Skeletal Muscle Physiology I Flashcards
What are the physiological functions of skeletal muscle?
Maintenance of posture, purposeful movement in relation to external environment, respiratory movements, heat production, contribution to whole body metabolism
What is the largest type of tissue in the body?
Muscle
What are the three kinds of muscle in the body?
Skeletal, cardiac, smooth
What can all three types of muscle do?
Capable of developing tension and producing movement through contraction
Are all muscle types striated?
No - skeletal and cardiac muscle are striated, smooth muscle is unstriated
What does striation look like under a light microscope?
Alternating dark (thick myosin) and light (thin actin) bands
What are skeletal muscles innervated by?
Somatic nervous system (subject to voluntary control)
What are cardiac and smooth muscle innervated by?
Autonomic nervous system (subject to involuntary control)
What are some features of skeletal muscle?
Neurogenic initiation of contraction, motor units, neuromuscular junction present, no gap junctions
How does excitation contraction coupling work in skeletal muscle?
Ca++ entirely from sarcoplasmic reticulum
How is gradation of contraction controlled in skeletal muscle?
By motor units and summation of contractions
What are some features of cardiac muscle?
Myogenic (pacemaker potential) initiation of contraction, no neuromuscular junction, gap junctions present
How does excitation contraction coupling work in cardiac muscle?
Ca++ from ECF and sarcoplasmic reticulum (ca++ induced Ca++ release)
How is gradation of contraction controlled in cardiac muscle?
Depends on the extent of heart filling with blood (preload)
Is there continuity of the cytoplasm between nerve and skeletal muscle cells?
No
What is the transmitter of neurogenic initiation of skeletal muscle contraction?
Acetylcholine at neuromuscular junction (stimulates alpha motor neuron)
How is skeletal muscle organised?
Organised into motor units = motor unit is a single alpha motor neuron and all the skeletal muscle fibre it innervates
What determines the number of muscle fibres per motor unit?
The functions served by the muscle = muscles which serve fine movements have fewer fibres per motor unit (about 10 per unit)
What does skeletal muscle consist of?
Parallel muscle fibres (skeletal muscle cells) bundled by connective tissue
How much of the muscle do skeletal muscle fibres usually cover?
The entire length of the muscle
What normally attaches the skeletal muscle to the skeleton?
Tendons
What makes up the lever system that allows the body to move?
Bones, muscles and joints
What does each muscle fibre contain?
Many myofibrils = specialised contractile intercellular structures, predominant structure of skeletal muscle
What is within each myofibril?
Actin and myosin, arranged into sarcomeres
What is the functional unit of an organ?
The smallest component capable of performing all the functions of that organ
What is the functional unit of skeletal muscle?
Sarcomeres
Where are sarcomeres found in the skeletal muscle?
Between two lines, connect the thin filaments of two adjoining sarcomeres
What are the four zones of sarcomeres?
A-band, H-zone, M-line, I-band
What is the A-band of sarcomeres?
Made up of thick filaments along portions of thin filaments that overlap in both ends of thick filaments
What is the H-zone of the sarcomere?
Lighter area within the A-band where thin filaments don’t reach
What is the M-line of the sarcomere?
Extends vertically down the middle of A-band within the centre of H-zone
What is the I-band of the sarcomere?
Consists of remaining portion of thin filaments that do not project in A-band
How is muscle tension produced?
By sliding of actin filaments on myosin filaments
What does force generation depend on?
ATP-dependent interaction between myosin and actin filaments
What is ATP required for in force generation?
For both contraction and relaxation, and to pump Ca++ back into the sarcoplasmic reticulum during relaxation
What is Ca++ required for in force generation?
Switching on cross bridge formation
What is excitation contraction coupling?
Process whereby the surface action potential results in activation of the contractile structures of the muscle fibre
When is Ca++ released from the lateral sacs of the sarcoplasmic reticulum?
When the surface action potential spreads down the transverse (T) tubules
What are T-tubules?
Extensions of the surface membrane that dip into the muscle fibre
What happens when the muscle is relaxed?
No cross bridge binding because the bridge binding site on actin is physically covered by the troponin-tropomyosin complex
What happens when the muscle is excited?
Ca++ binds with troponin, pulling troponin-tropomyosin complex aside to expose cross bridge binding site, cross bridge binding occurs
What does the binding of the actin and myosin cross bridge cause?
A power stroke that pulls actin inward during excitation
What is the first step of the excitation contraction coupling process?
Acetylcholine released by axon of motor neurons crosses cleft and binds to receptors/channels on motor end plate
What is the second step in the excitation contraction coupling process?
Action potential generated in response to binding of acetylcholine and subsequent end-plate potential is propagated across surface membrane and down T-tubules of muscle cells
What is the third step of excitation contraction coupling?
Action potential in T-tubules triggers Ca++ release from sarcoplasmic reticulum
What is the fourth step in the excitation contraction coupling process?
Calcium ions released from lateral sacs bind to troponin on actin filaments, leads to tropomyosin being physically moved aside to uncover cross bridge binding sites on actin
What is the fifth step of excitation contraction coupling?
Myosin cross bridges attach to actin and bend, pulling actin filaments towards centre of sarcomere, powered by energy provided by ATP
What is the sixth step in the excitation contraction coupling process?
Ca++ actively taken up by sarcoplasmic reticulum when there is no longer local action potential
What is the final step of excitation contraction coupling?
With Ca++ no longer bound to troponin, tropomyosin slips back to its blocking position over binding sites on actin, contraction ends, actin passively slides back to original resting postition
