this chapter covered the basics of muscle contraction, the neuromuscular junction, and the prioprioceptors involved in reflex loops. a review of the skeletal muscle structural hierarchy: skeletal muscle is surrounded by epimysium connective tissue and contains bundles of fascicles, which are surrounded by perimysium and contain muscle fibers, which are surrounded by endomysium and contain myofibrils, which are surrounded by sarcoplasmic reticulum and are composed of the basic contractile unit of muscles, the sarcomeres, which are made up of thin and thick filaments.
the neuromuscular junction is the site at which the motor neuron synapses with the muscle, at the motor plate. muscle contraction occurs when an action potential propagates down to the end of the axon, which causes release of acetylcholine, which migrates to the motor plate and binds to receptor channels, causing a membrane potential which propagates down the t tubules of the myofibrils, causing release of calcium from the terminal cisternae of the sarcoplasmic reticulum, which initiates the contraction of the myofilaments. relaxation occurs which calcium is sequested back into the sarcoplasmic reticulum and acetylcholine esterase breaks down the acetylcholine that was released into the NMJ.
muscle contraction can be classified in several different manners depending on frequency, intensity, and relaxation between contractions. a twitch is a single contraction and a full relaxation. within a twitch there are three stages: the latent, contraction, and relaxation phase. "treppe" is repeated contractions of increasing intensity with a return to full relaxation in between. "wave summation" is similar to treppe but without the full relaxation in between. "tetanus" is a higher frequency, maximal intensity contraction with barely any or no relaxation in between contractions.
there are several different types of axons which provide motor and sensory innervation to muscles. A-alpha, thickest and fastest conducting, seen in motor efferents and muscle spindle receptor afferents. A-beta, seen in touch and pressure mechanoreceptor afferents. A-gamma, seen in muscle spindle efferents. A-delta, seen in skin temperature and pain afferents. B fibers, seen in sympathetic preganglionics, and C fibers (unmyelinated), seen in sympathetic postganglionics.
there are three main types of proprioceptors: muscle spindles receptors, golgi tendon organs, and joint kinesthetic organs. muscle spindles are located within the muscle belly (called "extrafusal fibers") and are composed of intrafusal fibers, either "bags" or "chains", which are wrapped with a sensory nerve ending. nuclear bag fibers have contractile ends that are attached to the extrafusal fibers, nuclei in the center of the fiber, and detect fast changes in muscles length-- whereas nuclear chain fibers are attached to the nuclear bag fibers themselves and detect slow, steady chains. primary "Ia" sensory endings are wrapped around the center of spindle receptors and respond to change in muscle length; secondary "flower spray" sensory endings are slower conducting and respond to the overall length of the fiber.
spindle receptors are involved in both rapid adjustments to maintain balance as well as slower adjustments to maintain posture. the reflex they are involved in is called the "quick stretch reflex"-- extrafusal muscle causes stretching of the muscle spindle, which activates the primary or secondary nerve, which activates an interneuron which activates the motor innervation of the muscle itself-- in essence providing a balancing / feedback system for quickly contracting muscles. spindle receptors also mediate the reciprocal inhibition reflex pathway, which inhibit the antagonist muscles.
golgi tendon organ receptors are embedded within the tendon of the muscles and are involved in a complementary reflex loop- at the extremes of contraction, when tension is high in the tendon of the muscle, golgi tendon organs stimulate 1b sensory nerves, which activate the inhibitory interneurons for the muscle, causing it to relax (while simultaneously activating the antagonist muscles).
the third type of proprioceptor is the joint kinesthetic receptor, which are located within joints and detect joint movement, tension, and angle. pacinian corpuscles are located within connective tissue and sense rapid pressure changes, stretch, and acceleration of the joint motion. ruffini corpuscles are located in synovial capsules and ligaments and sense deep/rapid sustained pressure, lateral stretch, and joint angle changes. free nerve endings are found throughout and sense both rapid and sustained pressure.
questions
what are these functional units surrounded by and what do they contain?
1. skeletal muscle
2. muscle fascicle
3. muscle fiber
4. myofibril
5. sarcomere
NMJ...
6. what is a neuromuscular junction?
7. what are "motor points" and where are they generally?
8. what is the neurotransmitter that is released from axons at the NMJ?
9. what is involved in the relaxation of a muscle in the NMJ?
10. describe the process of contraction starting from release of neurotransmitter from the axon at the NMJ.
11. describe the process of muscle relaxation.
contraction types...
12. what is a "motor unit"?
13. what are examples of small and large motor units?
14. what is a muscle "twitch"?
15. what is a "treppe"?
16. what is a "wave summation"?
17. what is "tetanus"?
18. what is the difference between incomplete and complete tetanus?
19. what are the three phases to the muscle twitch?
axonal classifications...
20. what are three types of neurons?
21. describe the A-alpha classification of nerve fiber.
22. ...A-beta.
23. ...A-gamma.
24. ...A-delta.
25. ...B fibers.
26. ...C fibers.
27. what are the 5 components to a spinal cord reflex arc?
spindles...
28. what are the three types of proprioceptors?
29. where are muscle spindle receptors located? what do they measure?
30. what are intrafusal and extrafusal fibers?
31. what are the two types of intrafusal fibers and what is the difference between them?
32. what are the two types of sensory neurons from muscle spindle receptors and what is the difference between them?
33. what are the two types of responses to stretch that muscle spindle receptors are involved in?
34. describe the role of gamma motor neurons in maintaining balance.
golgi tendon organs...
35. what are golgi tendon organs?
36. what do golgi tendon organs detect?
37. describe how a golgi tendon organ can protect a muscle from damage.
38. describe the two types of stretch responses that a golgi tendon organ can be involved in.
joint kinesthetic receptors...
39. what are the three types of joint kinesthetic receptors?
40. what do joint kinesthetic receptors sense?
41. describe the different locations and functions of the different types of joint kinesthetic receptors.
proprioceptors affect on reflex muscle tone...
42. what are the three types of reflexes that proprioceptors mediate?
43. describe the quick stretch reflex.
44. describe the reciprocal inhibition reflex.
answers
1. surrounded by epimysium, contains muscle fascicles.
2. surrounded by perimysium, contains muscle fibers.
3. surrounded by endomysium, contains myofibrils.
4. surrounded by sarcoplasmic reticulum, consists of sarcomeres.
5. contains thick and thin filaments.
6. where a motor nerve synapses with a muscle fiber.
7. the actual point at which the nerve synapses- generally in the center of the muscle belly.
8. AcH.
9. breakdown of AcH by AcHesterase.
10. acetylcholine released from axon into NMJ, which bind to receptors on motor end plate and creates action potential, which propogates through the t tubules, causing calcium to be released from sarcoplasmic reticulum, which initiates the contraction of the myofilaments by binding to troponin and exposing binding sites on tropomyosin.
11. calcium is sequestered into the sarcoplasmic reticulum, acetylcholine broken down by esterase in the NMJ.
12. a nerve and all the muscle fibers it innervates.
13. extraocular muscles (20 fibers) vs. gastrocnemius (1,000 fibers)
14. a single brief stimulus that produces a short contraction and relaxation.
15. repeated stimuli in which the intensity of the contraction increases but returns back to full relaxation in between.
16. repeated stimuli which increase in intensity and do not return to full relaxation.
17. higher frequency stimuli which maintains nearly maximum intensity of contraction.
18. incomplete has partial relaxation.
19. latent, contraction, relaxation.
20. sensory, motor, interneuron.
21. fastest conducting (100ms), largest diameter, seen in motor efferents and muscle spindle afferents.
22. (50ms), touch and pressure mechanoreceptor axons.
23. (20ms), muscle spindle efferents.
24. (15ms), skin temperature and pain.
25. slower myelinated fibers seen in sympathetic preganglionics.
26. unmyelinated fibers seen in sympathetic postganglionics.
27. sensory receptor, sensory neuron, interneuron, motor neuron, effector organ.
28. muscle spindle receptors, golgi tendon organs, joint kinesthetic receptors.
29. located in between muscle fibers, measure length of muscle and speed of contraction.
30. the muscle spindle receptors have intrafusal fibers which are embedded within the extrafusal muscle fibers of a muscle.
31. nuclear bag fibers and nuclear chain fibers. bag fibers have nuclei in the center, contractile elements attached to extrafusal fibers (which receive motor efferents from A-gamma neurons), detect fast changes in muscle length. chain fibers have nuclei spread throughout, contractile elements attached to the bag fibers, and detect slow and steady changes in muscle length.
32. primary (Ia) sensory neurons are wrapped around the center of muscle spindle receptors, are faster conducting, and respond to change in muscle length. secondary "flower spray" sensory endings are smaller diameter and slower conducting and respond to the overall length of fiber.
33. monosynaptic spinal reflexes (rapid adjustments to prevent falls) and cerebellum controlled stretch response (to regulate muscle tone.
34. firing of gamma motor neurons causes shortening of the contractile elements within the spindle fibers, increasing their sensitivity to stretch; useful when trying to maintain balance because quick and precise feedback is needed.
35. free nerve endings located at junction between tendon and muscle.
36. force of muscle contraction / tension exerted at the tendon.
37. it can detect excess force applied to a muscle, which fires the 1b sensory fiber, which activates the inhibitory interneuron, which has an inhibitory effect on the motor neuron that innervates the muscle, causing it to relax and reduce tension.
38. dynamic response, an involuntary relaxation due to a sudden increase in muscle tension, and static response; an ongoing relaxation in response to a gradual increase in muscle tension.
39. pacinian corpuscles, ruffini corpuscles, free nerve endings.
40. movement, tension, acceleration, strain, postural changes in joint.
41. pacinian corpuscles are located within connective tissue and sense rapid pressure changes, stretch, acceleration/deceleration. ruffini corpuscles are located in synovial capsules and ligaments and sense deep rapid/sustained pressure, lateral stretch, and joint angle changes. free nerve endings are ubiquitous and sense rapid and sustained pressure.
42. quick stretch reflex, reciprocal inhibition, autogenic inhibition.
43. mediated by muscle spindle receptors, produces a quick contraction of muscle fiber in response to stretching.
44. also mediated by muscle spindle receptors, producing an inhibition in the antagonist muscles in response to stretching.
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