Fundamentals of the Nervous and Muscular Systems

Chapter 11: Fundamentals of the Nervous System

Characteristics of the Nervous System

The nervous system is responsible for coordinating and controlling body activities through rapid communication. It consists of specialized cells and organs that detect stimuli, process information, and elicit responses.

• Basic Functions: Sensory input (detecting changes), integration (processing information), and motor output (initiating responses).

• Key Organs: Brain, spinal cord, nerves, and sensory receptors.

• Neuronal Tissue: The neuron is the functional unit, consisting of a cell body (soma), dendrites (receive signals), and an axon (transmits signals).

• Functional Types of Neurons:

  • Sensory (Afferent) Neurons: Carry impulses from receptors to the CNS; typically unipolar or bipolar; located in sensory ganglia.

  • Interneurons: Integrate information within the CNS; multipolar; found entirely within the brain and spinal cord.

  • Motor (Efferent) Neurons: Transmit impulses from CNS to effectors (muscles/glands); multipolar; cell bodies in CNS, axons in PNS.

• Glial Cells: Support neurons. Six types:

  • Central Nervous System (CNS): Astrocytes (support, blood-brain barrier), oligodendrocytes (myelinate CNS axons), microglia (immune defense), ependymal cells (line ventricles, produce CSF).

  • Peripheral Nervous System (PNS): Schwann cells (myelinate PNS axons), satellite cells (support neuron cell bodies in ganglia).

• Myelin: Lipid-rich sheath that insulates axons, increasing conduction speed. Oligodendrocytes myelinate CNS axons; Schwann cells myelinate PNS axons.

Resting Membrane Potential

Neurons maintain a voltage difference across their membrane, essential for electrical signaling.

• Ion Channels: Leak channels (always open), voltage-gated (open with voltage changes), ligand-gated (open with chemical binding), mechanically gated (open with physical deformation). Location and ion flow depend on channel type.

• Resting Membrane Potential: The steady-state voltage (typically -70 mV) due to differential ion distribution and membrane permeability, mainly to K+ and Na+.

Equation:

$V_m = \frac{RT}{F} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$

Additional info: This is the Nernst equation for a single ion; the Goldman-Hodgkin-Katz equation considers multiple ions.

Graded Potentials

Graded potentials are small, localized changes in membrane potential that vary in amplitude and decay with distance.

• Definition: Local changes in membrane potential, produced by stimulus-induced opening of ion channels.

• Location: Occur in dendrites and cell bodies of neurons.

• Stimuli: Neurotransmitters, sensory input, or mechanical changes.

• Amplitude: Proportional to stimulus strength; decays with distance from stimulus site.

• Summation: Multiple graded potentials can combine (summate) to influence action potential generation.

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

• Spatial Summation: Simultaneous stimuli at different locations.

Action Potentials

Action potentials are rapid, large changes in membrane potential that propagate along axons, enabling long-distance communication.

• Comparison with Graded Potentials:

  • Location: Action potentials in axons; graded in dendrites/cell bodies.

  • Amplitude: Action potentials are all-or-none; graded potentials vary.

  • Duration: Action potentials are brief; graded can be longer.

  • Channels: Action potentials use voltage-gated Na+ and K+ channels.

• Definition: A self-propagating wave of depolarization and repolarization along the axon.

• Phases:

  • Depolarization: Voltage-gated Na+ channels open; Na+ influx.

  • Repolarization: Na+ channels inactivate, K+ channels open; K+ efflux.

  • Hyperpolarization: K+ channels remain open briefly.

• Refractory Periods:

  • Absolute: No new action potential possible (Na+ channels inactivated).

  • Relative: Stronger stimulus needed (K+ channels still open).

• Conduction: Myelinated axons conduct faster (saltatory conduction) than unmyelinated; larger diameter axons conduct faster.

Action Potential Graph:

$\text{Depolarization} \rightarrow \text{Repolarization} \rightarrow \text{Hyperpolarization}$

Additional info: The graph plots membrane potential (y-axis) vs. time (x-axis).

Synapses

Synapses are specialized junctions where neurons communicate with other cells.

• Definition: A synapse is the site of communication between a neuron and another cell.

• Chemical Synapse Structure: Presynaptic terminal, synaptic cleft, postsynaptic membrane.

• Transmission Events:

  1. Action potential arrives at axon terminal.

  2. Voltage-gated Ca2+ channels open; Ca2+ influx.

  3. Neurotransmitter released into synaptic cleft.

  4. Neurotransmitter binds to receptors on postsynaptic cell, causing graded potentials.

• Termination Mechanisms: Reuptake, enzymatic breakdown (e.g., acetylcholinesterase), diffusion away from synapse.

• Electrical vs. Chemical Synapses: Electrical synapses use gap junctions for direct ion flow; chemical synapses use neurotransmitters.

• Postsynaptic Responses: Excitatory postsynaptic potentials (EPSPs) depolarize; inhibitory postsynaptic potentials (IPSPs) hyperpolarize.

Neurotransmitters

Neurotransmitters are chemical messengers that transmit signals across synapses.

Neurotransmitter	Chemical Composition	Receptors	Mechanism of Action
Acetylcholine (ACh)	Ester of acetic acid and choline	Nicotinic (ionotropic), Muscarinic (metabotropic)	Excitatory at neuromuscular junctions; can be inhibitory or excitatory elsewhere
Norepinephrine (NE)	Catecholamine (derived from tyrosine)	Adrenergic (alpha, beta)	Excitatory or inhibitory depending on receptor subtype

Chapter 9: Muscular System

Characteristics of Muscle Tissue

Muscle tissue enables movement, posture, and heat production. There are three types: skeletal, cardiac, and smooth muscle.

• Major Functions: Movement, posture maintenance, joint stabilization, heat generation.

• Types of Muscle:

  • Skeletal: Voluntary, striated, attached to bones.

  • Cardiac: Involuntary, striated, found in heart.

  • Smooth: Involuntary, non-striated, found in walls of hollow organs.

• Gross Structure: Muscle → Fascicle → Muscle fiber (cell) → Myofibril → Myofilament (actin/myosin).

Anatomy of a Muscle Fiber and Sliding Filament Model

Skeletal muscle fibers are highly organized for contraction, with specialized structures and proteins.

• Key Components:

  • Sarcolemma: Muscle cell membrane.

  • Transverse Tubules (T-tubules): Invaginations of sarcolemma; conduct impulses.

  • Sarcoplasmic Reticulum: Stores and releases Ca2+.

  • Myofibrils: Bundles of myofilaments (actin, myosin).

  • Sarcomere: Functional contractile unit; defined by Z-discs.

  • Myofilaments: Thick (myosin) and thin (actin, troponin, tropomyosin).

• Sarcomere Structure:

  • A-band: Length of thick filaments; unchanged during contraction.

  • I-band: Thin filaments only; shortens during contraction.

  • H-zone: Center of A-band; thick filaments only; shortens during contraction.

  • Z-disc: Boundary of sarcomere.

  • M-line: Center of sarcomere.

• Sliding Filament Mechanism: Myosin heads bind to actin, pulling thin filaments toward the center; regulated by troponin and tropomyosin in response to Ca2+.

Skeletal Muscle Excitation-Contraction Coupling

Excitation-contraction coupling links the electrical signal in a motor neuron to muscle contraction.

• Neuromuscular Junction (NMJ): Synapse between motor neuron and muscle fiber; includes axon terminal, synaptic cleft, and motor end plate.

• Contraction Events:

  1. Action potential arrives at NMJ.

  2. Acetylcholine released, binds to receptors on sarcolemma.

  3. Muscle action potential generated, travels via T-tubules.

  4. Ca2+ released from sarcoplasmic reticulum.

  5. Ca2+ binds troponin, shifting tropomyosin, exposing actin sites.

  6. Myosin binds actin; contraction cycle begins.

• Relaxation: Ca2+ pumped back into SR; ATP required for detachment of myosin; rigor state occurs if ATP is depleted.

• Muscle Tension and Twitch: Tension is force generated; twitch is a single contraction; muscle tone is baseline tension; motor unit is a motor neuron and all fibers it innervates.

• Recruitment: Increasing number of active motor units increases force.

• Muscle Twitch Graph:

  • Latent Period: Events of excitation-contraction coupling.

  • Contraction Period: Cross-bridge cycling, tension rises.

  • Relaxation Period: Ca2+ reuptake, tension falls.

• Length-Tension Relationship: Optimal overlap of actin and myosin yields maximal tension.

• Summation and Tetanus: Increased frequency of stimulation leads to temporal summation, unfused (incomplete) tetanus, or fused (complete) tetanus.

• Contraction Types:

  • Isotonic: Muscle changes length (concentric: shortens; eccentric: lengthens).

  • Isometric: Muscle length unchanged; tension increases.

• Muscle Fatigue: Decline in ability to generate force; due to ATP depletion, ion imbalances, or lactic acid buildup.

Smooth Muscle

Smooth muscle differs from skeletal muscle in structure and contraction mechanism.

• Myofilament Arrangement: Smooth muscle lacks sarcomeres; actin and myosin arranged diagonally; contracts more slowly but can sustain contractions longer.

Abnormalities of the Neuromuscular System

Disorders can affect neuromuscular transmission, muscle contraction, or both. Examples include myasthenia gravis (autoimmune attack on ACh receptors) and muscular dystrophies (genetic defects in muscle proteins).