Anatomy Chapter 6: Muscular System

3 types of muscles – cardiac, skeletal, and smooth

o Cardiac

Only found in the heart

Involuntarily controlled

Striated, branched, cylindrical cells with intercalated discs

Has a single nucleus in each cell

Found in the walls of the heart

Slow contraction speed

Changes the internal volume of the heart as it contracts

o Skeletal

Found attached to bones

Voluntarily controlled

Striated

Has several nuclei per cell

Contains long, non-branching cylindrical cells

Slow to fast contraction speed

Concerned with locomotion of the body as a whole

o Smooth

Found in the walls of the stomach, uterus, and arteries

Involuntarily controlled

Has a single nucleus in each cell

Contains spindle-shaped cells

Changes internal volume of an organ as it contracts

Very slow contraction speed

Anatomy of a microscopic muscle cell o Perimysium

a sheath of connective tissue which groups individual muscle fibers (anywhere between 10 to 100 or more) into bundles or fascicles.

o Epimysium Connective tissue that covers the entire muscle cell

o Endomysium literally meaning within the muscle, is a layer of connective tissue that unsheathes a muscle

fiber and is composed mostly from reticular fibers. It also contains capillaries, nerves and lymphatics.

o Fascicle Bundle of muscle cells

o Fiber Muscle cell

o Myofilament Actin or myosin-containing structure

o Myofibril A long filamentous organelle found within

muscle cells that has a banded appearance, one of many contractile filaments that make up a striated muscle fiber

o Sarcolemma Plasma membrane of the muscle

o Sacromere Basic contracting unit of muscle cell

consists of actin and myosin filaments between z-lines in a muscle cell

o Tendon Cordlike extension of connective tissue

beyond the muscle, serving to attach it to the bone

Muscle functions o Producing movement

Responsible for all locomotion of the body Walking, running, swimming ect.

Allow us to respond quickly to the external environment Allow us to express our emotions Smooth and cardiac muscle work together to circulate blood and maintain blood pressure Smooth muscle of hollow organs force fluids and other substances through internal body canals

o Maintaining posture Functioning continuously, the muscles in our body maintain an erect or seated posture

o Stabilizing jointso Generating heat

Heat is a bi-product of muscular activity Microscopic anatomy of skeletal muscle

o Sarcolemma Plasma membrane of the muscle

o Myofibril Long, ribbonlike organelles that fill the cytoplasm Bundles of myofilaments

o Light (I) bandso Dark (A) bandso Sarcomeres

Tiny contractile units Basic contracting unit of muscle cell consists of actin and myosin filaments between z-lines in a

muscle cell

o Myofilament Actin or myosin-containing structure

o Thick filaments/myosin filaments Made mostly of bundles of the protein myosin and also contain ATPase enzymes Have heads (extensions of cross bridges) Myosin and actin overlap

Skeletal muscle activityo Nerve stimulus and the action potential

Stimulated by nerve impulse to contact

One motor neuron may stimulate a few muscle cells or hundreds of them, depending on the particular muscle or action

Motor unit = one neuron and all the skeletal muscle cells it stimulates

Axon = nerve fiber Axon terminal

Forms junctions (neuromuscular junctions) with the sarcolemma of a different cell

The nerve endings and the muscle cell never toucho The gap between them is called the

synaptic cleft When nerve impulses reach the axon terminals, a chemical (neurotransmitter) is released

Acetylcholine (ACh) = the specific neurotransmitter that stimulates skeletal muscle cellso Diffuses across the synaptic cleft and attaches to the receptors (membrane

proteins) Stimulation of muscle cells

Step 1: release of acetylcholineo Vesicles in the synaptic terminal release their contents into the synaptic cleft

Step 2: Ach binding at the motor end plateo The binding of ACh to the receptors changes the membrane permeability and

induces an action potential in the sarcolemma Step 3: action potential conduction by the sarcolemma

o The action potential spreads across the membrane surface and travels down the transverse tubules, triggering the release of calcium ions at the cistemae. While this occurs, AChE removes the acetylcholine from the synaptic cleft

Contraction of muscle (1) Ach is released, binding to receptors (2) Action potential reaches T tubule (3) Sarcoplasmic reticulum releases Ca+ (4) Active-site exposure, cross-bridge binding (5) Contraction begins (6) Ach removed by AChE (7) Sarcoplasmic reticulum recaptures Ca (8) Active sites covered, no cross-bridge interaction (9) Contraction ends (10) Relaxation occurs, passive return to resting length

Providing energy for muscle contraction – as a muscle contracts, the bonds of ATP molecules are hydrolyzed to release the needed energy. ATP is the only energy source that can be used directly to power muscle activity, and it must be continuouisly be regenerated if contraction is to continue

o Three ways ATP is produced Direct phosphorylation of ATP by creatine phosphate –

Direct phosphorylation o Muscle cells contain creatine phosphate (CP)

CP is a high-energy moleculeo After ATP is depleted, ADP is left

o CP transfers energy to ADP, to regenerate ATPo CP supplies are exhausted in about 20 seconds

Aerobic respiration – Series of metabolic pathways that occur in the mitochondria Glucose is broken down to carbon dioxide and water, releasing energy This is a slower reaction that requires continuous oxygen

Anaerobic glycolysis and lactic acid formation – Reaction that breaks down glucose without oxygen Glucose is broken down to pyruvic acid to produce some ATP Pyruvic acid is converted to lactic acid This reaction is not as efficient, but is fast

o Huge amounts of glucose are neededo Lactic acid produces muscle fatigue

Types of body movements o Flexion o Extensiono Rotationo Abduction o Adduction o Circumduction

o Special movements Dorsiflexion (up) and plantar flexion (down)

up and down movements of your foot inversion (turn sole medially) and eversion (turn sole laterally) supination (“turning backward”, ulna and radius are parallel) and pronation (radius across the

ulna, forming an X) opposition (pinching you thumb and index finger together; opposable thumbs)

interactions of the skeletal muscles in the bodyo muscles can’t push, they can only pull as they contract, most body movements are the result of two or

more muscles acting against each other or togethero prime mover

muscle that has the major responsibility for causing a particular movement o antagonist

muscles that oppose or reverse a movement can also be prime movers in their own right (ex: biceps of the arm is antagonized by the triceps)

o synergists (syn = together, erg = work) help prime movers by producing the same movement or by reducing undesirable movements

o fixators specialized synergists that hold a bone still or stabilize the origin of a prime mover so all the

tension can be used to move the insertion bone naming skeletal muscles

o direction of muscle fibers – some muscles are named in reference to some imaginary line, usually the midline of the body

ex: rectus (straight); fibers run parallel to an imaginary line rectus femoris (rectus describes the direction, femoris describes the location of the

muscle, which is on the femur) o relative size of the muscle

maximus = largest minimus = smallest longus = long magnus = large

o location of the muscle – some muscles are named for the bone which they are associated ex: temporalis and frontalis overlie the temporal and frontal bones of the skull

o number of origins ex: biceps, triceps, quadriceps (bi=2, tri=3, quad=4)

o location of the muscles origin ex : sternocleidomastoid has its origin on the sternum (sterno) and clavicle (cleido)

o shape of the muscle ex: deltoid = triangular

o action of the muscle extensor, flexor, and adductor

Anterior muscles

Posterior muscles