Muscle Physiology

Muscle Physiology: Learning Objectives

  • Describe skeletal muscle physiology and the somatic motor system through the lens of the core concepts of cell-cell communication, structure-function, and levels of organization.
  • Predict the impact of disease (such as myasthenia gravis) on skeletal muscle function.
  • Integrate sensory and motor system function to describe simple spinal reflexes.

Physical World Flow-Down Gradients (Flux)

  • Energy: Refers to the capacity to do work.
  • Mass Balance: The principle that mass cannot be created or destroyed.
  • Biological World: Understanding biological systems in terms of interactions and dynamics.
  • Homeostasis: The maintenance of stable internal conditions despite external changes.

Core Concepts

  • Cell Theory: All living organisms are composed of cells. Cells are the basic unit of life.
  • Cell Membrane: A biological membrane that separates and protects the interior of all cells from the outside environment.
  • Cell-Cell Communication: The process where cells use signaling molecules to communicate with each other.
  • Structure & Function: The relationship between the shape of a structure and its function within a biological system.
  • Levels of Organization: Biological organization ranges from cells to tissues, organs, organ systems, and organisms.
  • Systems Integration: Understanding how different biological systems work together.
  • Scientific Reasoning: The process of using the scientific method to analyze and interpret data.

Key Terminology for Physiology

  • Afferent: Information moving toward the central nervous system (CNS) or sensory input.
  • Efferent: Information moving away from the CNS or motor output.

The Nervous System

  • Central Nervous System (CNS): Comprises the brain (cerebral hemispheres, diencephalon, cerebellum, brainstem) and spinal cord; integrates sensory and motor information.
  • Peripheral Nervous System (PNS): Consists of cranial nerves, spinal nerves, sensory components, and motor components, including:
    • Sensory Ganglia and Nerves: Relay sensory information to CNS.
    • Motor Nerves: Carry command signals from CNS to effectors (i.e., muscles and glands).
  • Somatic Motor System: Controls voluntary movements.
  • Visceral Motor System: Controls involuntary responses; includes sympathetic, parasympathetic, and enteric divisions.

Descending Tracts

  • Corticospinal Tract: Involves several components:
    • Primary Motor Area of Cerebral Cortex: Initiates motor commands.
    • Internal Capsule: Pathway for signals from the cortex.
    • Medulla Oblongata & Pyramids: Important for motor pathway decussation.
    • Cervical & Lumbar Spinal Cord: Pathways to skeletal muscle.
  • Extrapyramidal Tract: Coordinates involuntary muscle functions; involves brain structures like the basal ganglia and cerebellum.

Central and Peripheral Nervous System Functions

  • CNS Functions:
    • Receives and processes sensory information.
    • Initiates responses, stores memories, generates thoughts and emotions.
  • Motor Neurons: Carry efferent information from CNS to muscles and glands.
  • Sensory Neurons: Carry afferent information from sensory organs to CNS.
  • Autonomic Nervous System: Divided into Sympathetic ('Fight or Flight') and Parasympathetic ('Rest and Digest') divisions.

Three Muscle Types

  • Skeletal Muscle: Voluntary, striated muscle responsible for movement.
  • Smooth Muscle: Involuntary, non-striated muscle found in walls of hollow organs.
  • Cardiac Muscle: Involuntary, striated muscle found in the heart.

Skeletal Muscle Function

  • Movement: Skeletal muscle movement is vital for mobility.
  • Support: Muscles in the abdominal wall support visceral organs.
  • Posture: Muscle contractions help maintain posture.
  • Temperature Regulation: Skeletal muscles generate heat through contractions.
  • Communication: Facilitate modes of interpersonal communication, such as facial expressions and gestures.

Gross Anatomy of Skeletal Muscle

  • Components:
    • Bone: Skeletal muscles pull on bones to create movement.
    • Tendon: Connect muscles to bones.
    • Epimysium: Surrounds the entire muscle.
    • Perimysium: Surrounds bundles of muscle fibers (fascicles).
    • Endomysium: Surrounds individual muscle fibers (myocytes).
    • Sarcolemma: Muscle cell membrane.
    • Myosin (thick) & Actin (thin): Contractile proteins.
    • Muscle Belly, Fasciculus, Myofibril, Myofilaments: Key structural features of muscle.

The Motor Unit

  • Definition: A motor unit consists of a single motor neuron and all muscle fibers it innervates.
  • Typical Innervation: Between 100 to 1,000 muscle cells per motor neuron.
  • Characteristics: Muscle fibers in the same motor unit are of the same type, defined by speed, strength, and fatigability.

The Neuromuscular Junction (NMJ)

  • Definition: A specialized synapse between the somatic motor neuron and the skeletal muscle cell’s motor end plate.
  • Function: Action potentials from the motor neuron trigger action potentials in muscle cells, initiating contraction.
  • E/C Coupling: The process where an electrical signal results in muscle contraction at the sarcomere level.

Key Neurotransmitters at NMJ

  • Acetylcholine (ACh): The primary neurotransmitter at NMJs.
  • Other Neurotransmitters: Dopamine (DA), GABA, Glutamate (Glu), Serotonin (5HT), Epinephrine (Epi), Norepinephrine (NE), and others categorized under biogenic amines and catecholamines.

Nicotinic Acetylcholine Receptor (nAChR)

  • Type: A ligand-gated ion channel that allows for ion passage.
  • Mechanism:
    1. Closed until ACh binds to it.
    2. Opens to allow Na+ and K+ diffusion, contributing to depolarization in muscle cells.

Synaptic Cleft at NMJ

  • Components: Contains vesicles releasing ACh into the synaptic cleft, where it interacts with nicotinic receptors.
  • Durations: ACh is degraded in the cleft by acetylcholinesterase to terminate the signal.

From Motor Neuron APs to Skeletal Muscle Contractions

  1. ACh binds to skeletal muscle fiber.
  2. Action potential moves into T-tubules.
  3. Ca2+ is released from the sarcoplasmic reticulum.
  4. Ca2+ binds to troponin; active cross-bridge cycling begins.
  5. Muscle contraction occurs.
  6. ACh is degraded by acetylcholinesterase.
  7. Ca2+ is pumped back to ECF and the sarcoplasmic reticulum.
  8. Tropomyosin blocks cross-bridge formation, preventing contraction when muscle is not stimulated.

Excitation/Contraction Coupling

  • Description: Muscle action potentials sensed in T-tubules trigger Ca2+ release from the sarcoplasmic reticulum, leading to muscle contraction.
  • Components Involved:
    • T-Tubules: Invaginate the myofibrils and are continuous with the sarcolemma.
    • Sarcoplasmic Reticulum (SR): A complex network that stores and releases calcium ions.

The Sarcomere

  • Definition: The smallest contractile unit of a muscle cell; consists of thick (myosin) and thin (actin) filaments.
  • Structure: Comprises Z-discs, A-bands, M-line, I-band, and H-zone characterized by specific contractile processes during muscle contraction.

The Sliding Filament Model of Muscle Contraction

  • Mechanism: Muscle contraction occurs when thin filaments slide toward the M-lines, while thick and thin filament lengths remain constant.
  • Changes During Contraction: I-bands and H-zones narrow, while A-bands stay the same length.

Cross-Bridge Cycle in Muscle Contraction

  1. Resting Fiber: Low intracellular calcium concentration.
  2. Calcium Release: Increases intracellular calcium concentration, allowing cross-bridge formation.
  3. Contraction: Cross-bridge cycling occurs as myosin heads pull on actin filaments.

Ca2+ Dependence of Cross-Bridge Attachment

  • In relaxed muscle, tropomyosin prevents cross-bridging.
  • Upon stimulation, Ca2+ binds to troponin, causing tropomyosin displacement and allowing attachment points for myosin heads.

Length/Tension Curve

  • Optimal tension production occurs when sarcomeres are at an ideal length (degree of filament overlap).
  • Deviations lead to suboptimal force production.

Force/Velocity Curve in Skeletal Muscle

  • Types of Contractions:
    • Concentric Contraction: Muscle shortening.
    • Isometric: No change in length.
    • Eccentric Contraction: Muscle lengthening.

Muscle Twitch

  • The response of a muscle to a single action potential; higher frequency stimulation leads to summation of twitches.
  • Stimulation rates yield different types of responses:
    • Low AP frequency: muscles partially relax between contractions.
    • High AP frequency: sustained contraction without relaxation.

Overall Muscle Contraction Strength

  • Increases through:
    • Recruitment of more motor units.
    • Increased number of myofibers activated.
    • Increased frequency of stimulation.
    • Increased thickness of myofibers.
    • Muscle fiber length at rest.

Muscle Fiber Types in Humans

  • Type I: Slow-twitch; fatigue-resistant, low contraction force.
  • Type IIA: Fast-twitch; intermediate fatigue resistance and contraction force.
  • Type IIX: Fast-twitch; high force production but fatigue-prone.

Energy Sourcing for Skeletal Muscles

  • Utilization of ATP for various functions during contraction and relaxation:
    • Myosin ATPases for contraction.
    • Ca2+-ATPases for relaxation.
    • Na+/K+ ATPases for maintaining gradients.
  • ATP Production Methods:
    • Aerobic Respiration: Utilizes oxidative phosphorylation in mitochondria.
    • Anaerobic Respiration: Includes glycogenolysis and fermentation to lactate.
    • Creatine Phosphate: Provides quick energy during short bursts of activity.

Types of Motor Units (Muscle Fibers)

  • Type I (Slow-Twitch): Small diameter, high fatigue resistance, aerobic enzyme-rich.
  • Type IIA (Fast-Twitch): Intermediate diameter, moderate force production, both aerobic and anaerobic enzyme capabilities.
  • Type IIX (Fast-Twitch): Large diameter, high force production, primarily anaerobic enzymes.

Muscle Fiber Type Distribution in Various Populations

  • Sprinters, jumpers, throwers, weightlifters, mid-distance runners, and long-distance runners show varying distributions of muscle fiber types, highlighting the adaptations of muscle to different athletic demands.

Proprioception and Reflexes

  • Muscle Spindle Apparatus: Senses muscle stretch and relays information to initiate reflex actions through afferent pathways.
  • Knee-Jerk Reflex: A monosynaptic stretch reflex illustrating reciprocal innervation between agonist (quadriceps) and antagonist (hamstrings) muscles, demonstrating fundamental reflex pathways in the spinal cord.