Muscular System

Main function of muscle tissue

To contract or shorten, making movement possible.

Functions of the muscular system

Produces body movement, maintains posture, causes respiration, produces body heat, performs movements involved in communication, constricts organs and vessels, and pumps blood.

Contractility

Ability of muscle to shorten forcefully or contract.

Excitability

Capacity of muscle to respond to stimuli.

Extensibility

Ability of muscle to stretch beyond its normal resting length and still contract.

Elasticity

Ability of muscle to return to its original resting length after being stretched.

Cause of muscle contraction

Contractile proteins located within muscle cells.

Three types of muscle tissue

Skeletal, cardiac, and smooth muscle tissue.

Skeletal muscle

Attaches to the skeleton and enables the body to move.

Appearance of skeletal muscle

Striated or banded because of the arrangement of contractile proteins.

Control of skeletal muscle

Voluntary.

Skeletal muscle nuclei

Has many nuclei according to the module.

Cardiac muscle

Muscle tissue of the heart.

Appearance of cardiac muscle

Striated and cylindrical but shorter than skeletal muscle cells.

Cardiac muscle nuclei

Usually one nucleus per cell.

Control of cardiac muscle

Involuntary.

Smooth muscle

Forms the walls of hollow organs.

Examples of organs containing smooth muscle

Esophagus, stomach, bladder, and intestines.

Appearance of smooth muscle

Tapered at each end, single nucleus, and non-striated.

Control of smooth muscle

Involuntary.

Peristalsis

Movement of the esophagus mentioned in the module as an example involving smooth muscle.

Fascicle

A group of muscle fibers or muscle cells.

Muscle fiber

A muscle cell, also called a myocyte.

Myocyte

Another term for a muscle cell.

Myofibril

Important structure inside a muscle fiber where contraction occurs.

Basic organization emphasized in the module

Fascicles contain muscle fibers or myocytes, which contain myofibrils.

Motor neuron

Neuron that sends a signal to muscle cells to initiate contraction.

Axon

Part of the neuron that releases neurotransmitters to signal muscle cells.

Acetylcholine

Neurotransmitter released by neurons that triggers ligand gates on muscle cells.

Effect of acetylcholine

Triggers sodium ligand gates, beginning depolarization of the muscle cell.

Sodium ligand gates

Gates opened by acetylcholine that allow the process of muscle-cell depolarization to begin.

Depolarization

Change initiated after sodium ligand gates open that leads toward muscle contraction.

Action potential

Electrical signal involved in initiating muscle contraction.

Condition stated for contraction in the module

All myofibrils must be depolarized.

T-tubules

Structures that help spread the signal/sodium ions to the myofibrils according to the module.

Sarcoplasmic reticulum

Specialized structure surrounding myofibrils that contains large amounts of calcium ions.

Role of the sarcoplasmic reticulum

Releases calcium ions when triggered by muscle-cell depolarization.

Calcium ions

Ions released from the sarcoplasmic reticulum that are important for muscle contraction.

Z-disc

Structure located at the ends of the contractile region described in the module.

Myofilaments

Contractile filaments including actin and myosin.

Actin

One of the two important myofilaments involved in muscle contraction.

Myosin

One of the two important myofilaments involved in muscle contraction.

What causes muscle striations?

Overlapping arrangement of the myofilaments according to the module.

Troponin

Protein associated with actin that helps prevent actin and myosin from binding when the muscle is at rest.

Tropomyosin

Protein associated with actin that prevents actin and myosin from binding when the muscle is at rest.

Role of calcium in actin-myosin interaction

Calcium interacts with the regulatory proteins, freeing actin so it can bind with myosin.

ATP

Energy involved in muscle contraction and produced by mitochondria.

ATP breakdown

ATP is broken down into ADP and phosphate during the contraction process described in the module.

ADP and phosphate

Attach to the myosin head as part of the contraction process.

Myosin head

Part of myosin that interacts with actin to produce movement.

Power stroke

Movement produced when the myosin head binds with actin and influences actin movement.

Role of new ATP after the power stroke

Removes the binding between myosin and actin, allowing return toward resting length.

Titin

Spring-like protein described in the module that is associated with pulling/positioning of the Z-disc.

Simplified muscle contraction sequence

Motor neuron signal → acetylcholine → sodium gates open → depolarization → T-tubules spread the signal → sarcoplasmic reticulum releases Ca²⁺ → actin and myosin can interact → power stroke → contraction.

Aerobic respiration

Used for continuous muscle activity and produces a large amount of ATP.

Anaerobic respiration

Used for emergency situations and produces emergency ATP according to the module.

Example of anaerobic respiration in the module

An emergency situation such as being chased by a dog.

Lactic acid

Produced during continued anaerobic ATP production according to the module; accumulation is associated with muscle cramps.

Muscle fatigue

Worn-out or fatigued muscles associated in the module with using large amounts of ATP.