The muscular system

The muscular system

The human body has >600 distinct skeletal muscles

The face includes 60 muscles in which >40 are used to frown and 20 to smile

Functions of muscle tissue

Movement-Walking, running,talking

Stabilising body positions-Standing, sitting, keeping head up

Regulating organ volumes-Sustained contraction of sphinceters to prevent an outflow from hollow organs

Movement of substances-Contraction/relaxation of muscles in blood vessels,gastrointestinal tract, reproductive system,cardiac muscles for blood flow

Heat production-Contraction of muscle tissues,involtunatry contractions of skeletal muscle (shivering)

Properties of mucle tissue

• Excitability – responds to chemicals released from nerve cells​

• Conductivity – ability to propagate electrical signals over membrane​

• Contractibility – ability to contract and generate force​

• Extensibility – ability to be stretched without damaging the tissue​

• Elasticity – ability to return to original shape after being stretched

Types of muscle tissue

Skeletal muscle tissue – attached to bones of skeleton​

Smooth muscle tissue – forms the walls of hollow internal structures​

Cardiac muscle tissue – forms the wall of the heart

3 types of muscle tissue

Skeletal muscle tissue

Cardiac muscle tissue

Smooth muscle tissue

Skeletal muscle

Attaches to bonds, skin or fascia

Conscious control of muscles (voluntary tissue)

Cells are long and threadlike with alternate light and dark cross-markings called striations​

Cells are multinucleated​

Skeletal muscles move the head, trunk and limbs

Smooth muscle

They are shorter than skeletal muscle cells and are spindle shaped​

Cells do not have striations (unlike skeletal muscle cells) = smooth​

Cells are mononucleated​

Located in the walls of hollow internal organs (stomach, intestines, urinary bladder, uterus, blood vessels)​

Involuntary ​muscle movements

Cardiac muscle

• Found in the heart & makes up the bulk of the heart​

• Cells are striated and branched, joining end to end and form complex networks​

• Cells are mononucleated​

• Involuntary

What is the skeletal muscle composed of

Composed of a variety of tissues including layers of connective tissue

Fascia (dense connective tissue) covers the surface of the muscle

The epimysium lies beneath the fascia

Perimysium extends into the structure of the muscles to group muscle cells into fascicles (bundles of skeletal muscle fibres)

Endomysium separates individual muscle fibres within fascicles

Skeletal muscles at a microscopic level

• Beneath its cell membrane (sarcolemma) is the ​

cytoplasm (sarcoplasm), which has many small, oval ​

nuclei and mitochondria​

• The sarcoplasm also contains many threadlike ​

myofibrils (essential in muscle contraction)​

• Myofibrils consist of 2 types of protein filaments:​

• Thick filaments composed of myosin​

• Thin filaments composed of mainly actin ​

(others: troponin & tropomyosin)​

• Myofibrils consist of repeating units called sarcomeres

Sarcomere at a microscopic level

• A sarcomere is the functional unit of muscle contraction​

• The striations of skeletal muscle result from a repeating pattern of units (sarcomeres)​

• The myofibrils are essentially sarcomeres joined end to end​

• I bands:​

• Composed of thin filaments attached to structures called Z lines​

• A bands:​

• Have a central region (H zone) – only thick filaments​

• Two regions on either side of H zone are where the thick and thin filaments overlap​

• The M line consists of proteins which help hold thick filaments in place

Role of actin and myosin

The actin and myosin slide into each other for muscle contraction

Neuromuscular junction

• Neurons (nerve cells) are important for communication within the body through a conduction of electrical impulses.​

• A skeletal muscle fibre usually contracts only when simulated by a motor neuron (Neurons that control effectors).​

• Each skeletal muscle fibre is connected (through a synapse) to the axon of a motor neuron that passes outward from the brain or the spinal cord.​

• A neuromuscular junction is the synapse between a motor neuron and the muscle fibre that it controls.

Structures of NMJ region

Synaptic and bulbs are swellings of axon terminals

The end bulbs contain synaptic vesicles filled with acetyl chloride (ACh)

Motor and plate membrane contains 30 million ACh receptors

Interaction of skeletal muscles

They function in groups and arrange in opposing pairs at joints with apposing movements (Flexion and extension)

Isometric contraction

A muscle contraction without motion. Isometric contractions are used to stabilize a joint like when a weight is held at waist level neither raising and lowering it

Isotonic contraction

When a muscle shortens to overcome resistance. When a muscle shortens, at least one joint moves, and body movement occurs. The resistance comes from lifting a weight, pulling up your body.

Dynamic contractions

Muscle contractions with a fixed amount of weight.

What is skeletal muscle fibre?

A single cell that contracts in response to stimulation and then relaxes when the stimulation ends​

Muscle movement

When the upper limb straightens at the elbow

Rigid bar= Forearm bones

Fulcrum= Elbow joint

Object moved against resistance= Hand moved against resistance provided by the weight

Force=Supplied by the posterior muscles of thr arm (triceps brachii)

When the weight is lowered (rope pull down) the biceps relax and the triceps contract

• Agonist (prime mover) – generates the majority of the force to cause the desired action​

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Antagonist

muscle that brings about the opposite reaction​

Synergist

aids the prime mover during the desired reaction or inhibits the opposing action​