Muscle Tissue, Muscular System, and Fundamentals of the Nervous System (Chapters 9–11) – Study Guide

Muscle Tissue

Types of Muscle Tissue

Muscle tissue is specialized for contraction and is classified into three types, each with distinct structural and functional characteristics.

• Skeletal Muscle: Voluntary, striated, multinucleated, long fibers; responsible for body movement and posture.

• Cardiac Muscle: Involuntary, striated, usually one or two nuclei per cell, contains intercalated discs; found only in the heart.

• Smooth Muscle: Involuntary, non-striated, single nucleus, short spindle-shaped cells; found in walls of hollow organs.

Key Differences: Control (voluntary/involuntary), presence of striations, number of nuclei, and location in the body.

Table: Comparison of Muscle Tissue Types

Feature	Skeletal	Cardiac	Smooth
Control	Voluntary	Involuntary	Involuntary
Striations	Yes	Yes	No
Nuclei	Multiple	1–2	1
Special Features	Long fibers	Intercalated discs	Caveolae, calmodulin

Special Properties of Muscle Tissue

• Excitability (Irritability): Ability to receive and respond to stimuli.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to stretch without damage.

• Elasticity: Ability to recoil to resting length after stretching.

Functions of Muscle Tissue

• Movement of the body and substances within the body

• Maintenance of posture

• Stabilization of joints

• Heat generation (thermogenesis)

Connective Tissue Layers of Skeletal Muscle

• Epimysium: Surrounds entire muscle

• Perimysium: Surrounds fascicles (bundles of muscle fibers)

• Endomysium: Surrounds individual muscle fibers

Microscopic Anatomy of Skeletal Muscle

• Sarcomere: Structural and functional unit of muscle contraction; bordered by Z discs.

• Major Regions: H zone (only myosin), A band (myosin and actin overlap), I band (only actin), Z disc (sarcomere boundary).

• Sarcolemma: Muscle cell plasma membrane; conducts action potentials.

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

• Myofibril: Contractile organelle composed of repeating sarcomeres.

• T (Transverse) Tubules: Invaginations of sarcolemma; conduct impulses deep into muscle fiber.

• Triad: T tubule plus two terminal cisternae of SR.

• Myoglobin: Oxygen-binding protein in muscle.

• Glycosomes: Glycogen storage organelles.

Sliding Filament Model of Contraction

• Thin filaments (actin) slide past thick filaments (myosin), shortening the sarcomere.

• Myosin heads bind to actin, forming cross-bridges, and pull actin toward the center (power stroke).

Neuromuscular Junction and Excitation-Contraction Coupling

• Neuromuscular Junction: Synapse between motor neuron and muscle fiber; ACh is the neurotransmitter.

• Excitation-Contraction Coupling: Action potential travels along sarcolemma and T tubules, triggering Ca2+ release from SR, leading to contraction.

• Key Steps: Depolarization (Na+ influx), repolarization (K+ efflux), Ca2+ release, cross-bridge cycling.

Muscle Metabolism

• Direct Phosphorylation: Creatine phosphate + ADP → ATP (lasts ~15 sec).

• Anaerobic Respiration: Glycolysis (glucose → lactic acid); 2 ATP/glucose (lasts ~60 sec).

• Aerobic Respiration: Glycolysis + Krebs cycle + ETC; 36 ATP/glucose (lasts hours).

Equation for Aerobic Respiration:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + 36\ \text{ATP}$

Muscle Twitch and Contraction Types

• Muscle Twitch: Single contraction in response to a single stimulus; phases: latent, contraction, relaxation.

• Graded Responses: Varying strength of contraction due to frequency and number of motor units recruited.

• Treppe: Increased contraction efficiency with repeated stimulation.

• Tetanus: Incomplete (unfused) or complete (fused) sustained contraction.

• Isometric Contraction: Tension increases, muscle length unchanged.

• Isotonic Contraction: Muscle changes length (concentric = shortens, eccentric = lengthens).

Types of Skeletal Muscle Fibers

Fiber Type	Oxygen Use	Fatigue	ATPase Speed	Metabolism
Slow Oxidative	High	Resistant	Slow	Aerobic
Fast Glycolytic	Low	Fatigable	Fast	Anaerobic
Fast Oxidative	Intermediate	Intermediate	Fast	Aerobic & Anaerobic

Muscle Adaptations

• Hypertrophy: Increase in muscle size due to increased fiber size.

• Atrophy: Decrease in muscle size due to disuse.

• Hyperplasia: Increase in number of muscle fibers (rare in humans).

Smooth Muscle

• Found in walls of hollow organs; contracts in sheets (circular and longitudinal layers).

• Key differences: Ca2+ binds to calmodulin (not troponin), neurotransmitter release via varicosities, caveolae instead of T-tubules, slower contraction.

• Functions: Blood pressure regulation, GI propulsion, peristalsis.

Muscular System

Organization and Naming of Muscles

• Muscles are organized into functional groups: prime mover (agonist), antagonist, synergist, fixator.

• Named by location, shape, size, direction of fibers, number of origins, attachments, and action.

• Fascicle arrangements: parallel, fusiform, circular, convergent, unipennate, bipennate, multipennate.

Muscle Attachments and Levers

• Tendon: Cordlike connective tissue attaching muscle to bone.

• Aponeurosis: Sheetlike connective tissue attachment.

• Origin: Attachment to stationary bone.

• Insertion: Attachment to moveable bone.

• Levers: 1st, 2nd, 3rd class; consist of effort, load, and fulcrum.

Major Muscles and Their Functions

• Head: Buccinator (cheek compression), frontalis (raises eyebrows), masseter (elevates mandible), orbicularis oculi (closes eye), orbicularis oris (purses lips), temporalis (closes jaw), zygomaticus (smiling).

• Neck: Sternocleidomastoid (flexes/rotates head), trapezius (stabilizes scapula).

• Shoulder: Deltoid (abduction), latissimus dorsi (extension/adduction), levator scapula (elevates scapula).

• Rotator Cuff: Supraspinatus (abduction), infraspinatus/teres minor (lateral rotation), subscapularis (medial rotation), rhomboid major (stabilizes scapula), teres major (extends/adducts humerus).

• Arm: Biceps brachii (flexes elbow), brachialis (forearm flexor), triceps brachii (extends elbow).

• Forearm: Brachioradialis (flexes forearm), flexor/extensor carpi radialis/ulnaris (wrist movement), extensor digitorum (finger extension), pronator teres (pronation), palmaris longus (wrist flexion).

• Thorax: Diaphragm (breathing), intercostals (inspiration/expiration), pectoralis major/minor (arm flexion, scapula movement), serratus anterior (scapula rotation).

• Abdomen: External/internal obliques, rectus abdominis, transversus abdominis (flex/compress abdomen).

• Hip: Gluteus maximus (extends thigh), gluteus medius (abducts/medially rotates thigh).

• Thigh: Adductors (adduction), gracilis (adduction/flexion), hamstrings (extension/flexion), quadriceps (knee extension), sartorius (flexes/abducts/laterally rotates thigh), tensor fascia lata (steadies knee/trunk).

• Leg: Gastrocnemius/soleus/fibularis longus (plantar flexion), tibialis anterior (dorsiflexion), extensor digitorum longus (toe extension).

Fundamentals of the Nervous System

Nervous System Overview

• Works with the endocrine system to maintain homeostasis.

• Uses electrical impulses and neurotransmitters for rapid communication (milliseconds).

• Endocrine system uses hormones for slower, longer-lasting effects.

Functions of the Nervous System

• Sensory Input: Detects changes inside/outside the body.

• Integration: Processes and interprets sensory input.

• Motor Output: Activates effector organs (muscles/glands).

Organization of the Nervous System

• Central Nervous System (CNS): Brain and spinal cord.

• Peripheral Nervous System (PNS): Cranial/spinal nerves, ganglia.

• PNS Divisions: Sensory (afferent) and motor (efferent).

• Motor Division: Somatic (voluntary, skeletal muscle) and autonomic (involuntary, cardiac/smooth muscle, glands).

• Autonomic Subdivisions: Sympathetic and parasympathetic.

Neuroglia (Glial Cells)

• CNS: Astrocytes (support, chemical environment), microglia (phagocytosis), ependymal cells (circulate CSF), oligodendrocytes (myelinate axons).

• PNS: Schwann cells (myelinate axons), satellite cells (support neuron cell bodies).

Neurons

• Structural units of the nervous system; conduct action potentials.

• Parts: Dendrites (receive input), cell body (soma, integration), axon hillock (trigger zone for action potentials), axon (conducts impulses).

• Types: Multipolar (most common, CNS), bipolar (special senses), unipolar (sensory neurons).

Membrane Potentials and Action Potentials

• Resting Membrane Potential: $-70\ \text{mV}$ (ICF more negative than ECF).

• Depolarization: Na+ influx; threshold at $-55\ \text{mV}$.

• Peak: $+30\ \text{mV}$; repolarization (K+ efflux).

• Hyperpolarization: More K+ exits, ICF becomes more negative.

• Refractory Periods: Absolute (no new AP), relative (stronger stimulus needed).

• Na+/K+ Pump: Restores ion balance (3 Na+ out, 2 K+ in).

White and Gray Matter

• White Matter: Myelinated axons; fast, saltatory conduction (Node of Ranvier to Node of Ranvier).

• Gray Matter: Cell bodies, dendrites, unmyelinated fibers; slower conduction.

Graded Potentials vs. Action Potentials

Feature	Graded Potentials	Action Potentials
Location	Dendrites/cell body	Axon
Distance	Short	Long
Amplitude	Variable, decreases	All-or-none, constant
Channels	Chemically gated	Voltage-gated

• EPSP: Excitatory, depolarizes membrane (Na+ in, K+ out).

• IPSP: Inhibitory, hyperpolarizes membrane (K+ out or Cl- in).

Synaptic Transmission

• Occurs at chemical synapses (most common); electrical synapses are rare.

• Action potential arrives at axon terminal, Ca2+ influx triggers neurotransmitter release.

• Neurotransmitter binds to postsynaptic receptors, opening ion channels (EPSP or IPSP).

• Neurotransmitter removal: enzymatic breakdown, reuptake, or diffusion.

Types of Synapses

• Axodendritic: Axon to dendrite

• Axosomatic: Axon to soma

• Axoaxonic: Axon to axon

Neurotransmitters

• Acetylcholine (ACh): EPSP in skeletal muscle, IPSP in cardiac muscle.

• Norepinephrine (NE): EPSP or IPSP; enhanced by amphetamines.

• Dopamine: EPSP or IPSP; reward/pleasure, deficient in Parkinson's disease.

• Serotonin: IPSP; mood regulation, targeted by SSRIs.

• GABA: IPSP; main inhibitor in brain, effects increased by alcohol/antianxiety drugs.

• Glutamate: EPSP; learning/memory, excessive release in stroke.

• Endorphins: IPSP; natural painkillers, mimic opiates.

• Substance P: EPSP; mediates pain in PNS.

EPSP: Depolarizes postsynaptic membrane. IPSP: Hyperpolarizes postsynaptic membrane.

Additional info:

• Muscle contraction and relaxation require both Ca2+ and ATP.

• Rigor mortis occurs post-mortem due to lack of ATP to break cross-bridges.

• Oxygen debt is the extra oxygen needed to restore muscle to resting state after exercise.