Muscular System

Anatomy of a Muscle

Major Functions of the Muscular System

• Produce Movement • Heat production• Maintain Posture

Skeletal Muscle-Voluntary MuscleThreadlike cells that have multiple

nuclei Many cross striations (stripes)

Attached to bones to produce body movement or facial expressions

Smooth Muscle-Involuntary Muscle Elongated narrow cells with single nucleus

No cross striations Found in the walls of hollow organs to

propel food through them by peristalsis

Cardiac Muscle-Also involuntaryFound only in the heart

Branching cells joined by intercalated disks with gap junctions that allow ions to

pass freely from cell to cell=rapid conduction of electrical impulse.

The functional unit of a muscle

Striated Muscle

Multinucleated

Striations are visible Caused by the sarcomeres.

Terms

Sarcomere,Sarcolemma-cell membraneSarcoplasmic Reticulum- like an endoplasmic reticulum ofany cell.

Physiology of Muscles contractionWhat Happens and Why?

1. A nerve impulse reaches the end of a motor neuron – releases acetylcholine

2. Acetylcholine diffuses across the gap of the neuromuscular junction and binds to receptors on the motor endplate of the muscle fiber.

3. Impulse travels along the sarcolemma, to the T tubules to sacs of the SR.

THEN What HAPPENS?

4. Ca++ is released from the SR into the sarcoplasm, where it binds to troponin mlcs in the thin myofilaments.

5. Tropomyosin mlcs in the myofilaments shift, exposing actin’s active sites.

6. Energized myosin cross bridges of the thick myofilaments bind to actin and pull the thin myofilaments together.

7. Muscles Shortens.

Sarcomere structure

MyosinActin

Sliding Filament Theory

Types of Muscle contractions

• Isotonic –muscle shortens and movement occurs.

• Isometric- muscle does not shorten but tension increases.

Twitch contraction

• Threshold stimulus – electrical stimulus of sufficient intensity must be applied to the muscle.

• A single brief threshold stimulus produces a quick jerk of the muscle.

2msec

15msec

25msec

Treppe: A Staircase phenomenon

• A gradual step-like

increase in the strength of contraction that can be observed in a series of twitch contractions that occur about 1 second apart.

• Muscle Fatigue - Repeated stimulation of muscle in time lessens its excitability and contractility.

TetanusSmooth sustained contractions

• Stimuli comes in rapid succession the muscle does not have time to relax completely before the next contraction.

• Normal skeletal muscles exhibit this most of the time.

Looks like one continuous contraction

Rapid repeated Twitch contractions

Muscle Tone--tonic contractions

A continual partial contraction.• Maintaining posture – sitting or standing

• When you pass out/faint muscle tone is lost

• Muscles with Less tone are flaccid.• Muscles with More tone are spastic.• Tone is maintained by a negative

feedback mechanisms centered in the nervous system (in spinal cord)

Muscle Problems

• Cramps-painful muscle spasms (involuntary twitches) caused by any irritation or ion and water imbalance

• Convulsions-uncoordinated tetanic contractions of varying groups of muscles.

May result from a disturbance in the brain or seizure. AP along motor nerve increases and becomes disorganized

• Fibrillation – abnormal contraction in which

individual fibers contract out

of sync.(asynchronously)

instead of at the same time.

Muscular Disorders

• Sprains – Joint and ligament damage

• Myalgia – Muscle pain

• Contusions – bruises – localized bleeding

• Poliomyelitis – Viral infection of the nerves that control skeletal muscles.

• Muscular Dystrophy – A group of Genetic

Disease characterized by atrophy of skeletal muscle tissues. Some forms Fatal

• Myasthenia Gravis – Autoimmune disease in which the immune system attacks muscle cells at the neuromuscular junction

So nerve impulses are unable to fully stimulate the affected muscle.

• Hernias – Weakness of abdominal muscles can lead to protrusion of an abdominal organ.

• Strangulated Hernia – Cuts off blood supply to an organ which can lead to gangrene – death of the organ or individual – Requires emergency surgery.