University:
Kentucky State University
Course:
BIO 301 | Human Anatomy
Academic year:

2019
Views:

35

Pages:

29
Author:

Adowj7b

Physiology of Skeletal muscles L14, L15, L16, L17 Connective tissue layers • They allow voluntary movements • They allow functions, such as swallowing, urination, and defecation, to be under voluntary control • They protect internal organs • They maintain homeostasis in the body by generating heat • Each skeletal muscle has 3 layers of connective tissue (provide structure to the muscle and compartmentalize the muscle fibers within the muscle) • Each muscle is wrapped in a sheath of dense, irregular connective tissue called the epimysium • They are supplied by somatic motor neuron(from CNS), which signals the fiber to contract Sheet Note 1 >> skeletal muscles functions: allow voluntary movements like swallowing ,walking and talking , ECT >> has 3 layers of connective tissue : Epimysium covers skeletal muscle Perimysium covers muscle fascicle Endomysium covers muscle fiber (muscle cell) >> Muscle fascicle : group of muscle fibers (muscle cells ) >> muscle fiber : group of muscle fibrils(the smallest) which made of actin and myosin myofibrils that surrounded by sarcolemma. >> Sarcolemma : acts like plasma membrane for other cells , but its special for muscle cells (muscle fibers ). 1. 2. 3. 4. 5. • • Excitability: respond to chemical released from nerve Conductivity: ability to propagate electrical signals over Contractility: ability to shorten and generate force Extensibility: ability to be stretched without damaging the tissue Elasticity: ability to return to original shape after being stretched Motor Unit: The basic functional units of skeletal muscle, it includes the motoneuron and all of its associated muscle fibers. Its activity represents the final output of the central nervous system Each individual muscle fiber is innervated by only one motor neuron (single nerve cell) or motoneuron, but one motor neuron may innervate several muscle fibers Sheet Note 2 Motor units in small muscles Motor units in large muscles • Permit very fine motor control of the muscle • Have low number of muscle fibers • React to stimulation rapidly • Have low threshold that allow them to fire first • Generate the smallest force • Allow functions not requiring fine motor control in muscles with gross vigorous movements • Have a larger number of muscle fibers • Have the highest thresholds that allow them to fire last • Generate the largest force E.g. 1) laryngeal muscle, in which a motor unit has approximately 2-3 muscle fibers per motor neuron 2) The small motor units of the extraocular eye muscles that move the eyeballs • E.g. 1) Gastrocnemius muscle; in which a motor unit contains approximately 500 muscle fibers per motor neuron Sheet Note 3 >> the lower the threshold the faster the excitation happens , that’s why motor units in small muscles give faster response . The small motor units of the extra ocular eye muscles that move the eyeballs and contain less than 10 muscle fibers . >> in motor units of large muscles needs gross vigorous movement >> the greater the force utilized in skeletal muscle ,the greater excitation it needs Gastrocnemius muscle; in which a motor unit contains approximately 500 muscle fibers per motor neuron. It is a highly specialized chemical synapse between a motor neuron nerve terminal and its muscle fiber that are responsible for converting electrical impulses generated by the motor neuron into electrical activity in the muscle fibers. It is structure is composed of: 1. Presynaptic membrane of the neural cell - Represent the axon terminal of motor neurons - It contains Na+, K+ and Ca2+ voltage gated ion channels. - It has vesicles that contains the neurotransmitter - The secreted neurotransmitter (NT) is always acetylcholine (ACh) Sheet Note 4 >> we studied the synapses in general which can happen in any cell in the body , but now we’re gonna study it specifically in skeletal muscles with some specific details in skeletal muscles (SM) . >> motor end plate : postsynaptic membrane of the muscle cell (fiber). >> As we know neurotransmitters (specially acetylcholine) are stored in the terminals of neuron into vesicles when the action potential arrives the terminals of neuron Ca+2 voltage gated channels open then Ca+2 enters presynaptic terminals to bind with sensor proteins (synaptotagmin proteins ) making Ca+2 synaptotagmin complex this complex then binds to NT vesicles docking them to the presynaptic cell membrane ACH will be released then in the Neuromuscular junction then ACH will bind to ACH receptors that found on motor end plate of the postsynaptic cell membrane . 2. Postsynaptic membrane of the muscle cell - It is the part of sarcolemma that lies directly under the terminal button - It has receptors for the NT - Synaptic gutter(synaptic trough): The invaginated muscle membrane - Synaptic cleft: The space between the terminal and the fiber membrane (20-30 nm) - Sub-neural clefts: smaller folds of the muscle membrane that increase the surface area at which the synaptic transmitter can act Sheet Note 5 >> once Ach binds to it’s receptors on the end plate the cell membrane(end plate ) then will make invaginations called (synaptic gutter or synaptic trough) their function is to increase the surface area which consequently increases the response process of NT. 1. At the presynaptic neural end: - New vesicles that has ACh are formed: - They are synthesized by the Golgi apparatus at the neural cell body - then transported by axoplasm along the axon to the neuromuscular junction - About 300,000 vesicles collect in the nerve terminals of a single skeletal muscle end plate. - ACh is synthesized in the nerve fiber terminal then transported into the vesicles where it stored (about 10,000 Ach molecules in each vesicle) Sheet Note 6 >> ACH are synthesized by the Golgi apparatus at the neural cell body then it will be stored in the terminal of neuron into vesicles fixed in cytoskeleton of the presynaptic terminal by Synapsin proteins (so the vesicles of ACH are not free in the terminals until an action potential comes ). - When the AP arrives to the neural terminals - voltage- gated Ca2+-channels open promoting a calcium influx to the nerve terminal. - Ca2+ binds to calmodulin protein forming complex - Ca2+-calmodulin complex activate protein kinase - protein kinase phosphorylates Synapsin proteins that anchor the acetylcholine vesicles to the cytoskeleton of the presynaptic terminal - Phosphorylated synapsin protein detach the ACh vesicles from the cytoskeleton - The detached ACh vesicles fuse to the membrane and the ACh is released into the synaptic cleft - About 125 vesicles usually rupture with each AP AP :action potential Sheet Note 7 >> As we said vesicles of Ach are fixed in the terminals by Synapsin proteins , so once the synapsin proteins are phosphorylated by protein kinase they will detach Ach vesicles Then they will be able to be docked , fusion ….etc. the process mentioned in sheet #4. >> About 125 vesicles usually rupture with each AP - After the ACh action is done, it split by Acetylcholinesterase into acetate and choline - the Choline is reabsorbed actively into the neural terminal to be reused to form new Ach - The synaptic vesicle membrane is recycled to form new ACh vesicles …. Why? …. How? A. Clathrin protein coats the remnants of the vesicular membrane B. Dynamine protein pinches off the coated vesicular membrane C. The coated vesiclular membrane is now carried into the cytoplasm of presynaptic terminal D. ATPase removes the coating from the vesicle (uncoating of clathrin) E. Synapsin protein binds to the vesicle and attaches it into the actin filaments in the cytoskeleton (where they refilled with Ach and get ready for the next AP) AP :action potential Sheet Note 8 >> not only ACH will be recycled also its vesicle will be recycled too >> and that’s to shorten the time and make the process neurotransmitting faster . >> vesicles of ACH won’t be completely fused in the membrane and that’s why it has the ability to be recycled . >> vesicle will be uncoated by ATPase enzyme after the endocytosis cause it wont need clathrin anymore . >> after anchoring the vesicle in the cytoskeleton it will be empty and ready to receive new ACH . - - In skeletal muscles, ACh binds to the Nicotinic ACh receptors (Ligand gated channels) Two ACh must bind to open the channel When open Na diffuses in and K diffuses out Sheet Note 9 >> When the channel is opened Na diffuses in and K diffuses out Na diffuses in large amount so this increase the membrane potential so we can call it excitatory process. • It is an integral membrane protein • Formed tubular channels molecular weight of approximately 275,000 Dalton • Receptor life span is 10 days • Synthesized constantly • Get internalized by endocytosis and the degraded by lysosome activity • In fetus: it is composed of five subunit proteins, two alpha proteins and one each of beta, delta, and an epsilon proteins. • In adult: an epsilon protein substitutes for the gamma protein in this receptor complex. Sheet Note 10 >> the two Ach will bind exclusively with two alpha α subunits on the receptor , once they bind the channel will open . - - Binding of two Ach will active the receptor causing Na influx results in a local depolarization of the motor endplate (the endplate potential) The electrotonic spread of the endplate potential to the prejunctional zone activates Na and K voltage-gated channels If the threshold of excitability of the muscle fiber is achieved, an action potential is triggered Transmission of action potential along the muscle fiber by Ttubules Starting the process of muscle contraction The muscles resting membrane potential is about -80 to -90 mV skeletal muscle cell resting membrane potential is due to Cl conductance. Sheet Note 11 >> resting membrane potential for muscle cells is less than resting membrane potential for neural cells ,and that’s due to Cl conductance (the membrane is permeable to cl) so it decreases the membrane potential of the cell . – T tubules • • • • • located at the junctions of A bands and I bands Small and run transverse to the myofibrils Begin at the cell membrane and penetrate all the way from one side of the muscle fiber to the opposite side Branch among themselves and form entire planes of T tubules interlacing among all the separate myofibrils Originate from the cell membrane when an AP spreads over a muscle fiber membrane, a potential change also spreads along the T tubules to the deep interior of the muscle fiber. The electrical currents surrounding these T tubules then elicit the muscle contraction. Sheet Note 12 >> T-tubule is abbreviation for transverse tubules >> their function : they allow the action potential to spread along the muscle fiber from one t-tubule to another . A band &I band The contraction unit (myofibril) length or area is from Z line to the next Z line – Sarcoplasmic Reticulum (SR) It is the internal tubular structure that is the site of Ca2+ storage and release for excitation-contraction coupling. It is composed of two major parts: 1. Terminal cisternae: Large chambers that abut the T tubules One T-tubule and two terminal cisternae is called Triad 2. L tubule: longitudinal tubules that surround all surfaces of the actual contracting myofibrils. Sheet Note 13 >> Triad : 2 terminal cisternae and one T-tubule between them • Skeletal muscles are composed of many individual cells known as muscle fibers that are composed of a number of smaller myofibrils • Each muscle fibers is: 1) 1 to 4 cm long 2) Has cylindrical shape (10-100 micron in diameter) 3) It is multinucleated cell. • Muscle fiber cell membrane is called (sarcolemma or myolemma) and cytoplasm is called sarcoplasm. In the sarcoplasm embedded the nuclei, Golgi apparatus, mitochondria, sarcoplasmic reticulum, ribosomes, and glycogen and occasional lipid droplets. • The sarcoplasm contains number of myofibrils which form the main contains of muscle fiber.

Physiology of Skeletal Muscles

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