Muscular System link

Muscle types

Skeletal Muscle Organ of the muscular system Covered by connective tissue called fascia Separated into epi-, peri-, and endomysium (figure 10.1)

Skeletal Muscle Fibers (cells) Thin, elongated cylinder with rounded ends Cell membrane called sarcolemma Many nuclei and mitochondria

Anatomy of a muscle “Bundles” Muscle Fascicles Fibers (cells) Myofibrils Myofilaments

Movement Functions sphincter Storing and moving substances Stabilizing joints Functions sphincter Storing and moving substances Stabilizing posture Thermogenesis

Excitability Extensibility Properties Elasticity Contractility

Myofibrils play fundamental role in contraction Myosin– thick (A bands with H zone) Actin– thin (I bands with Z lines)

Z Line to Z Line called Sarcomere

Connected to motor neuron at neuromuscular junction Stimulated by neurotransmitters Entire unit called motor unit One neuron, many muscle cells

Must be stimulated by the neurotransmitter acetylcholine (from motor neuron)

Skeletal Muscle Contraction Sliding filament theory (figure 10.11) cross bridges formed between actin and myosin filaments when Ca2+ present Using energy from ATP cross-bridges bend then break then form another bridge further down the actin filament animation

Detailed animation

Muscle impulse creates increased permeability to calcium ions Reversed when nerve impulses cease and muscle relaxes (acetylcholine broken down)

Steps of muscle contraction Electric signal sent from brain down motor neuron to neuromuscular junction 2. Neurotransmitter signal sent from neuron received by sarcolemma of muscle cell 3. Calcium ion movement into muscle cell 4. Myosin forms cross bridges with actin 5. ATP allows cross bridges to bend, brake, and reform (power stroke) 6. Filaments continue to slide past each other until actin overlaps (sarcomere shortens)

Enzyme ATPase catalyzes breakdown of ATP– regenerated by creatine phosphate

Oxygen needed to effectively contract When muscles strenuously exercised anaerobic respiration occurs and muscles develop oxygen debt Lactic acid produces May lead to muscle fatigue or inability to contract

Heat released as only 25% of energy used

Muscle Responses Threshold stimulus- minimal stimuli needed to cause contraction All-or-none response Can be recorded on myogram (figure 10.15)

Series of stimuli causes summation (sustained contraction) All fibers in motor unit stimulated at the same time Entire muscle contracts as motor units recruit others link

Skeletal Muscle Actions (main movements) Origin- immovable end Insertion- movable end Pulled toward origin with contraction Muscles can have more than one of either Ex. Biceps brachii Belly

Muscles almost always function in groups Prime mover- provides most movement (contracts) Synergist- assists prime mover Antagonist- relaxes (opposite movement)

Muscle Names Often describe them Size- pectoralis major Shape- deltoid Location- sternocleidomastoid Action- extensor digitorum Number of attachments- triceps brachii Direction of fibers- external obliques

Size terms Major Vastus Maximus Medius

Location Frontalis Temporalis Tibialis anterior Biceps femoris

Action Adductor Extensor digitorum Adductor Extensor digitorum

Number of attachments Biceps brachii Also give location Biceps femoris Triceps brachii Also give location

Direction of fibers External oblique Rectus abdominus

Arrangement of fascicles table 11.1

Parallel muscle Sternocleidomastoid

Parallel muscle Sartorius

Fusiform Biceps Brachii                                                  

Fusiform Tensor Fasciae Lata

Fusiform Semitendinosus

Triangular (convergent) muscle Pectoralis Major

Bipennate muscle Rectus Femoris

Unipennate Extensor Digitorum Longus

Multipennate Deltoids

Circular Muscles Orbicularis oculi Orbicularis oris

link