CNS
• Ganglion = collection of neuron somata in PNS
  • Sensory ganglia contain unipolar neuron bodies
  • Somatic sensory neurons monitor outside world / position
  • Visceral sensory neurons monitor internal organs

Interneurons

• Entirely in CNS; between sensory & motor neurons
• Roles: distribution, coordination, higher functions (learning, memory, planning)

Motor Neurons

• Somata in CNS; axons in PNS nerves
• Somatic motor neurons → skeletal muscle (voluntary)
• Visceral motor neurons → all other effectors; synapse in autonomic ganglia; axons called efferent fibers

Module 11.4 Neuroglia of CNS

• Glia ≈ 50 % of nervous tissue volume
• Types (4)
  1. Ependymal cells
  • Line ventricles & central canal; produce, circulate, monitor cerebrospinal fluid (CSF)
  1. Microglia
  • Phagocytic; remove debris, wastes, pathogens
  1. Astrocytes
  • Maintain blood–brain barrier (BBB)
  • Structural support
  • Regulate interstitial ion, nutrient, gas concentrations
  • Absorb & recycle NTs
  • Form scar tissue after injury
  1. Oligodendrocytes
  • Produce myelin sheaths in CNS
  • One cell myelinates segments of several axons; increases conduction speed

Module 11.5 Neuroglia of PNS

• Schwann cells (neurolemmocytes)
  • Enclose all peripheral axons (myelinated or not)
  • Assist in axon repair
• Satellite cells
  • Surround PNS neuron cell bodies in ganglia
  • Regulate environment (functionally akin to astrocytes)

Module 11.6 Membrane Potential

• Transmembrane (plasma) potential → unequal charge distribution
  • Inside: slightly −
  • Outside: slightly +
• Resting potential (neuron) ≈ -70\,\text{mV}
• Neuronal activities originate as changes from rest:
  1. Graded potential — local, decays with distance
  2. Action potential — propagated electrical event along axon
  3. Synaptic activity — NT release → graded potentials in postsynaptic cell
  4. Information processing — postsynaptic integration

Ion Gradients & Equilibrium

• Example diagrams (Fig. 11.7):
  • Sodium chemical vs electrical gradients → net \text{Na}^+ electrochemical drive into cell
  • Potassium gradients → net \text{K}^+ drive out of cell

Sodium–Potassium Exchange Pump

• Active transport: 3\ \text{Na}^+{\text{out}} : 2\ \text{K}^+{\text{in}} per ATP consumed
• Maintains stable -70\,\text{mV} rest

Module 11.8 Gated Channels

• Closed at rest; open/close to stimuli → alter permeability & potential
• Three types
  1. Chemically (ligand) gated — open when specific chemical binds (e.g., ACh at NMJ); mainly on dendrites & soma
  2. Voltage-gated — open/close with membrane potential changes; abundant on axon hillock, initial segment, axolemma (Na⁺, K⁺, Ca²⁺)
  3. Mechanically gated — open via physical distortion (pressure, stretch)

Module 11.9 Graded Potentials

• Local potentials; cannot spread far
• Opening of chemically gated Na⁺ channels typical example
  • Influx of \text{Na}^+ → depolarization (less −)
  • Degree of depolarization proportional to stimulus magnitude (more channels open ⇒ bigger change)
  • Effect greatest at stimulus site and diminishes with distance (local currents)
• Removal of stimulus + active transport → repolarization (return to rest)
• Opening gated K⁺ channels → hyperpolarization (more − than rest)

Four universal characteristics (Fig. 11.9-6):

1. Largest change at origin, decays with distance
2. Spread is passive via local current
3. Can be depolarizing or hyperpolarizing depending on ion movement
4. Stronger stimulus → greater potential change + area affected

Module 11.10 Action Potential Generation

• APs affect entire excitable membrane; triggered when local depolarization reaches threshold (≈ -60\,\text{mV})

Sequence (Fig. 11.10-2):

1. Resting (-70\,\text{mV})
2. Depolarization to threshold via graded potential
3. Rapid depolarization
   • Voltage-gated Na⁺ channels open → \text{Na}^+ influx → membrane to +30\,\text{mV}
4. Na⁺ channel inactivation; K⁺ channel activation → repolarization starts
5. K⁺ outflow continues; may overshoot → hyperpolarization (~-90\,\text{mV})
6. Voltage-gated K⁺ channels close; membrane returns to rest

• Absolute refractory period – Na⁺ channels inactivated; no AP possible
• Relative refractory period – K⁺ channels open; larger-than-normal stimulus needed

Module 11.11 Action Potential Propagation

• AP is regenerated at adjacent membrane segments — propagation

Continuous Propagation

• Unmyelinated axons
• Tiny consecutive steps along entire length
• Speed ≈ 1\,\text{m\,s}^{-1}

Saltatory Propagation

• Myelinated axons; current jumps node-to-node (Nodes of Ranvier)
• Faster; velocity ↑ with axon diameter (larger diameter ⇒ lower resistance)

Module 11.12 Synapses

• Synapse: site of information transfer neuron → neuron/effector
• Presynaptic (sending) vs Postsynaptic (receiving)
• Types
  1. Chemical synapse (dominant)
  • NT release (e.g., cholinergic synapse releasing ACh)
  • Steps (Fig. 11.12-2):
    1. AP arrives → terminal depolarization
    2. Voltage-gated \text{Ca}^{2+} entry → exocytosis of ACh
    3. ACh binds postsynaptic receptors → graded depolarization
    4. AChE breaks ACh → acetate + choline → depolarization ends
    5. Choline re-uptake → ACh synthesis
  • Synaptic fatigue: prolonged activity depletes NT faster than re-synthesis ⇒ transmission failure until replenished
  1. Electrical synapse
  • Gap junctions; direct ion flow; rare but rapid, synchronized (e.g., certain brain regions, retina, cardiac muscle)

Module 11.13 Information Processing in a Neuron

• Postsynaptic potentials (PSPs) = graded potentials produced in postsynaptic membrane
  • Excitatory PSP (EPSP) → depolarization (membrane facilitated)
  • Inhibitory PSP (IPSP) → hyperpolarization (membrane inhibited)
• Summation = algebraic integration of PSPs at axon hillock
  • If net depolarization ≥ threshold → AP

Summation Types

1. Temporal Summation
   • Repeated stimuli at the same synapse in rapid succession
   • Successive ACh release builds depolarization → threshold
2. Spatial Summation
   • Simultaneous stimuli at multiple synapses
   • Combined depolarization depends on
     • Number of active excitatory synapses
     • Distance from initial segment

• Net effect may be facilitation, inhibition, or cancellation (EPSPs + IPSPs of equal magnitude)

Neuronal Integration

• Single neuron can have thousands of synapses (excitatory & inhibitory)
• Simplest level of CNS information processing
• Allows neurons to respond dynamically to extracellular changes (O₂, nutrients, drugs, toxins, etc.)