Muscle Tissue and Physiology

Types and Characteristics of Muscle Tissue

Muscle tissue is classified into three main types: skeletal, cardiac, and smooth muscle. Each type has distinct structural and functional characteristics.

• Skeletal muscle: Voluntary, striated, attached to bones, responsible for body movement.

• Cardiac muscle: Involuntary, striated, found only in the heart, responsible for pumping blood.

• Smooth muscle: Involuntary, non-striated, found in walls of hollow organs (e.g., intestines, blood vessels).

Common functions: Movement, posture, heat production, and stabilization of joints.

Muscle Fiber Structure and Organization

Muscle fibers are composed of myofibrils, which contain repeating units called sarcomeres. Sarcomeres are the functional units of muscle contraction.

• Myofibrils: Cylindrical structures within muscle fibers, made up of actin (thin) and myosin (thick) filaments.

• Sarcomere: The segment between two Z-lines; contains bands (A, I) and zones (H, M).

• Connective tissue: Endomysium (surrounds individual fibers), perimysium (bundles fibers into fascicles), epimysium (surrounds entire muscle).

Muscle Contraction Mechanism

Muscle contraction is initiated by the sliding filament theory, involving the interaction of actin and myosin filaments.

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

• Role of ATP: Provides energy for cross-bridge cycling and detachment.

• Calcium ions: Released from the sarcoplasmic reticulum, bind to troponin, exposing binding sites on actin.

Equation:

Muscle Twitch and Myogram Phases

A muscle twitch consists of three phases: latent, contraction, and relaxation. A myogram records these phases.

• Latent period: Time between stimulus and onset of contraction.

• Contraction period: Muscle fibers shorten and tension increases.

• Relaxation period: Muscle tension decreases as calcium is reabsorbed.

Motor Units and Recruitment

A motor unit consists of a motor neuron and all the muscle fibers it innervates. Recruitment refers to the activation of additional motor units to increase muscle force.

• Small motor units: Fine control (e.g., eye muscles).

• Large motor units: Gross movements (e.g., thigh muscles).

Muscle Fatigue and Recovery

Muscle fatigue is the decline in ability of a muscle to generate force. It can result from depletion of ATP, accumulation of lactic acid, or ionic imbalances.

• Recovery: Involves removal of lactic acid, replenishment of ATP and glycogen stores.

Neurophysiology and Action Potentials

Neuron Structure and Function

Neurons are specialized cells for transmitting electrical signals. Key parts include the cell body, dendrites, and axon.

• Dendrites: Receive incoming signals.

• Axon: Transmits action potentials away from the cell body.

• Axon terminals: Release neurotransmitters to communicate with other cells.

Resting Membrane Potential

The resting membrane potential is the voltage difference across the neuron's membrane at rest, typically around -70 mV.

• Maintained by: Sodium-potassium pump ( out, in), and selective permeability of the membrane.

Equation:

\text{ATPase}: 3 \text{Na}^+ \text{out}, 2 \text{K}^+ \text{in}$

Action Potential Generation and Propagation

An action potential is a rapid change in membrane potential that travels along the axon.

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

• All-or-none principle: Action potentials either occur fully or not at all.

• Phases: Depolarization (Na+ influx), repolarization (K+ efflux), hyperpolarization.

Ion Channels and Membrane Permeability

Different types of ion channels regulate membrane permeability and action potential dynamics.

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

• Ligand-gated channels: Open in response to binding of neurotransmitters.

• Leak channels: Always open, contribute to resting potential.

Synaptic Transmission

Synaptic transmission is the process by which neurons communicate with each other or with effector cells.

• Chemical synapses: Use neurotransmitters to transmit signals across the synaptic cleft.

• Electrical synapses: Direct flow of ions through gap junctions.

• Neurotransmitter removal: By reuptake, enzymatic degradation, or diffusion.

Neurotransmitters and Their Effects

Neurotransmitters can be excitatory or inhibitory, affecting the likelihood of action potential generation in the postsynaptic cell.

• Excitatory: Glutamate, acetylcholine (at neuromuscular junction).

• Inhibitory: GABA, glycine.

Comparisons and Classifications

Comparison of Muscle Types

Comparison of Synaptic Types

Additional Info

• ATP production: Occurs via glycolysis, Krebs cycle, and oxidative phosphorylation.

• Motor end plate: Specialized region of muscle fiber membrane at the neuromuscular junction.

• Summation: Increased force of contraction by combining multiple stimuli.

• Refactory period: Time during which a neuron or muscle fiber cannot respond to a new stimulus.