Muscle Tissue and Physiology

Muscle Fiber Structure and Organization

The muscle fiber is the basic unit of skeletal muscle, composed of specialized structures that enable contraction and force generation.

• Muscle fiber: A single muscle cell, elongated and multinucleated, surrounded by a plasma membrane called the sarcolemma.

• Myofibril: Cylindrical organelles within muscle fibers, made up of repeating units called sarcomeres.

• Myofilament: Protein filaments (actin and myosin) that make up myofibrils.

• Actin: The main protein of thin filaments; interacts with myosin for muscle contraction.

• Myosin: The main protein of thick filaments; has heads that bind to actin during contraction.

• Sarcomere: The functional contractile unit of a myofibril, defined by Z-discs.

• Striated: Appearance of muscle fibers due to the regular arrangement of actin and myosin.

• Troponin and Tropomyosin: Regulatory proteins on thin filaments; control access of myosin to actin binding sites.

• Thick filament: Composed mainly of myosin molecules.

• Thin filament: Composed mainly of actin, with troponin and tropomyosin.

• Transverse (T) tubule: Invaginations of the sarcolemma that transmit action potentials into the muscle fiber.

• Terminal cisternae: Enlarged areas of the sarcoplasmic reticulum (SR) adjacent to T-tubules; store calcium ions.

• Sarcoplasmic reticulum (SR): Specialized endoplasmic reticulum that stores and releases calcium ions for contraction.

• Triad: Structure formed by a T-tubule and two adjacent terminal cisternae.

Connective Tissue Organization

• Endomysium: Connective tissue surrounding individual muscle fibers.

• Perimysium: Connective tissue surrounding bundles of muscle fibers (fascicles).

• Fascicle: A bundle of muscle fibers within a muscle.

• Epimysium: Connective tissue surrounding the entire muscle.

• Fascia: Sheet of connective tissue separating muscles from each other and from skin.

• Tendon: Cord-like structure connecting muscle to bone.

• Aponeurosis: Broad, flat tendon.

Neuromuscular Junction and Motor Units

• Neuromuscular junction (NMJ): Synapse between a motor neuron and a muscle fiber.

• Motor unit: A single motor neuron and all the muscle fibers it innervates.

• Synaptic cleft: The gap between the axon terminal and the muscle fiber membrane.

• Junctional folds: Folds in the sarcolemma at the NMJ, increasing surface area for neurotransmitter receptors.

• Acetylcholine (ACh): Neurotransmitter released by motor neurons to stimulate muscle contraction.

• Axon terminal: The end of a motor neuron that releases ACh into the synaptic cleft.

Muscle Contraction: From Beginning to End

Muscle contraction involves a series of steps from neural stimulation to the sliding of filaments within the sarcomere.

1. Action potential arrives at the axon terminal of the motor neuron.

2. Acetylcholine is released into the synaptic cleft and binds to receptors on the sarcolemma.

3. Depolarization of the sarcolemma triggers an action potential that travels along the T-tubules.

4. Calcium ions are released from the sarcoplasmic reticulum into the cytosol.

5. Calcium binds to troponin, causing tropomyosin to move and expose binding sites on actin.

6. Myosin heads bind to actin, forming cross-bridges, and perform a power stroke using ATP.

7. Filaments slide past each other, shortening the sarcomere and contracting the muscle.

8. Relaxation occurs when calcium is pumped back into the SR, and the muscle returns to its resting state.

Muscle Physiology Concepts (PhysioEx 2)

• Muscle twitch: A single, brief contraction and relaxation cycle in a muscle fiber.

• Three phases of a twitch:

  • Latent period: Time between stimulus and contraction onset.

  • Contraction period: Muscle fibers shorten and tension increases.

  • Relaxation period: Muscle tension decreases as fibers return to resting length.

• Excitation-contraction coupling: The process linking muscle fiber excitation to contraction via calcium release.

• Recruitment: Increasing the number of active motor units to generate greater force.

• Threshold voltage: The minimum stimulus required to elicit a muscle contraction.

• Stimulus frequency and contraction: Higher frequency leads to increased force due to summation.

• Wave summation: Successive stimuli before relaxation increase contraction strength.

• Treppe: Gradual increase in contraction strength with repeated stimulation ("staircase effect").

• Tetanus: Sustained muscle contraction due to high-frequency stimulation.

  • Fused tetanus: No relaxation between stimuli; maximal, smooth contraction.

  • Unfused tetanus: Partial relaxation between stimuli; contraction is not smooth.

• Muscle fatigue: Inability to maintain contraction strength after prolonged activity.

• Length-tension relationship: The force a muscle can produce depends on its initial length.

The Muscular System

Muscle Roles and Classification

• Agonist (prime mover): Muscle primarily responsible for a movement.

• Antagonist: Muscle that opposes the action of the agonist.

• Synergist: Muscle that assists the agonist in performing its action.

• Fixator: Muscle that stabilizes the origin of the agonist.

Students should be able to identify and know the functions of essential skeletal muscles, as well as their locations on diagrams or models.

Introduction to the Nervous System and Nervous Tissue

Neurons and Neuroglia

The nervous system is composed of two main cell types: neurons and neuroglia.

• Neurons: Excitable cells that transmit electrical signals.

• Neuroglia (glial cells): Support, protect, and nourish neurons; do not conduct impulses.

Types of Neuroglia

Neuron Structure and Function

• Dendrites: Receive incoming signals and transmit them to the cell body.

• Axon: Conducts electrical impulses away from the cell body.

• Axon terminals: Release neurotransmitters to communicate with other neurons or effectors.

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

• Schwann cells: Myelinate axons in the PNS by wrapping around them.

Transmission of Nerve Impulses

1. Action potential travels down the axon to the axon terminal.

2. Neurotransmitter is released into the synaptic cleft.

3. Neurotransmitter binds to receptors on the postsynaptic cell, initiating a new action potential.

4. At the neuromuscular junction, acetylcholine stimulates muscle fiber contraction.

Classification of Neurons

• Structural:

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

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

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

• Functional:

  • Sensory (afferent): Transmit impulses toward the CNS.

  • Motor (efferent): Transmit impulses away from the CNS to effectors.

  • Interneurons: Connect sensory and motor neurons within the CNS.

Other Key Terms

• Tract: Bundle of axons in the CNS.

• Nucleus (CNS): Cluster of neuron cell bodies within the CNS.

• Nerve: Bundle of axons in the PNS.

• Ganglia: Clusters of neuron cell bodies in the PNS.

Connective Tissue Sheaths of Nerves

• Endoneurium: Surrounds individual axons.

• Perineurium: Surrounds bundles of axons (fascicles).

• Epineurium: Surrounds the entire nerve.

Nerve Physiology (PhysioEx 3)

Resting Membrane Potential and Action Potentials

• Resting membrane potential: The voltage difference across the membrane of a resting neuron, typically about -70 mV.

• Generation: Maintained by sodium-potassium pumps and differential permeability to ions.

• Local potential: Small, localized change in membrane potential.

• Depolarization: Membrane potential becomes less negative due to sodium influx.

• Action potential: Rapid, large change in membrane potential that propagates along the axon.

• Trigger zone: Region where action potentials are initiated (axon hillock).

• Threshold: Minimum depolarization needed to trigger an action potential.

Voltage-gated ion channels open or close in response to changes in membrane potential, allowing specific ions to cross the membrane.

• Inactivated sodium channel: Channel is closed and cannot be reopened until it returns to the resting state.

• Refractory period: Time during which a neuron cannot fire another action potential.

  • Absolute refractory period: No action potential can be generated, regardless of stimulus strength.

  • Relative refractory period: A stronger-than-normal stimulus is required to generate an action potential.

• Myelination: Increases speed of neural transmission by allowing saltatory conduction (impulse jumps between nodes of Ranvier).

• Neurotransmitter: Chemical messenger that facilitates synaptic transmission between neurons or from neuron to muscle.

Key Equations

• Nernst Equation (for equilibrium potential of an ion):

• Goldman-Hodgkin-Katz Equation (for membrane potential):

Study Tips

• Use pre-lab quizzes and online anatomy lab resources for practice.

• Focus on understanding diagrams and figures referenced in your lab manual.

• Pay attention to muscle names, locations, and functions, especially those highlighted in essential muscle lists and tables.