Contraction and Relaxation

Stretching

• passive

• no brain involvement

pandiculation

• subject is active

• brain is involved

• increases sensation and awareness of muscles

what muscles compose the muscular system?

• smooth muscle

• cardiac muscle

• skeletal muscle

what is made of smooth muscle in the body?

• walls of visceral organs

• walls of blood vessels

• walls of respiratory passageways

smooth muscle identification

• cells are spindle shaped

• cells have a centrally located nucleus

• no visible striations (filaments appear smooth)

smooth muscle function

regulates the diameter of blood vessels and produces contractions of visceral organs (e.g., peristalsis)

how is smooth muscle regulated/activated?

regulated by the autonomic nervous system. may also contract in response to hormonal stimulation, stretching, or other stimuli

smooth muscle receptors

ACh and/or norepinephrine receptors on the membrane lead to excitation or inhibition 

what is made of cardiac muscle in the body

the wall of the heart

cardiac muscle identification

• striated

• one nucleus per cell

• dark lines between the cells (intercalated disks)

cardiac muscle function

involuntary, rhythmic contraction to pump blood through the body

intercalated disks

membranes between cardiac muscle cells that allow electrical impulses to travel rapidly through them to better coordinate contractions

pacemaker cells

initiate electrical impulses to set the heart rate and rhythm

contractile cells

contract in response to electrical signals to pump blood

where is skeletal muscle found in the body?

• attached to bones

• abdominal wall muscles

• facial expression muscles

skeletal muscle primary function

produce movement at the joints

skeletal muscle identification

• heavily striated

• cells are very long and cylindrical

• multiple nuclei per cell

• cells can contract as a large unit when stimulated

how is skeletal muscle activated?

stimulated by the nervous system to contract voluntarily

muscle contraction overview

• motor neuron sends an action potential to neuromuscular junction between nerve and muscle

• neuron releases ACh, which binds to receptors and triggers an action potential in muscle fiber and T-tubules

• sarcoplasmic reticulum releases Ca2+ ions into muscle fiber cytoplasm

• calcium binds to troponin protein causing tropomyosin protein to move and expose myosin-binding sites on actin (thin filament). myosin heads (thick filament) attach to actin, forming cross-bridges.

• myosin heads pivot, pulling actin filaments toward the center of the sarcomere. this shortens the sarcomere, generating force and causing contraction.

megaesophagus

dilation of the esophagus; prevents food from adequately traveling from the mouth to stomach. more common in dogs than in cats.

aspiration pneumonia

food or water is regurgitated, then inhaled, which can lead to fluid/bacteria getting into the lungs

myopathies

diseases that primarily cause damage to muscles or muscle tissues

myositis

inflammatory reaction in muscle, commonly caused by infections, parasitic diseases, etc.

myositis symptoms

• muscle tremors

• pelvic limb stiffness

• progressive weakness

• muscle atrophy

what breed of dog is more at risk for myositis?

dutch shepherd

masticatory muscle myositis

inflammatory condition that affects the muscles used to chew, preventing patient from opening their mouth or eating