Muscle Types, Structure, and Contraction Mechanics: A Comprehensive Biology Guide

Skeletal muscle

One of three basic muscle types in the body. Most are attached by tendons to bones, have cells that are multinucleated, are striated (have visible banding), and are voluntary (subject to conscious control). Its functions include supporting the body to remain upright, providing movement of bones, providing pressure (blood/lymph), protecting internal organs/bones, and stabilizing joints.

Cardiac muscle

One of three basic muscle types. Has striations, usually has a single nucleus, consists of branching cells, is joined to another muscle cell at an intercalated disc, is involuntary, and is found only in the heart.

Smooth muscle

One of three basic muscle types. Lacks striations, consists of spindle-shaped cells, has a single nucleus, is involuntary (no conscious control), and is found mainly in the walls of hollow organs.

Endomysium

Connective tissue that encloses a single muscle fiber.

Perimysium

Connective tissue that wraps around a fascicle (bundle) of muscle fibers.

Epimysium

Connective tissue that covers the entire skeletal muscle.

Fascia

Connective tissue located on the outside of the epimysium.

Muscle

Composed of bundles of skeletal muscle fibers called FASCICLES.

Fascicles

Bundles of skeletal muscle fibers; surrounded by connective tissue.

Muscle fiber

A cell that possesses a plasma membrane (sarcolemma), a cytoplasm (sarcoplasm), and an endoplasmic reticulum (sarcoplasmic reticulum).

Sarcolemma

The specialized plasma membrane of a muscle fiber.

Sarcoplasm

The cytoplasm of a muscle fiber.

Myofibrils

Long organelles found inside the muscle cell; they are aligned to give distinct bands.

Sarcoplasmic reticulum (SR)

Specialized smooth endoplasmic reticulum; its function is to store and release calcium and it surrounds the myofibril.

I band (Light band)

A distinct band given by the alignment of myofibrils that contains only thin filaments.

A band (Dark band)

A distinct band given by the alignment of myofibrils that contains the entire length of the thick filaments.

Sarcomere

The contractile unit of a muscle fiber.

Myofilaments

The filaments within the sarcomere, which include thick and thin filaments.

Thick filaments

Also known as myosin filaments.

Thin filaments

Also known as actin filaments.

H zone

A bare zone that lacks actin filaments when the muscle is at rest.

Excitability (Responsiveness or Irritability)

The ability to receive and respond to a stimulus.

Contractility

The ability to shorten when an adequate stimulus is received.

Extensibility

The ability of muscle cells to be stretched.

Elasticity

The ability to recoil and resume resting length after stretching.

Motor neuron (nerve cell)

Skeletal muscles must be stimulated by this cell to contract.

Motor unit

One motor neuron and all the skeletal muscle cells stimulated by that neuron; defined as a nerve fiber plus all the muscle fibers it innervates.

All-or-none law (Motor Unit)

States that all muscle fibers in a motor unit are stimulated at once, so that they all contract or they don't contract.

Neuromuscular junction

The association site of the axon terminal of the motor neuron and the muscle.

Synaptic cleft

The gap between the nerve and muscle; the nerve and muscle do not make contact; the area is filled with interstitial fluid.

Neurotransmitter

A chemical released by the nerve upon the arrival of a nerve impulse.

Acetylcholine (ACh)

The specific neurotransmitter for skeletal muscle.

Muscle twitch

A single contraction, lasting a fraction of a second, that takes place when a motor unit is stimulated by infrequent electrical impulses.

Latent period

The period of time between CNS stimulation and the beginning of muscle contraction.

Contraction Period

The period of time during which the muscle is shortening.

Relaxation period

The period of time during which the muscle is returning to its original length.

Tetanus

A sustained contraction achieved if maximal contraction occurs; continues until fatigue sets in and the muscle relaxes even when stimulated.

Muscle tone

The condition where some motor units are always contracted, but not enough to cause movement.

Myoglobin

A substance contained within muscle cells that combines with and stores O2.

Glycogen and fat (triglycerides)

Sources of energy stored in muscle.

Blood glucose and plasma fatty acids

Sources of energy derived from the blood.

Creatine PO4 Pathway

An anaerobic pathway to produce ATP for energy for muscle contraction; it is the fastest and simplest way for muscles to produce ATP; CP is used for high-intensity exercise before cellular respiration takes over.

Fermentation

An anaerobic pathway to produce ATP; produces ATP by breaking down glucose, producing Lactate; begins by breaking down glycogen to glucose.

Lactate

Produced by fermentation; its formation results in muscle aches and fatigue; converted back to glucose with rest and O2 intake.

O2 debt

What limits fermentation; defined as the amount of additional O2 that is consumed following exercise to restore pre-exercise conditions.

Cellular Respiration (Aerobic pathway)

A pathway to produce ATP for muscle contraction; more likely to supply energy when exercise is sub-maximal in intensity; it is not an immediate source.

Fast twitch muscle fibers

Light in color (fewer mitochondria), have little myoglobin (to store O2), and have fewer blood vessels. They develop maximum tension more quickly and to a greater degree, but fatigue quickly and lactate builds up quickly because they depend on anaerobic energy. Associated with sprinting and power sports.

Slow twitch muscle fibers

Darker in color (more mitochondria), have lots of myoglobin, and are surrounded by dense capillary beds to take in lots of O2. They have more endurance (e.g., long distance running, swimming) because they produce most of their energy aerobically; they exhibit low maximum tension but are resistant to fatigue and maintain substantial reserves of glycogen and fat.

Concentric contraction of muscles

Contraction during which muscles shorten.

Eccentric contraction of muscles

Contraction during which muscles lengthen (the negative phase of the lift).

Origin (Muscle attachment)

Attachment to an immovable bone.

Insertion (Muscle attachment)

Attachment to a moveable bone.

Flexion

Decreases the angle of the joint; brings two bones closer together; typical of hinge joints.

Extension

The opposite of flexion; increases the angle between two bones.

Rotation

Movement of a bone around its longitudinal axis; common in ball-and-socket joints.

Abduction

Movement of a limb away from the midline.

Adduction

The opposite of abduction; movement of a limb toward the midline.

Circumduction

A combination of flexion, extension, abduction, and adduction; common in ball-and-socket joints.

Dorsiflexion

Lifting the foot so that the superior surface approaches the shin.

Plantar flexion

Depressing the foot (pointing the toes).

Inversion

To turn the sole of the foot medially.

Eversion

To turn the sole of the foot laterally.

Supination

Forearm rotates laterally so the palm faces anteriorly.

Pronation

Forearm rotates medially so the palm faces posteriorly.

Opposition

Moving the thumb to touch the tips of other fingers on the same hand.

Prime mover

The muscle with the major responsibility for a certain movement.

Antagonist

The muscle that opposes or reverses a prime mover.

Synergist

The muscle that aids a prime mover in a movement and helps prevent rotation.

Fixator

The muscle that stabilizes the origin of a prime mover.

Direction of muscle fibers

Example: Rectus (straight).

Relative size of the muscle

Example: Maximus (largest).

Location of the muscle

Example: Temporalis (temporal bone).

Number of origins

Example: Triceps (three heads).

Location of the muscle's origin and insertion

Example: Sterno (on the sternum).

Shape of the muscle

Example: Deltoid (triangular).

Action of the muscle

Example: Flexor and extensor (flexes or extends a bone).