Muscular System

Skeletal Muscle 3 types of muscle Voluntary Striated Muscle Tissue Multiple nuclei

Cardiac Muscle Involuntary Striated Muscle Tissue Intercalated discs: special connections between each muscle fiber, allowing for the proper muscle contraction. ONLY in the heart.

Smooth Muscle: Involuntary Not striated Spindle-shaped cells Single nuclei per cell.

Smooth muscle is held in position and compartmentalized by fascia. Connective Tissues, CT Smooth muscle is held in position and compartmentalized by fascia. Fascia: sheet like layers of CT.

The CT that extends beyond the muscle to form a cord like tissue is a tendon. Tendon: attach muscles to the periosteum of bones.

The CT that extends beyond the independent muscle and forms a broader fibrous sheet like covering over adjacent muscles is aponeurosis. Aponeurosis: spans more than one muscle.

Structure All the parts of skeletal muscle are enclosed in layers of CT. This forms a network that spans the entire muscular system.

Structure: skeletal muscle Fascicle Muscle fiber Myofibril Sarcomere

Each muscle has specific layers of membrane: Epimysium (around the muscle) Perimysium (around the fascicle) Endomysium (around the muscle fiber)

Each muscle fiber is a single cell of a muscle Each muscle fiber is a single cell of a muscle. It responds to stimulation by contracting and relaxing.

Immediately below the cell membrane or sarcolemma is the sarcoplasm of the fiber. This sarcoplasm contains the oval nuclei, mitochondria, and myofibrils.

The key to contraction Myofibrils, protein filaments Myosin: molecules composed of protein strands with globular parts. A band Dark band

Actin: molecules composed of protein with ADP attached to its surface. I band Light band

Sarcomere The segment of a myofibril between two consecutive Z-lines. Actin filaments are attached to the Z-lines. This arrangement leads to the striated appearance.

Carrying the impulse To carry the impulse there are membranous channels within the cytoplasm of each muscle fiber, sarcoplasmic reticulum and transverse tubules.

Sarcoplasmic reticulum Transverse tubules Run parallel to the myofibril Transverse tubules Extend inward passing through muscle fibers.

The Activators There must be a connection between the muscular system and the nervous system. Neuromuscular junction

Neuromuscular Junction Specialized connection of a nerve fiber and a muscle fiber. Motor end plate: specialized end of the muscle fiber

Follow a nervous impulse… Synaptic Clefts: cup like branches of the motor end plate specialized to receive neurotransmitter. Follow a nervous impulse…

The nerves we are dealing with are called motor fibers. One motor fiber may be connected to many muscle fibers.

Impulse transmission When a motor nerve fiber transmits an impulse, all of the muscle fibers connected to it, are stimulated to contract simultaneously.

Motor Unit The motor neuron and the muscle fibers controlled by it make up a motor unit.

How does a muscle contract? Myosin filament globular heads form crossbridges Actin filament attachment sites allow for attachment of these crossbridges.

Sliding Filament Theory Myosin head is bent back and up to attach with an actin active site. The myosin head pulls back contracting the sarcomere.

The cross bridge is broken. The myosin head straightens out The cross bridge is broken. The myosin head straightens out. A new linkage can now be formed for further contraction.

Stimulus for this contraction Stimulus via neurotransmitter: Acetylcholine. Synthesized in the cytoplasm of the motor neuron

Stored in synaptic vesicles at the distal end of the motor neuron. Travel: Diffuses across the gap, synaptic cleft. Received by receptors on the motor end plate.

Stimulus = Contraction

Control Troponin wants to attach to Ca Troponin is attached to tropomyosin Tropomyosin is attached to actin Myosin wants to attach to actin

The sarcoplasmic reticulum has a high [Ca] the membrane becomes permeable to the Ca ions and they flow into the sarcoplasm of the muscle fiber.

Tropomyosin blocks the actin attachment sites. Troponin on the tropomyosin has Ca attachment sites When [Ca] goes up the troponin binds to it.

Tropomyosin is lifted the myosin-actin binding sites become available  linkage  muscle contraction

Reverse… Ca ions are quickly moved from the sarcoplasm back to the sarcoplasmic reticulum by a Ca pump. Transport?

Troponin and tropomyosin return to myosin Myosin and actin attachment is broken Myosin and actin slide apart  relaxation of the muscle fiber

Acetylcholine is broken down by acetylcholinesterase Don’t forget about… rhigamortis Acetylcholine is broken down by acetylcholinesterase Botulism

Botulism Prevents the release of acetylcholine. NO muscle stimulation NO contraction  NO…

ATP is the energy source of the muscle Regeneration of ATP requires creatine phosphate and the cellular respiration of glucose.

Respiration of glucose If the muscle is lacking O Glucose  pyruvic acid Pyruvic acid  lactic acid Lactic acid  muscle soreness

If O had become available at any time during this cycle, it could be reversed.

Hemoglobin carries O in the blood = red. Hold onto the O. Hemoglobin carries O in the blood = red. Myoglobin holds O in the muscle = reddish brown. This compensates for the blood lost during contraction.

Muscular Response Threshold stimulus: minimal strength needed to cause contraction

All-or-none Response: the stimulated fiber always contracts completely, or not at all. If stimulus is increased, recruitment occurs.

Recruitment: multiple fibers contracting simultaneously.

Types of Contraction Twitch: a single contraction Tetanic: sustained forceful contraction without relaxation Muscle Tone: a continuous slight contraction

Muscle tone Caused by continuous nervous impulse to a few fibers from the spinal cord (the brain is not involved)

Skeletal Muscle Actions Teamwork: Prime mover: responsible for the bulk of movement Synergist: helpers Antagonist: work against, move in the opposite direction

Cardiac Only in the heart Intercalated discs have a very low threshold, act on impulse rapidly. Self-exciting, responsible for the rhythmic continuous beating.

Slower to contract and slower to relax. Smooth Muscle Slower to contract and slower to relax. Maintains force for a longer period of time with the same amount of ATP. Force is not effected by stretch.

2 Types of Smooth Muscle Multi-unit: Less organized Contains blood vessels

Visceral: Sheets of spindles Hollow organs Self-exciting wave like rhythmic contraction Tubular organs  peristalsis