Muscle Tissue

Types of Muscle Tissue

Muscle tissue is classified into three main 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 per cell, short spindle-shaped cells. Found in walls of hollow organs (e.g., blood vessels, GI tract).

Comparison Table:

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 being damaged.

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

Functions of Muscle Tissue

• Movement: Locomotion and manipulation (skeletal), movement of substances (smooth, cardiac).

• Posture: Continuous contraction maintains posture.

• Stabilizing Joints: Muscles reinforce and stabilize joints.

• Heat Generation: Muscle contraction produces heat, aiding in temperature regulation.

Connective Tissue Layers

• 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; contains H zone, A band, Z disc, I band.

• 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-tubules: Invaginations of sarcolemma; transmit action potentials into muscle fiber.

• Triad: T-tubule flanked by two terminal cisternae of SR.

• Myoglobin: Oxygen-binding protein in muscle.

• Glycosomes: Organelles storing glycogen.

Sliding Filament Model of Contraction

Muscle contraction occurs as thin (actin) filaments slide past thick (myosin) filaments, shortening the sarcomere.

• Thick Filament: Myosin (darker, thicker).

• Thin Filament: Actin (lighter, thinner).

Neuromuscular Junction and Excitation-Contraction Coupling

• Neuromuscular Junction: Site where motor neuron communicates with muscle fiber.

• Process: ACh released → binds to receptors on motor end plate → Na+ influx → action potential in sarcolemma.

• Excitation-Contraction Coupling: Action potential travels down sarcolemma and T-tubules → Ca2+ released from SR → contraction.

Action Potential Phases

• Depolarization: Na+ enters cell.

• Repolarization: K+ exits cell.

Motor Units

• Definition: One somatic motor neuron and all muscle fibers it innervates.

• Fine Control: Few fibers per neuron (e.g., eye muscles).

• Coarse Control: Many fibers per neuron (e.g., gluteus maximus).

Cross-Bridge Cycle and Muscle Relaxation

• Requirements: Ca2+ (binds troponin) and ATP (energizes myosin).

• Regulatory Proteins: Troponin and tropomyosin block myosin binding sites on actin.

• Cycle: Myosin binds actin → power stroke → ATP binds myosin (detachment) → cycle repeats.

• Relaxation: Ca2+ reabsorbed into SR, ACh broken down by acetylcholinesterase.

• Rigor Mortis: No ATP to break cross-bridges after death.

Muscle Twitch and Graded Responses

• Muscle Twitch: Single, brief contraction in response to one action potential.

• Phases: Latent (Ca2+ release), contraction (cross-bridge activity), relaxation (Ca2+ reuptake).

• Graded Responses: Vary by motor unit recruitment and frequency of stimulation.

• Treppe: Successive contractions become stronger due to increased Ca2+ and heat.

• Tetanus: Incomplete (unfused) vs. complete (fused) tetanus based on frequency of stimulation.

Types of Muscle Contractions

• Isometric: Tension increases, but muscle length does not change.

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

• Muscle Tone: Slight, constant contraction due to spinal reflexes.

Muscle Metabolism

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

• Anaerobic Respiration: Glycolysis (no O2), produces lactic acid, 2 ATP/glucose (lasts ~60 sec).

• Aerobic Respiration: Glycolysis + Krebs cycle + ETC (with O2), 36 ATP/glucose (lasts hours).

• Oxygen Debt: Extra O2 needed to restore muscle to resting state.

Equation for Aerobic Respiration:

Types of Skeletal Muscle Fibers

Muscle Adaptations

• Hypertrophy: Increase in muscle size.

• Atrophy: Decrease in muscle size due to disuse.

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

Smooth Muscle

• Functions: Maintains blood pressure, propels food, regulates organ volume.

• Layers: Circular (inner) and longitudinal (outer) around hollow organs.

• Key Differences from Skeletal Muscle: Ca2+ binds calmodulin (not troponin), uses varicosities, has caveolae, contracts slowly, adapts to stretch.

• Peristalsis: Alternating contraction/relaxation to move contents (e.g., GI tract).

• Neurotransmitter Effects: ACh contracts, norepinephrine relaxes bronchioles/contracts blood vessels.

Muscular System

Organization and Functional Groups

• Prime Mover (Agonist): Main muscle responsible for movement.

• Antagonist: Opposes or reverses movement.

• Synergist: Assists prime mover, reduces unnecessary movement.

• Fixator: Stabilizes origin of prime mover.

Muscle Naming Criteria

• Location, shape, size, direction of fibers, number of origins, attachment sites, action.

• Examples: Rectus (straight), oblique (diagonal), maximus (large), biceps (two origins), sternocleidomastoid (attachments), flexor carpi radialis (action).

Fascicle Arrangements

Muscle Attachments

• Tendon: Cordlike connective tissue attaching muscle to bone.

• Aponeurosis: Sheetlike connective tissue.

• Origin: Attachment to stationary bone.

• Insertion: Attachment to movable bone.

Lever Systems

• Parts: Effort (muscle force), load (resistance), fulcrum (joint).

• Types: First, second, and third class levers (distinguished by relative positions 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 (abducts arm), latissimus dorsi (extends/adducts arm), levator scapula (elevates scapula).

• Rotator Cuff: Supraspinatus (abducts arm), infraspinatus/teres minor (lateral rotation), subscapularis (medial rotation).

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

• Forearm: Brachioradialis (flexes forearm), flexor/extensor carpi radialis/ulnaris (flex/extend/abduct/adduct wrist), extensor digitorum (extends fingers), pronator teres (pronates forearm), palmaris longus (wrist flexor).

• Thorax: Diaphragm (breathing), external/internal intercostals (inspiration/expiration), pectoralis major (flexes/adducts arm), pectoralis minor (draws scapula forward), serratus anterior (rotates scapula).

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

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

• Thigh: Adductor longus/magnus (adducts thigh), gracilis (adducts thigh/flexes leg), hamstrings (biceps femoris, semitendinosus, semimembranosus: extend thigh/flex knee), quadriceps (rectus femoris, vastus lateralis/medialis/intermedius: extend knee), 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 (extends toes).

Fundamentals of the Nervous System

Nervous vs. Endocrine System

• Similarities: Both maintain homeostasis and control body functions.

• Nervous System: Uses electrical impulses and neurotransmitters; rapid (milliseconds).

• Endocrine System: Uses hormones in blood/ECF; slower (seconds to months).

Functions of the Nervous System

• Sensory Input: Detects internal/external changes.

• Integration: Processes and interprets sensory input.

• Motor Output: Activates effectors (muscles/glands).

Organization of the Nervous System

• CNS: Brain and spinal cord.

• 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, regulate environment), microglia (phagocytosis), ependymal cells (circulate CSF), oligodendrocytes (myelinate axons).

• PNS: Schwann cells (myelinate axons), satellite cells (regulate ion concentration).

Neurons

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

• Nissl Bodies: Ribosomes in soma.

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

Membrane Potentials and Action Potentials

• Resting Membrane Potential: -70 mV (ICF more negative).

• Depolarization: Na+ influx; threshold at -55 mV.

• Peak: +30 mV, then repolarization (K+ efflux).

• Hyperpolarization: Excess K+ leaves cell.

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

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

Na+/K+ Pump Equation:

Neural Organization

• Nuclei: Cell bodies in CNS.

• Ganglia: Cell bodies in PNS.

• Tracts: Axon bundles in CNS.

• Nerves: Axon bundles in PNS.

White Matter vs. Gray Matter

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

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

Graded Potentials vs. Action Potentials

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

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

Synaptic Transmission

• Chemical Synapse: Most common; neurotransmitter released from presynaptic neuron, binds to postsynaptic receptor.

• Process: AP arrives at axon terminal → Ca2+ influx → vesicle fusion → neurotransmitter release → receptor binding → ion channel opening (EPSP/IPSP).

• Termination: Neurotransmitter degraded (e.g., acetylcholinesterase), reuptake, or diffusion away.

Types of Synapses and Channels

• Synapse Types: Axodendritic (axon to dendrite), axosomatic (axon to soma), axoaxonic (axon to axon).

• Channel Types: Ligand-gated (postsynaptic, for EPSP/IPSP), voltage-gated (axon, for AP).

Selected Neurotransmitters

Summary: Chapters 9-11 cover the structure and function of muscle tissue, organization of the muscular system, and the fundamentals of the nervous system, including cellular anatomy, physiology, and neurotransmission. Mastery of these concepts is essential for understanding human movement and neural control.