Fundamentals of the Nervous System and Muscles: Study Guide

Chapter 11: Fundamentals of the Nervous System

Major Functions of the Nervous System

The nervous system is the master controlling and communicating system of the body. It is responsible for integrating sensory input, coordinating motor output, and maintaining homeostasis.

• Sensory Input: Gathering information from sensory receptors about internal and external changes.

• Integration: Processing and interpreting sensory input to determine an appropriate response.

• Motor Output: Activating effector organs (muscles and glands) to cause a response.

Divisions of the Nervous System

• Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for integration and command.

• Peripheral Nervous System (PNS): Consists of cranial and spinal nerves; connects the CNS to the rest of the body.

• Functional Divisions of PNS:

  • Sensory (Afferent) Division: Transmits impulses from receptors to the CNS.

  • Motor (Efferent) Division: Transmits impulses from the CNS to effector organs.

    • Somatic Nervous System: Voluntary control of skeletal muscles.

    • Autonomic Nervous System (ANS): Involuntary control of smooth muscle, cardiac muscle, and glands.

      • Sympathetic Division: Mobilizes body systems during activity.

      • Parasympathetic Division: Conserves energy and promotes housekeeping functions.

Neuroglial Cells

Neuroglia are supporting cells in the nervous system, essential for neuron function and health.

• CNS Neuroglia:

  • Astrocytes: Support neurons, regulate the blood-brain barrier, and maintain the extracellular environment.

  • Microglia: Act as phagocytes, removing debris and pathogens.

  • Ependymal Cells: Line ventricles of the brain and central canal of the spinal cord; circulate cerebrospinal fluid (CSF).

  • Oligodendrocytes: Produce myelin sheaths in the CNS.

• PNS Neuroglia:

  • Schwann Cells: Form myelin sheaths in the PNS.

  • Satellite Cells: Surround neuron cell bodies in ganglia; regulate environment.

Key Terms: Nucleus, Ganglion, Nerve, and Tract

• Nucleus: Cluster of neuron cell bodies in the CNS.

• Ganglion: Cluster of neuron cell bodies in the PNS.

• Nerve: Bundle of axons in the PNS.

• Tract: Bundle of axons in the CNS.

Neurons: Structure and Function

• Definition: Neurons are excitable cells that transmit electrical signals.

• Components:

  • Cell Body (Soma): Contains the nucleus and organelles.

  • Dendrites: Receive signals from other neurons.

  • Axon: Conducts impulses away from the cell body.

  • Axon Hillock: Initiates action potentials.

  • Axon Terminals: Release neurotransmitters to communicate with other cells.

• Myelin Sheath: Insulating layer that increases the speed of impulse conduction.

  • Formation in CNS: By oligodendrocytes.

  • Formation in PNS: By Schwann cells.

Types of Neurons

• Multipolar: Many dendrites, one axon (most common in CNS).

• Bipolar: One dendrite, one axon (sensory organs).

• Unipolar (Pseudounipolar): Single process that splits into two branches (sensory neurons in PNS).

Ohm’s Law in Neurophysiology

Ohm’s Law describes the relationship between voltage, current, and resistance in neurons:

$V = I \times R$

• V: Voltage (potential difference)

• I: Current (flow of ions)

• R: Resistance (hindrance to current flow)

Ion Channels and Resting Membrane Potential

• Types of Ion Channels:

  • Leak Channels: Always open; maintain resting potential.

  • Gated Channels: Open in response to stimuli (voltage, ligand, or mechanical).

• Resting Membrane Potential: The voltage difference across the membrane of a resting neuron (typically -70 mV).

• Significance: Essential for the generation of action potentials.

• Establishment: Due to differential permeability of the membrane to Na+ and K+, and the Na+/K+ ATPase pump.

Action Potentials

• Voltage-Gated Ion Channels: Open or close in response to changes in membrane potential; essential for action potential generation.

• Phases of Action Potential:

  • Depolarization: Na+ influx causes membrane potential to become less negative.

  • Repolarization: K+ efflux restores negative membrane potential.

  • Hyperpolarization: Membrane potential becomes more negative than resting.

Graded Potentials vs. Action Potentials

• Graded Potentials: Short-distance, variable-strength signals; decrease with distance.

• Action Potentials: Long-distance, all-or-none signals; do not decrease in strength.

Refractory Periods

• Absolute Refractory Period: No new action potential can be initiated.

• Relative Refractory Period: A stronger stimulus can initiate another action potential.

Conduction of Action Potentials

• Continuous Conduction: Occurs in unmyelinated axons; slower.

• Saltatory Conduction: Occurs in myelinated axons; action potentials jump between nodes of Ranvier; faster.

• Factors Affecting Velocity: Larger axon diameter and myelination increase conduction speed.

Chemical Synapses

• Definition: Junctions where neurons communicate via neurotransmitters.

• Components: Presynaptic terminal, synaptic cleft, postsynaptic membrane.

• Events of Transmission:

  1. Action potential arrives at axon terminal.

  2. Voltage-gated Ca2+ channels open.

  3. Neurotransmitter released into synaptic cleft.

  4. Neurotransmitter binds to postsynaptic receptors.

  5. Postsynaptic potential generated.

Postsynaptic Potentials

• EPSP (Excitatory Postsynaptic Potential): Depolarizes membrane, increases likelihood of action potential.

• IPSP (Inhibitory Postsynaptic Potential): Hyperpolarizes membrane, decreases likelihood of action potential.

• Summation:

  • Temporal Summation: Multiple signals from one neuron over time.

  • Spatial Summation: Signals from multiple neurons at the same time.

Neurotransmitter Receptors and Types

• Structural Classification: Based on chemical structure (e.g., acetylcholine, amino acids, peptides).

• Functional Classification: Excitatory or inhibitory effects.

• Receptors: Ionotropic (ligand-gated ion channels) and metabotropic (G-protein coupled receptors).

Neuronal Pools and Processing

• Neuronal Pool: Functional groups of interconnected neurons that integrate incoming information.

• Processing Types:

  • Serial Processing: Input travels along one pathway (e.g., reflex arc).

  • Parallel Processing: Input is segregated into multiple pathways (e.g., higher-level mental functioning).

• Neural Circuits: Diverging, converging, reverberating, and parallel after-discharge circuits.

Chapter 9: Muscles and Muscle Tissue

Major Functions of Muscle Tissue

• Movement: Skeletal muscles move the body; cardiac muscle pumps blood; smooth muscle moves substances through organs.

• Posture Maintenance: Muscles maintain body posture and position.

• Joint Stabilization: Muscles reinforce and stabilize joints.

• Heat Generation: Muscle contractions produce heat to maintain body temperature.

Structural Elements and Properties of Muscle Cells

• Excitability: Ability to receive and respond to stimuli.

• Contractility: Ability to shorten forcibly.

• Extensibility: Ability to be stretched.

• Elasticity: Ability to recoil to resting length.

Types of Muscle Tissue

• Skeletal Muscle: Voluntary, striated, multinucleated.

• Cardiac Muscle: Involuntary, striated, branched, intercalated discs.

• Smooth Muscle: Involuntary, non-striated, spindle-shaped.

Definitions:

• Agonist: Prime mover of a movement.

• Synergist: Assists the agonist.

• Flaccidity: Loss of muscle tone.

• Convulsion: Involuntary muscle contractions.

• Fibrillation: Rapid, irregular muscle contractions.

Structure of Skeletal Muscle Fiber

• Connective Tissue Sheaths: Epimysium (surrounds muscle), perimysium (surrounds fascicles), endomysium (surrounds fibers).

• Myofibril: Rod-like contractile elements; contain sarcomeres.

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

• Sarcoplasmic Reticulum (SR): Stores and releases Ca2+.

• T-tubules: Invaginations of the sarcolemma; conduct impulses deep into the fiber.

• Filaments:

  • Thick Filaments: Composed of myosin.

  • Thin Filaments: Composed of actin, tropomyosin, and troponin.

  • Elastic Filaments: Composed of titin; maintain sarcomere structure.

Sliding-Filament Mechanism

Muscle contraction occurs as thin filaments slide past thick filaments, shortening the sarcomere without changing filament length.

Anatomy of the Neuromuscular Junction (NMJ)

• Components: Axon terminal, synaptic cleft, motor end plate.

• Events:

  1. Action potential arrives at axon terminal.

  2. Ca2+ influx triggers acetylcholine (ACh) release.

  3. ACh binds to receptors on sarcolemma.

  4. Na+ influx depolarizes muscle fiber, generating an action potential.

Muscle Fiber Action Potential

• Ion Concentrations: Higher Na+ outside, higher K+ inside.

• Concentration Gradient: Difference in ion concentration across membrane.

• Electrical Potential: Difference in charge across membrane.

• Na+/K+ ATPase: Maintains resting membrane potential by pumping 3 Na+ out and 2 K+ in per ATP hydrolyzed.

Excitation-Contraction Coupling

• Sequence linking muscle fiber excitation to contraction via Ca2+ release from SR.

• Steps: Action potential spreads along sarcolemma → T-tubules → SR releases Ca2+ → Ca2+ binds troponin → Cross-bridge cycle begins.

Cross-Bridge Cycle

1. Myosin head attaches to actin (cross-bridge formation).

2. Power stroke: Myosin head pivots, pulling actin filament.

3. ATP binds to myosin, causing detachment.

4. ATP hydrolysis re-cocks myosin head.

Muscle Relaxation

• ACh is broken down; Ca2+ pumped back into SR; cross-bridges detach; muscle fiber returns to resting state.

Motor Unit and Recruitment

• Motor Unit: One motor neuron and all muscle fibers it innervates.

• Recruitment: Increasing number of active motor units to increase force.

• Treppe: Gradual increase in contraction strength with repeated stimulation.

Twitch Contraction and Types of Contractions

• Stages of Twitch: Latent period, contraction, relaxation.

• Types:

  • Isotonic: Muscle changes length.

    • Concentric: Muscle shortens.

    • Eccentric: Muscle lengthens.

  • Isometric: Muscle length does not change.

• Force, Velocity, Duration: Affected by sarcomere length, load, and fiber type.

• Muscle Tone: Slight, constant contraction of muscles.

Energy for Contraction

• Immediate Sources: ATP stored in muscle, creatine phosphate.

• Glycolytic Mechanism: Anaerobic breakdown of glucose; produces ATP quickly but less efficiently.

• Oxidative Mechanism: Aerobic metabolism; produces more ATP but slower.

• Muscle Fiber Types:

  • Slow Oxidative (Type I): Endurance, fatigue-resistant.

  • Fast Oxidative (Type IIa): Intermediate properties.

  • Fast Glycolytic (Type IIb): Short, powerful bursts; fatigue quickly.

• Duration of ATP Sources: Creatine phosphate (10 sec), glycolysis (30–40 sec), aerobic respiration (minutes to hours).

Excess Post Exercise Oxygen Consumption (EPOC)

• Oxygen required to restore muscle to resting state after exercise (replenish ATP, remove lactic acid, restore oxygen reserves).

Muscle Fatigue

• Caused by ionic imbalances, lactic acid accumulation, ATP depletion, and other factors.

Effects of Physical Conditioning

• Endurance Training: Increases capillaries, mitochondria, and myoglobin; improves fatigue resistance.

• Resistance Training: Increases muscle size (hypertrophy) and strength.

Summary Table: Muscle Fiber Types

Fiber Type	Contraction Speed	ATP Pathway	Fatigue Resistance	Example
Slow Oxidative (Type I)	Slow	Aerobic	High	Marathon running
Fast Oxidative (Type IIa)	Fast	Aerobic (some anaerobic)	Intermediate	Middle-distance running
Fast Glycolytic (Type IIb)	Fast	Anaerobic glycolysis	Low	Sprinting

Additional info: Where specific figures or tables are referenced (e.g., Table 11.1, Figure 9.1), students should consult their textbook for detailed diagrams and data. This guide provides a comprehensive overview of the key concepts and terminology for exam preparation.