Muscular System

Anatomy of a Muscle

Major Functions of the Muscular System Produce Movement Heat production Maintain Posture

Skeletal Muscle-Voluntary Muscle Threadlike 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 involuntary Found 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 membrane Sarcoplasmic Reticulum- like an endoplasmic reticulum of any cell.

Physiology of Muscles contraction What Happens and Why? 1. A nerve impulse reaches the end of a motor neuron – releases acetylcholine Acetylcholine diffuses across the gap of the neuromuscular junction and binds to receptors on the motor endplate of the muscle fiber. Impulse travels along the sarcolemma, to the T tubules to sacs of the SR.

THEN What HAPPENS? 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. Energized myosin cross bridges of the thick myofilaments bind to actin and pull the thin myofilaments together. Muscles Shortens.

Sarcomere structure Actin Myosin

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 25msec 15msec

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.

Tetanus Smooth 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. Rapid repeated Twitch contractions Looks like one continuous contraction

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.