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Nervous Tissue – Detailed Study Notes

Introduction

  • Purpose of Chapter 12 (Nervous Tissue)
    • Show how the nervous system maintains controlled conditions in support of health & homeostasis
    • Survey the major anatomical/functional divisions (CNS vs. PNS; sensory vs. motor vs. integrative)
    • Identify every cell type present in nervous tissue and describe its role

Gross Organization of the Nervous System

  • Two master divisions
    • Central Nervous System (CNS)
    • Brain & spinal cord; integrative and higher‐order centers
    • Peripheral Nervous System (PNS)
    • Cranial nerves, spinal nerves, ganglia, enteric plexuses & sensory receptors outside the CNS
  • PNS functional subdivisions
    • Sensory (Afferent) Division
    • Somatic senses (touch, pain, proprioception, temperature)
    • Special senses (vision, taste, smell, hearing, equilibrium)
    • Motor (Efferent) Division
    • Somatic Nervous System (SNS): voluntary output to skeletal muscle
    • Autonomic Nervous System (ANS): involuntary output to smooth & cardiac muscle, glands
      • Sympathetic division | Parasympathetic division (antagonistic but cooperative)
    • Enteric Nervous System (ENS): intrinsic neural plexuses that govern the GI tract

Core Functions of the Nervous System

  • Sensory
    • Detect internal & external changes via specialized receptors → relay information to CNS
  • Integrative
    • Analyze sensory input, store portions, make decisions → formulate appropriate response
  • Motor
    • Send commands through effectors (muscle fibers, glands) to restore/maintain homeostasis

Overview of Nervous Tissue

  • Two broad cell classes
    • Neurons (nerve cells)
    • Electrically excitable; propagate action potentials
    • Possess distinct morphological regions (dendrites, soma, axon, terminals)
    • Neuroglia (glial cells, “nerve glue”)
    • Roughly one‐half the volume of the nervous system
    • Fill spaces, support, insulate & protect neurons; can divide mitotically; do NOT conduct impulses

Neurons in Detail

  • Electrophysiological hallmark: ability to generate & conduct the nerve impulse (action potential)
  • Structural classification – based on number of processes
    • Multipolar: many dendrites, one axon (most CNS neurons, all motor neurons)
    • Bipolar: one dendrite & one axon (retina, inner ear, olfactory epithelium)
    • Pseudounipolar (Unipolar): fused dendrite–axon with soma off to side (most sensory neurons)
  • Functional classification – based on impulse direction
    • Sensory / Afferent → convey information to CNS
    • Motor / Efferent → convey commands from CNS to effectors
    • Interneurons / Association → integrate sensory data, decide motor output (vast majority of neurons)

Neuroglia of the CNS (4 types)

  • Astrocytes
    • Most abundant; contact neurons & capillaries; regulate extracellular ion composition, help form blood–brain barrier, guide neuron migration, influence synaptic activity, store glycogen, react to injury (scar)
  • Oligodendrocytes
    • Myelinate multiple CNS axons → form insulating myelin sheaths; increase conduction velocity
  • Microglia
    • Small, mobile phagocytes; remove microbes, debris; secrete cytokines; form scar tissue after injury
  • Ependymal cells
    • Ciliated epithelial lining of ventricles & central canal; produce and circulate cerebrospinal fluid (CSF)

Neuroglia of the PNS (2 types)

  • Schwann Cells
    • Myelinate single axons (or envelop several unmyelinated) in PNS; aid axonal regeneration
  • Satellite Cells
    • Surround neuronal cell bodies in ganglia; regulate micro-environment; electrical insulation & protection

Myelination

  • Myelin sheath produced by Schwann cells (PNS) & oligodendrocytes (CNS)
  • Functions: electrical insulation, increased conduction speed, axonal maintenance

Electrical Signals in Neurons

  • Two major signal types
    • Graded Potentials (GP)
    • Small, localized voltage changes; occur in dendrites/soma; amplitude varies with stimulus; decremental (die out with distance); may summate
    • Action Potentials (AP)
    • Large, propagated voltage spikes along axon; all-or-none; constant amplitude (≈100 mV); capable of long-distance transmission
  • Requirements for either signal
    1. Establishment of a Resting Membrane Potential (RMP) (non-conducting neuron ≈-70 mV)
    2. Presence of selective ion channels in membrane

Ion Channels in Neurons

  • Leak (Passive) Channels – randomly open/close; set RMP; more K^+ than Na^+ leaks → negative interior
  • Ligand-Gated Channels – open when specific chemical (neurotransmitter) binds; typical in dendrites & soma
  • Mechanically-Gated Channels – open to vibration, pressure, stretch; e.g., touch receptors
  • Voltage-Gated Channels – open when membrane potential reaches threshold; abundant in axons

Resting Membrane Potential (RMP)

  • Results from:
    1. Unequal ion distribution – high extracellular Na^+/Cl^-; high intracellular K^+/anions (phosphates, proteins)
    2. Selective permeability – membrane more permeable to K^+ (leaks out) than Na^+ (enters)
    3. Na^+/K^+ ATPase pump – actively exports 3 Na^+, imports 2 K^+ per ATP → maintains gradients & contributes ~-3 mV to RMP

Graded Potentials (GP) in Depth

  • Produced by opening of ligand-gated or mechanically-gated channels
  • Can be depolarizing (EPSP) or hyperpolarizing (IPSP)
  • Stimulus strength ↔ amplitude (larger stimulus → larger GP)
  • Summation
    • Spatial – simultaneous stimuli at different locations add
    • Temporal – rapid succession of stimuli at one site add
  • If GP depolarization at axon hillock reaches threshold (≈-55 mV), an AP fires

Action Potentials (AP) in Depth

  • Phases
    1. Depolarization – voltage-gated Na^+ channels open (activation gates), Na^+ influx drives Vm → +30 mV
    2. Repolarization – Na^+ channels inactivate, voltage-gated K^+ channels open, K^+ efflux returns Vm toward -70 mV
    3. After-hyperpolarization – K^+ channels remain open briefly; Vm dips ~-90 mV before returning to RMP
  • Refractory Periods
    • Absolute (≈1 msec) – no AP possible (Na^+ channels either open or inactive)
    • Relative – a larger-than-normal stimulus may trigger AP (K^+ channels still open)
  • Stimulus encoding – intensity coded by AP frequency, not size (size invariant)

Propagation of Action Potentials

  • Continuous Conduction – unmyelinated axons; every adjacent segment depolarizes sequentially
  • Saltatory Conduction – myelinated axons; AP “jumps” Node of Ranvier to node; faster & energy-efficient
  • Propagation speed influenced by
    1. Axon diameter – larger caliber → lower resistance → faster
    2. Myelination – presence increases speed dramatically
    3. Temperature – higher temperature ↑ conduction velocity (within physiological range)

Synaptic Transmission

  • Synapse: junction between neuron–neuron or neuron–effector
  • Electrical Synapse
    • Gap junctions provide cytoplasmic continuity → direct ionic current; synchronize activity (e.g., cardiac & some smooth muscle, CNS circuits)
  • Chemical Synapse (most common)
    1. AP arrives at presynaptic terminal → depolarizes → opens voltage-gated Ca^{2+} channels
    2. Ca^{2+} influx triggers exocytosis of neurotransmitter (NT) vesicles into synaptic cleft
    3. NT diffuses, binds to receptors on postsynaptic membrane
    • Ionotropic receptor – ligand-gated ion channel integral to receptor
    • Metabotropic receptor – receptor coupled via G protein to separate channel/second messengers
    1. Postsynaptic membrane generates EPSP (depolarizing) or IPSP (hyperpolarizing) graded potentials
    2. NT removal terminates signal via diffusion, enzymatic degradation (e.g., AChE), or reuptake (neurons or glia)
  • Summation of EPSPs & IPSPs at trigger zone determines whether threshold is reached

Major Neurotransmitters

  • Small Molecule NTs
    • Acetylcholine (ACh) – neuromuscular junction; excitatory at NMJ, inhibitory in cardiac muscle via muscarinic receptors
    • Amino acids: Glutamate (major excitatory CNS; ↑Ca^{2+} → learning; excessive → excitotoxicity); GABA & Glycine (inhibitory via Cl^- channels)
    • Biogenic amines: Dopamine (motor control, reward); Norepinephrine, Epinephrine (ANS), Serotonin (mood, sleep); Histamine
    • ATP & other purines – co-transmitters; act on purinergic receptors
    • Gases: Nitric oxide (NO), Carbon monoxide (CO) – diffuse freely; modulate vasodilation, memory
  • Neuropeptides (3–40 amino acids)
    • Substance P – pain transmission enhancement
    • Enkephalins, Endorphins, Dynorphins – endogenous opioids; analgesia, mood, temperature, learning
    • Hypothalamic releasing/inhibiting hormones – regulate anterior pituitary
    • Angiotensin II – thirst, blood pressure regulation
    • Cholecystokinin (CCK) – “stop-eating” signal, GI function
    • Neuropeptide Y – stimulates food intake, stress response

Plasticity, Regeneration & Repair

  • Plasticity: nervous tissue can reorganize synaptic connections based on experience & learning
  • Regeneration potential
    • CNS: minimal; inhibited by oligodendrocyte molecules & rapid glial scar formation
    • PNS: may regenerate if soma intact, Schwann cells active, limited scar tissue; involves axonal sprouting, Wallerian degeneration, regeneration tube formation

Selected Neurological Disorders

  • Multiple Sclerosis (MS)
    • Autoimmune demyelination of CNS; variable symptoms—muscle weakness, double vision; genetic/environmental triggers suspected
  • Depression
    • Types: Major, Dysthymia, Bipolar, Seasonal Affective Disorder
    • Symptoms: anhedonia, sadness, helplessness; treated commonly with SSRIs (↑serotonin in synapse)
  • Epilepsy
    • Recurrent seizures from synchronous neuronal discharges; may be motor, sensory, psychological
  • Excitotoxicity
    • Neuronal death from excessive excitatory transmission (e.g., glutamate overload after ischemia/stroke)

Neural Integration: Summation & Circuits

  • Spatial vs. Temporal Summation – additive effect of multiple EPSPs/IPSPs in space/time
  • Neural Circuits (wiring patterns)
    • Simple Series – one-to-one relay
    • Diverging – one input → many outputs (amplification)
    • Converging – many inputs → one output (aggregation)
    • Reverberating – feedback loop; prolonged output (rhythm generation, breathing)
    • Parallel after-discharge – input via parallel chains reconverging later; bursts of output (high-level problem solving)

Comprehensive Structure–Function Summary (Neuron Parts)

  • Dendrites: ligand/mechanically-gated channels; receive stimuli & generate GPs (generator or receptor potentials in sensory neurons; EPSPs/IPSPs in others)
  • Cell Body (Soma): integrates inputs; contains organelles
  • Axon Hillock / Initial Segment (Trigger Zone): sums EPSPs & IPSPs; if net depolarization ≥ threshold → AP initiation
  • Axon: propagates APs (voltage-gated Na^+ / K^+)
  • Axon Terminals / Synaptic End Bulbs: voltage-gated Ca^{2+} entry triggers NT release

Key Numerical / Statistical References & Equations

  • Typical neuronal RMP V_{rest} \approx -70\,\text{mV}
  • AP amplitude \approx 100\,\text{mV} (from -70 to +30 mV)
  • Na^+/K^+ ATPase stoichiometry 3\,Na^+{out} : 2\,K^+{in} per ATP
  • Threshold potential V_{th} \approx -55\,\text{mV}
  • Absolute refractory period \sim 1\,\text{msec}; relative refractory follows
  • Propagation speeds: unmyelinated \sim 0.5\text{–}2\,\text{m/s}; myelinated Aα fibers up to 120\,\text{m/s} (diameter & myelination dependent)

Ethical, Philosophical & Practical Implications

  • Understanding ion channel pharmacology underlies development of anesthetics, antiepileptics, antidepressants
  • Demyelinating diseases (MS) highlight immune–nervous interdependence; prompts exploration of immunomodulatory therapies
  • Plasticity emphasizes lifelong learning capability & rehabilitation potential after injury
  • Excitotoxicity research guides neuroprotective strategies in stroke/head trauma

Connections to Earlier & Future Content

  • Builds on cellular membrane transport (diffusion, active transport)
  • Prepares for later chapters: sensory systems (graded potentials), motor control, autonomic physiology, higher brain functions

Real-World Applications & Examples

  • EEG measures summed postsynaptic potentials (clinical diagnosis of epilepsy, sleep patterns)
  • Local anesthetics (e.g., lidocaine) block voltage-gated Na^+ channels → inhibit APs → analgesia
  • Myelin loss in MS visible on MRI; correlates with conduction block & neurological deficits
  • SSRIs prolong serotonin presence in synaptic cleft → mood elevation in depression management

End of Notes