Skeletal Muscle Structure, Function, and Nervous System Integration
Functions of the Skeletal System
- Move the body
- Maintain posture
- Protect and support
- Regulate elimination of materials
- Produce heat
Characteristics of Skeletal Muscle
- Excitability: Ability to respond to a stimulus by changing electrical membrane potential
- Conductivity: Sending an electrical change down the length of the cell membrane
- Contractility: Enables movement when filaments slide past one another
- Extensibility: Ability to stretch
- Elasticity: Ability to return to original length after shortening or lengthening
Muscle Fiber: Bundled into fascicles. Fascicle: A bundle of muscle fibers. Whole Muscle: Contains many fascicles.
Connective Tissue: 3 Concentric Layers
- Epimysium: Dense irregular connective tissue wrapping the whole muscle
- Perimysium: Dense irregular connective tissue wrapping fascicles. Houses many blood vessels and nerves
- Endomysium: Areolar connective tissue wrapping individual fibers. Provides electrical insulation, capillary support, and binding of tendons
Tendons: Cordlike structure of dense regular connective tissue. Aponeuroses: Thin, flattened sheets of dense irregular tissue.
Deep Fascia: Dense irregular connective tissue superficial to the epimysium. Separates individual muscles and binds muscles with similar functions. Superficial Fascia: Areolar and adipose connective tissue superficial to deep fascia. Separates muscles from skin.
Innervation: Controlled by somatic motor neurons. Axons branch and terminate at neuromuscular junctions. Skeletal muscle is a voluntary muscle because contraction is voluntarily controlled.
Muscle Fiber
Sarcoplasm: Muscle cytoplasm. Contains organelles, contractile proteins, and other specializations. Multiple Nuclei: Individual cells are multinucleated. Formed in the embryo by the fusion of myoblasts. Undifferentiated myoblasts become satellite cells for muscle repair. Sarcolemma (Plasma Membrane): Voltage-gated ion channels allow conduction of electrical signals. Transverse tubules (T-tubules) extend deep into the cell and contain voltage-sensitive calcium channels.
Sarcoplasmic Reticulum (SR): Internal Membrane Complex
Like smooth ER. Terminal Cisternae: Blind sacs of the SR. Serve as reservoirs for calcium ions. Two cisternae with a T-tubule in between form a triad. Calcium Regulation: Pumps import calcium. Calcium binds to calmodulin and calsequestrin. Release channels triggered by electrical signals release calcium into the sarcoplasm.
Myofibrils: Hundreds to thousands per cell. Bundles of myofilaments enclosed in the SR. Myofilaments: Thick Filaments: Bundles of myosin protein molecules. Myosin heads point toward the ends of the filament. Thin Filaments: Twisted strands of actin. Tropomyosin and troponin regulate actin-myosin interaction.
Sarcomere
Sarcomeres are the functional units of myofibrils. Myofilaments are arranged in repeating units called sarcomeres. Structure: Z-Discs: Specialized proteins perpendicular to the myofilaments. Serve as anchors for thin filaments. I-Bands: Light-appearing regions containing only thin filaments. Bisected by Z-discs. Get smaller during contraction and may disappear during maximal contraction. A-Bands: Dark-appearing regions containing thick filaments and overlapping thin filaments. Central region of the sarcomere. Contains: H-Zone: Central portion with only thick filaments (disappears during contraction). M-Line: Middle of the H-zone; anchoring site for thick filaments.
Other Sarcomere Proteins
Connectin: Extends from Z-disc to M-line. Stabilizes thick filaments and provides elasticity to sarcomeres. Dystrophin: Links myofibrils to the sarcolemma. Defective dystrophin causes muscular dystrophy.
Clinical View: Muscular Dystrophy
A collective term for hereditary diseases where skeletal muscles degenerate. Most Common Type: Duchenne Muscular Dystrophy (DMD): Caused by defective or insufficient dystrophin. Sarcolemma is damaged during muscle contraction, allowing calcium to enter cells and damage proteins. Symptoms: Walking difficulties. Muscle atrophy. Postural issues. Prognosis: Problems begin in early childhood. Patients rarely live beyond age 30. No cure available.
Energy Production in Muscle Fibers
Mitochondria: Abundant in muscle fibers for aerobic ATP production. Myoglobin: Allows for oxygen storage within cells, supports aerobic ATP production. Glycogen: Stored for quick fuel needs. Creatine Phosphate: Provides immediate ATP by transferring phosphate to ADP.
Motor Unit
A motor neuron and all the muscle fibers it controls. Small motor units: Fewer than 5 muscle fibers. Allow for precise control (e.g., eye muscles). Large motor units: Thousands of muscle fibers. Allow for strong force production (e.g., leg muscles). Distribution: Muscle fibers of a motor unit are dispersed throughout the muscle, not clustered.
Neuromuscular Junction
The location where a motor neuron innervates a muscle fiber. Usually found in the mid-region of the muscle fiber. Synaptic Knob: Expanded tip of the motor neuron axon. Contains synaptic vesicles filled with acetylcholine (ACh). Voltage-gated Ca²⁺ channels in the membrane allow Ca²⁺ to enter. Synaptic Cleft: Narrow, fluid-filled space separating the synaptic knob from the motor end plate. Contains acetylcholinesterase (enzyme that breaks down ACh). Motor End Plate: Specialized region of the sarcolemma with ACh receptors. Allows Na⁺ to enter and K⁺ to exit when ACh binds.
Resting Membrane Potential (RMP) in Skeletal Muscle Fibers
Definition: Muscle fibers maintain an RMP of approximately -90 mV. Inside of the cell is more negative compared to the outside. Established By: Na⁺/K⁺ pumps and leak channels. State at Rest: Voltage-gated channels are closed.
Skeletal Muscle Contraction
Excitation of a Skeletal Muscle Fiber
Calcium Entry at Synaptic Knob: Nerve signal opens voltage-gated Ca²⁺ channels, Ca²⁺ diffuses into the synaptic knob and binds to proteins on synaptic vesicles. Release of ACh: Vesicles merge with the plasma membrane, releasing ACh into the synaptic cleft. Binding of ACh: ACh diffuses across the cleft and binds to receptors, exciting the fiber.
Excitation-Contraction Coupling
End-Plate Potential (EPP): ACh binding opens chemically gated ion channels, Na⁺ diffuses in; K⁺ diffuses out, cell membrane becomes less negative locally. Propagation of Action Potential: Voltage-gated Na⁺ channels open, leading to depolarization (+30 mV). Repolarization occurs as K⁺ channels open, restoring RMP. Calcium Release: Action potential travels down T-tubules, triggers the release of Ca²⁺ from the sarcoplasmic reticulum.
Crossbridge Cycling
Calcium Binding: Ca²⁺ binds to troponin, causing tropomyosin to expose actin binding sites. Steps: Crossbridge Formation: Myosin head attaches to actin. Power Stroke: Myosin head pivots, pulling thin filament toward the center of the sarcomere. ADP and Pi are released. Release: ATP binds to myosin, causing it to detach from actin. Reset: ATP is hydrolyzed, repositioning the myosin head. Result: Sarcomeres shorten as Z-discs move closer. I-band and H-zone narrow; A-band remains constant.
Skeletal Muscle Relaxation
Termination of Stimulation: Nerve signals cease, stopping ACh release. ACh Breakdown: Acetylcholinesterase removes ACh from the cleft. Calcium Return: Ca²⁺ is pumped back into the SR. Tropomyosin Blockade: Tropomyosin re-covers actin binding sites. Elasticity: Muscle returns to its original position.
Types of Skeletal Muscle Organization
Parallel Muscles: Fascicles run parallel to the muscle’s long axis. Often have an expanded central belly. High endurance but not as strong as other muscles. Example: Rectus abdominis. Circular Muscles: Fascicles are concentrically arranged to form a sphincter. Control material passage through openings. Example: Orbicularis oris (mouth). Convergent Muscles: Fascicles merge toward a common attachment site. Can pull in varying directions but are weaker than parallel muscles. Example: Pectoralis major. Pennate Muscles: Fascicles organized like a large feather. Fibers pull at an angle to the tendon. Generate more tension but have shorter contraction distances. Unipennate: Fascicles on one side of the tendon (e.g., extensor digitorum). Bipennate: Fascicles on both sides of the tendon (e.g., rectus femoris). Multipennate: Branches of the tendon within the muscle, with fascicles arranged around both sides (e.g., deltoid).
Muscle Actions
Agonist (Prime Mover): Muscle that contracts to produce a movement. Example: Triceps brachii is the agonist for forearm extension. Antagonist: Muscle whose contraction opposes that of the agonist. Allows for smooth movement of controlled speed. Example: Biceps brachii opposes the triceps brachii during forearm extension. Synergist: Muscle that assists the agonist by contributing tension or stabilizing the origin. Example: Biceps brachii and brachialis work synergistically to flex the elbow joint.
Muscles of Facial Expression
General Characteristics: Attach to the subcutaneous layer deep to the skin. Contraction causes the skin to move, producing facial expressions. Most are innervated by the facial nerve (CN VII). Muscles of the Forehead, Scalp, and Eyebrows: Occipitofrontalis Muscle: Frontal belly raises the eyebrows. Occipital belly retracts the scalp. Corrugator Supercilii Muscle: Draws eyebrows together. Muscles of the Eyelids: Orbicularis Oculi: Circular fibers surround the orbit and close the eye. Levator Palpebrae Superioris: Elevates the upper eyelid. Nasalis: Elevates corners of the nostrils (flaring). Procerus: Wrinkles the nose.
Pelvic Floor
Structure: Composed of three layers of muscles and associated fasciae. Collectively referred to as the pelvic diaphragm. Primary Function: Supports the pelvic viscera (organs within the pelvis). Perineum: Diamond-shaped region between the lower appendages. Boundaries: Pubic symphysis (anterior). Coccyx (posterior). Ischial tuberosities (lateral). Divided into two triangles: Urogenital Triangle: Contains the external genitalia and urethra. Anal Triangle: Contains the anus.
Muscles That Move the Pectoral Girdle
Anterior Thoracic Muscles
Pectoralis Minor: Location: Deep to the pectoralis major. Action: Helps depress and protract the scapula (e.g., hunching shoulders). Serratus Anterior: Fan-shaped muscle located between the ribs and scapula. Action: Protracts and stabilizes the scapula. Subclavius: Extends from the clavicle to the first rib. Action: Stabilizes and depresses the scapula.
Posterior Thoracic Muscles
Levator Scapulae: Attaches to cervical vertebrae and scapula. Action: Elevates and inferiorly rotates the scapula. Rhomboid Major and Rhomboid Minor: Location: Runs inferolaterally from vertebrae to the scapula, deep to the trapezius. Action: Elevates, retracts, and inferiorly rotates the scapula. Trapezius: Large, diamond-shaped muscle extending from the skull and vertebral column to the pectoral girdle. Action: Elevates, depresses, retracts, or rotates the scapula depending on the active region.
Glenohumeral Joint
General Overview: Eleven muscles cross the glenohumeral joint and move the humerus. Agonists (Prime Movers): Latissimus Dorsi: Broad, triangular muscle on the back. Action: Agonist for arm extension; also adducts and medially rotates the arm. Pectoralis Major: Thick, fan-shaped muscle on the superior, anterior thorax. Action: Agonist for arm flexion; also adducts and medially rotates the arm. Other Movers: Triceps Brachii: Long head originates on the scapula and spans the shoulder joint. Action: Helps extend and adduct the arm. Biceps Brachii: Both heads originate on the scapula and span the shoulder joint. Action: Assists in flexing the arm. Deltoid: Action: Primary abductor of the arm. Anterior fibers: Flex and medially rotate the arm. Lateral fibers: Abduct the arm. Posterior fibers: Extend and laterally rotate the arm. Coracobrachialis: Action: Flexes and adducts the arm. Teres Major: Action: Extends, adducts, and medially rotates the arm.
Rotator Cuff Muscles
Provide strength and stability to the glenohumeral joint. Composed of four muscles: Subscapularis: Action: Medially rotates the arm. Supraspinatus: Action: Abducts the arm. Infraspinatus and Teres Minor: Action: Adduct and laterally rotate the arm.
Muscles of the Arm (Brachium)
Anterior Compartment
Contains flexor muscles. Blood Supply: Deep brachial artery. Innervation: Musculocutaneous nerve. Example Muscles: Biceps Brachii: Two-headed muscle on the anterior humerus. Action: Flexes and supinates the forearm; weak flexor of the humerus. Brachialis: Location: Deep to the biceps brachii. Action: Most powerful flexor of the forearm. Brachioradialis: Location: Anterolateral forearm. Action: Involved in elbow flexion.
Posterior Compartment
Contains extensor muscles. Blood Supply: Deep brachial artery. Innervation: Radial nerve. Example Muscles: Triceps Brachii: Large, three-headed muscle on the posterior arm. Action: Major extensor of the forearm; helps extend the humerus. Anconeus: Action: Weak elbow extensor; crosses the posterolateral region of the elbow.
Forearm Pronators and Supinator
Pronator Teres: Action: Rotates the radius across the ulna, pronating the forearm. Location: Anterior compartment of the forearm. Pronator Quadratus: Action: Pronates the forearm. Location: Deep in the anterior compartment. Supinator: Action: Supinates the forearm, moving the palm to a forward-facing position. Location: Posterior compartment of the forearm.
Muscles of the Forearm (Antebrachium)
Anterior Compartment: Generally, flex the wrist and fingers. Most originate on the medial epicondyle of the humerus. Posterior Compartment: Generally, extend the wrist and fingers. Most originate on the lateral epicondyle of the humerus. Retinacula: Thick fibrous bands of fascia at the wrist hold tendons close to bones. Flexor Retinaculum: Covers the palmar surface of the carpal bones. Forms the carpal tunnel through which flexor tendons pass. Extensor Retinaculum: Located on the dorsal surface of the wrist. Holds extensor tendons close to the bones.
Muscles of the Anterior Forearm
Superficial Layer
Flexor Carpi Radialis: Prominent muscle on the lateral forearm. Action: Flexes the wrist and abducts the hand. Palmaris Longus: Narrow muscle on the anterior forearm. Action: Weakly assists in wrist flexion. Flexor Carpi Ulnaris: Action: Flexes the wrist and adducts the hand.
Intermediate Layer
Flexor Digitorum Superficialis: The only muscle in this layer. Action: Flexes the wrist, metacarpophalangeal (MP), and proximal interphalangeal (PIP) joints of fingers 2–5.
Deep Layer
Flexor Pollicis Longus: Action: Flexes the thumb at the metacarpophalangeal (MP) and interphalangeal (IP) joints; weakly flexes the wrist. Flexor Digitorum Profundus: Lies deep to flexor digitorum superficialis. Action: Flexes the wrist, MP, PIP, and distal interphalangeal (DIP) joints of fingers 2–5.
Muscles of the Posterior Forearm
Superficial Layer
Extensor Carpi Radialis Longus: Action: Extends the wrist and abducts the hand. Extensor Carpi Radialis Brevis: Action: Extends the wrist and abducts the hand. Extensor Digitorum: Action: Extends the wrist and the MP, PIP, and DIP joints of fingers 2–5. Extensor Digiti Minimi: Action: Extends the little finger. Extensor Carpi Ulnaris: Action: Extends the wrist and adducts the hand.
Deep Layer
Extensor Pollicis Brevis: Action: Extends the thumb at the MP joint. Extensor Pollicis Longus: Action: Extends the thumb at the MP and IP joints. Extensor Indicis: Action: Extends the index finger at the MP, PIP, and DIP joints.
Intrinsic Muscles of the Hand
Both proximal and distal attachments are within the hand. Support the arches of the hand and allow precise movements of the fingers. Thenar Group: At the base of the thumb. Includes: Flexor Pollicis Brevis: Flexes the thumb. Abductor Pollicis Brevis: Abducts the thumb. Opponens Pollicis: Opposes the thumb. Hypothenar Group: At the base of the little finger. Includes: Flexor Digiti Minimi Brevis: Flexes the little finger. Abductor Digiti Minimi: Abducts the little finger. Opponens Digiti Minimi: Opposes the little finger. Midpalmar Group: Occupies the space between the thenar and hypothenar groups. Includes: Lumbricals: Flex MP joints and extend PIP and DIP joints of fingers 2–5. Dorsal Interossei: Abduct fingers 2–5; flex MP joints and extend PIP and DIP joints. Palmar Interossei: Adduct fingers 2–5; flex MP joints and extend PIP and DIP joints. Adductor Pollicis: Adducts the thumb.
Muscles of the Hip and Thigh
Anterior Compartment
Iliopsoas (Psoas Major and Iliacus): Action: Flex the hip joint.
Medial Compartment
Adductor Longus, Adductor Brevis, and Gracilis: Action: Adduct and flex the thigh. Adductor Magnus: Action: Adducts, extends, and laterally rotates the thigh. Obturator Externus: Action: Laterally rotates the thigh.
Posterior Compartment
Gluteus Maximus: Action: Agonist of thigh extension; also laterally rotates the thigh. Gluteus Medius and Gluteus Minimus: Action: Abduct and medially rotate the thigh.
Nervous System: Communication and Control
Collect Information: Sensory input from receptors. Process Information: Brain/spinal cord determine responses. Initiate Response: CNS sends motor output to effectors.
Structural Organization of the Nervous System
CNS: Brain and spinal cord. PNS: Nerves and ganglia.
Functional Organization of the Nervous System
Sensory Nervous System (Afferent): Somatic: Conscious perception (e.g., vision). Visceral: Subconscious input (e.g., from kidneys). Motor Nervous System (Efferent): Somatic: Voluntary Autonomic: Involuntary
Nerve: Connective Tissue Layers
Epineurium: Dense irregular CT around the nerve. Perineurium: Dense irregular CT around fascicles. Endoneurium: Areolar CT surrounding axons.
Neurons
Characteristics: Excitability: Response to stimuli. Conductivity: Transmission of signals. Secretion: Neurotransmitter release. Longevity: Lifetime function. Amitotic: No division post-development. Structure: Cell Body (Soma): Contains nucleus. Dendrites: Receive signals. Axon: Transmits signals; ends in synaptic knobs. Axonal Transport: Anterograde: Soma → Synapse. Retrograde: Synapse → Soma. Fast (400mm/day): Via microtubules. Slow (0.1-3mm/day): Axoplasmic flow.
Synapse
Types: Electrical: Fast; gap junctions. Chemical: Neurotransmitter-mediated. Mechanism: Synaptic vesicles release neurotransmitter → binds postsynaptic receptor → generates EPSP/IPSP.
Glial Cells
CNS Glia: Astrocytes: BBB, ion regulation, structural support. Ependymal Cells: Line ventricles; produce CSF. Microglia: Immune defense; phagocytosis. Oligodendrocytes: Form myelin sheath. PNS Glia: Satellite Cells: Support ganglia. Schwann Cells: Myelinate PNS axons.
Resting Membrane Potential (RMP)
Voltage difference across a membrane (~-70mV). Factors: K+ outflux; limited Na+ influx. Na+/K+ pump maintains gradients.
Myelination
CNS: Oligodendrocytes wrap multiple axons. No neurilemma; gaps = nodes of Ranvier. PNS: Schwann cells form a single myelin segment. Neurilemma supports regeneration.
Regions of the Brain
Cerebrum: Hemispheres, gyri/sulci, lobes. Diencephalon: Thalamus, hypothalamus. Brainstem: Midbrain, pons, medulla. Cerebellum: Coordination. Matter Distribution: Gray Matter: Neuron bodies, dendrites. White Matter: Myelinated tracts.
Brain Development
Neurulation: Neural plate → Neural tube → CNS. Defects: Anencephaly: Brain absence. Spina Bifida: Neural tube fails to close.
Meninges
Pia Mater: Innermost; adheres to brain. Arachnoid Mater: Web-like; contains CSF. Dura Mater: Tough outer layer.
Cerebrospinal Fluid (CSF)
Produced by the choroid plexus. Provides buoyancy, protection, nutrient transport.
Brain Lobes and Functional Areas
Lobes: Frontal: Motor control, decision-making. Parietal: Sensory integration. Temporal: Hearing, memory. Occipital: Vision. Insula: Taste. Functional Areas: Motor: Precentral gyrus; Broca’s areas. Can understand but can’t produce words. Wernicke’s: can speak words but can’t understand the meaning.
Diencephalon
Thalamus: Relay for sensory signals. Hypothalamus: Autonomic and endocrine regulation. Epithalamus: Includes pineal gland (circadian rhythm).
Brainstem
Midbrain: Substantia nigra (dopamine). Pons: Respiratory control. Medulla: Autonomic functions (heart rate, respiration).
Cerebellum
Coordination; error correction.
Sleep and Memory
Sleep: Non-REM: Restorative. REM: Memory consolidation. Short-term: Hippocampus. Long-term: Distributed in cortex.