Muscle Types, Structure, and Contraction Mechanisms: A Detailed Overview

Excitability

Ability to generate an action potential.

Elasticity

Ability to return to original length.

Extensibility

Ability to be stretched.

Contractility

Ability to shorten and generate force.

Skeletal Muscle

Voluntary, striated, multinucleated muscle attached to bones.

Cardiac Muscle

Involuntary, striated, branched muscle found in heart.

Smooth Muscle

Involuntary, nonstriated muscle found in hollow organs.

Functions of Muscle

Movement, posture, joint stabilization, heat production.

Epimysium

Connective tissue surrounding entire muscle.

Perimysium

Connective tissue surrounding fascicles.

Fascicle

Bundle of muscle fibers.

Endomysium

Connective tissue surrounding individual muscle fibers.

Muscle Fiber

Individual muscle cell.

Sarcolemma

Plasma membrane of muscle fiber.

Sarcoplasm

Cytoplasm of muscle fiber.

Sarcoplasmic Reticulum (SR)

Stores and releases calcium.

T-Tubules

Extensions of sarcolemma carrying action potentials inward.

Terminal Cisternae

Enlarged SR regions adjacent to T-tubules.

Triad

Two terminal cisternae and one T-tubule.

Myofibril

Organelle containing sarcomeres.

Sarcomere

Functional contractile unit of muscle.

Actin

Thin filament.

Myosin

Thick filament.

Z Disc

Anchoring point for actin.

M Line

Anchoring point for myosin.

Troponin

Regulatory protein involved in contraction.

Tropomyosin

Regulatory protein covering actin binding sites.

A Band

Dark band containing thick filaments.

I Band

Light band containing only thin filaments.

H Zone

Region with only thick filaments.

Striation

Alternating pattern of A and I bands.

Resting Membrane Potential

Approximately -70 mV.

Depolarization

Membrane becomes less negative.

Repolarization

Return toward resting membrane potential.

Hyperpolarization

Membrane becomes more negative than resting potential.

Threshold

Approximately -55 mV.

Action Potential

Widespread depolarization along membrane.

Neuromuscular Junction

Site where motor neuron meets muscle fiber.

Motor Unit

One motor neuron and all muscle fibers it controls.

Sliding Filament Model

Actin slides past myosin causing contraction.

Role of Calcium

Allows actin and myosin interaction.

Contraction Begins When

Calcium is released from SR.

Latent Period

First stage of muscle twitch.

Contraction Phase

Second stage of muscle twitch.

Relaxation Phase

Third stage of muscle twitch.

Skeletal Muscle Histology

Long fibers, many nuclei, striations.

Cardiac Muscle Histology

Branched fibers, central nuclei, intercalated discs.

Smooth Muscle Histology

Spindle-shaped cells, no striations.

Intercalated Discs

Connections between cardiac muscle cells.

Cardiac Muscle Nuclei

Usually one central nucleus.

Smooth Muscle Shape

Spindle-shaped.

Skeletal Muscle Control

Voluntary.

Cardiac Muscle Control

Involuntary.

Smooth Muscle Control

Involuntary.