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 skeletal muscle due to the regular arrangement of actin and myosin.

• Troponin: Regulatory protein on thin filaments; binds calcium to initiate contraction.

• Tropomyosin: Regulatory protein that blocks myosin-binding sites on actin at rest.

• 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.

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

• 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: Dense connective tissue that separates muscles from each other and from the skin.

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

• Aponeurosis: Broad, flat tendon.

Neuromuscular Junction and Muscle Contraction

The neuromuscular junction (NMJ) is the synapse between a motor neuron and a muscle fiber, essential for initiating muscle contraction.

• Neuromuscular junction (NMJ): Site where a motor neuron communicates with a muscle fiber.

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

• Synaptic cleft: The gap between the axon terminal and the muscle fiber's sarcolemma.

• 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 Sequence:

1. An action potential travels down the motor neuron to the axon terminal.

2. ACh is released into the synaptic cleft and binds to receptors on the muscle fiber's sarcolemma.

3. This triggers an action potential in the muscle fiber, which travels along the sarcolemma and down T-tubules.

4. The action potential causes the SR to release calcium ions.

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

6. Myosin heads bind to actin, forming cross-bridges, and perform a power stroke, shortening the sarcomere.

7. ATP binds to myosin, causing it to detach from actin and reset for another cycle.

8. When stimulation ends, calcium is pumped back into the SR, and the muscle relaxes.

Example: Lifting a weight involves repeated cycles of muscle contraction and relaxation, coordinated by the NMJ and the release of calcium.

Muscle Physiology Concepts (PhysioEx 2)

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

• 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 (action potential) to contraction (cross-bridge cycling).

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

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

• Stimulus frequency and contraction: Higher frequency stimuli can lead to stronger contractions due to summation.

• Wave summation: Increased muscle tension due to repeated stimulation before complete relaxation.

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

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

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

  • Unfused tetanus: Partial relaxation between stimuli; tension fluctuates.

• Muscle fatigue: Inability of muscle to maintain force of contraction after prolonged activity.

• Length-tension relationship: The force a muscle can generate depends on its initial length; optimal overlap of actin and myosin produces maximal force.

Example: During rapid, repeated lifting, muscles may enter tetanus and eventually fatigue.

The Muscular System

Muscle Roles and Classification

Skeletal muscles work together to produce movement, each playing specific roles.

• 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.

Example: In elbow flexion, the biceps brachii is the agonist, triceps brachii is the antagonist, and brachialis acts as a synergist.

Essential Muscles and Their Functions

Students should be able to identify and describe the function of major skeletal muscles, as listed in the Essential Muscle list and illustrated in figures (e.g., 13.2, 13.3).

• Muscles are grouped by region (head, arms, legs, etc.) and function (movement, posture, etc.).

• Comments in tables provide additional functional details.

Example: The deltoid muscle abducts the arm at the shoulder joint.

Introduction to the Nervous System and Nervous Tissue

Neurons and Neuroglia

The nervous system consists of two main cell types: neurons and neuroglia, each with distinct functions.

• Neurons: Excitable cells that transmit electrical signals.

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

• Functional differences: Neurons are responsible for communication; neuroglia provide structural and metabolic support.

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.

Example: A motor neuron transmits an impulse from the spinal cord to a muscle fiber, causing contraction.

Transmission of Nerve Impulses

• Impulse transmission involves the movement of ions across the neuron's membrane, generating action potentials.

• At the synapse (including the NMJ), neurotransmitters like ACh facilitate communication between neurons or between neuron and muscle.

Classification of Neurons

• Structural: Based on the number of processes (multipolar, bipolar, unipolar).

• Functional: Sensory (afferent), motor (efferent), interneurons (association).

Key Nervous System 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 neuron's membrane at rest, typically about -70 mV.

• Generation: Due to differences in ion concentrations and selective permeability of the membrane.

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

• Depolarization: Membrane potential becomes less negative (more positive).

• 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 membrane potential needed to trigger an action potential.

Key Equations:

• Resting membrane potential (Nernst equation):

Ion Channels and Refractory Periods

• Voltage-gated ion channels: Open or close in response to changes in membrane potential.

• Inactivated sodium channel: Temporarily unable to open, even if the membrane is depolarized; differs from the closed (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.

Example: Myelinated axons conduct impulses much faster than unmyelinated axons, enabling rapid reflexes.

Additional Study Tips

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

• Focus on figures and tables in your textbook and lab manual for visual identification and understanding of structures.