BT_PO 1.98a - Skeletal Muscle Phys Flashcards

Question

What are the main physiological processes in muscle that require energy?

Answer 1

to drive the Na+/K+ ATPase pump, which maintains the ionic gradients across the sarcolemma (muscle cell membrane) to requester Ca++ into the sarcoplasmic reticulum (cisternae) to cause muscle contraction.

Answer 2

ATP is the only energy source that contractile proteins can use, but muscles only store enough for eight twitches (1-2 secs max)

Answer 3

First source of energy to reconstitute ATP is phosphocreatine phosphocreatine is instantly cleaved, and its released energy causes bonding of a new phosphate ion to ADP to reconstitute the ATP. the total amount of phosphocreatine in the muscle fiber is also small, only about 5 times as great as the ATP. Therefore, the combined energy of both the stored ATP and the phosphocreatine in the muscle is capable of causing maximal muscle contraction for only 5 to 8 seconds. Second source for energy is glycolysis The breakdown of glycogen previously stored in muscle cells Rapid enzymatic breakdown of the glycogen to pyruvic acid and lactic acid liberates energy that is used to convert ADP to ATP. Process is important for two reasons glycolytic reactions can occur even in the absence of oxygen, so muscle contraction can be sustained for many seconds and sometimes up to more than 1 minute, even when oxygen delivery from the blood is not available. the rate of ATP formation by glycolysis is about 2.5 times as rapid as ATP formation in response to cellular foodstuffs reacting with oxygen. However, so many end products of glycolysis accumulate in the muscle cells that glycolysis also loses its capability to sustain maximum muscle contraction after about 1 minute. Third source is Oxidative metabolism means combining oxygen with the end products of glycolysis and with various other cellular foodstuffs to liberate ATP. Process for > 95% of all energy used by the muscles for sustained long- term contraction foodstuffs that are consumed are carbohydrates, fats, and protein For extremely long- term maximal muscle activity—over a period of many hours—the greatest proportion of energy comes from fats but, for periods of 2 to 4 hours, as much as one half of the energy can come from stored carbohydrates Products of anaerobic respiration During intense exercise, anaerobic respiration and the breakdown of creatine phosphate can only provide energy for 10–20 seconds, limited by the depletion of glucose and creatine phosphate and lactic acid accumulation. Some of the lactic acid is transported to the liver, where it is converted to glucose. The glucose can then be utilized by the muscles which produced the lactic acid. (the cori cycle)

Answer 4

Single-twitch contraction brief contraction of a motor unit – e.g. in response to brief threshold stimulus – followed by complete relaxation Single twitch duration usu. 10 ~ 100 ms depending on fibre type There is a short refractory period of the transmembrane potential ~3msec, however the contractile mechanism of skeletal muscle has no refractory period Tetanic contraction sustained maximum possible contraction of a motor unit – e.g. in response to repeated high frequency threshold stimuli With each stimuli --> build up of intracellular [Ca2+] --> ↑cross-bridges --> ↑tension ∴ tetanic contraction up to 4x tension of single twitch contraction ∴ tetanic contraction consumes more energy (ATP) than single twitch Critical frequency required for tetany Depends on the contraction-relaxation time of muscle fibres Slow twitch fibres – contraction time ~ 100 ms ∴ require repeated stimuli > 10 Hz for tetanus Fast twitch fibres – contraction time ~ 10 ms ∴ require repeated stimuli > 100 Hz for tetanus

Answer 5

cessation of electrical stimuli --> muscle action potential stops ATP depletion --> muscle fatigue Inhibitory effect from local lactic acidosis

Answer 6

Proper control of muscle function requires not only excitation of the muscle by spinal cord anterior motor neurons but also continuous feedback of sensory information from each muscle to the spinal cord, also to the cerebellum and even to the cerebral cortex indicating the functional status of each muscle at each instant. Info The length of the muscle, what is its instantaneous tension, how rapidly is its length or tension changing?

Answer 7

distributed throughout the belly of the muscle Parallel orientation with the muscle send information to the nervous system about muscle length or rate of change of length 3–10 mm in long consists of 3–12 tiny intrafusal muscle fibers that are pointed at their ends and attached to the glycocalyx of the surrounding large extrafusal skeletal muscle fibers

Answer 8

is a tiny skeletal muscle fiber. the central region has few / no actin and myosin filaments --> does not contract when ends do --> functions as a sensory receptor End portions that contract are excited by type A 𝛄 motor neuron fibers (𝛄 efferent fibers) in anterior horns of spinal cord There are two types of intrafusal muscle fibres: Nuclear BAG fibres longer and thicker nuclei aggregated in the central region 1-3 in each spindle Nuclear CHAIN fires shorter and thinner with nuclei arranged in a chain (row) along the centre of the fibre. 3-9 in each spindle

Answer 9

Receptor portion is central portion the muscle spindle receptor can be excited in two ways: Lengthening the whole muscle stretches the midportion of the spindle and, therefore, excites the receptor. Even if the length of the entire muscle does not change, contraction of the end portions of the spindle’s intrafusal fibers stretches the midportion of the spindle and therefore excites the receptor. Primary ending / Ia afferent / ‘dynamic’ fibres coil around the central region of both nuclear bag and nuclear chain fibres to form the primary annulospiral ending. the largest nerves with the fastest conduction velocities, 1 nerve per spindle Respond to the rate of change in length (stretch) --> ↑ rate of stretch ∝ impulse frequency. Secondary ending / group II afferent / ‘static’ fibres terminate on nuclear chain fibres to form flower-spray or secondary endings. nerves are smaller and slower conducting, respond to different degrees of stretch (length) on the muscle spindle --> impulse frequency ∝ muscle length. Both primary Ia and secondary II endings respond to static changes in the muscle spindle. The Primary Ending (but Not the Secondary Ending) Responds to Rate of Change of Receptor Length—“Dynamic” Response.

Answer 10

γ-motor neurons supply the contractile ends of the spindle and set the sensitivity of the afferent endings in the middle. Two roles adjust the sensitivity required by different muscle loading conditions maintain the sensitivity of the muscle spindle during muscle shortening two types: gamma- dynamic (gamma- d) Associated with plate endings ( found at poles of BAG and CHAIN fibers) excitation --> dynamic response (veolocity) of the muscle spindle becomes enhanced, static response is hardly affected. gamma-static (gamma- s) Associated with trail endings (mainly BAG-2 and CHAIN fibers) Excitation --> enhances the static response, little influence on static response

Answer 11

Loss of supraspinal inhibition occurs in an upper motor neuron lesion. Cortical damage resulting in a loss of inhibitory impulses can lead to spasticity as a result of overactive input from the γ motor neurons.

Answer 12

Continuous afferent discharge under normal conditions Stretching spindle --> ↑ rate of firing --> +ve signal to spinal cord Shortening --> ↓ rate of firing --> -ve signals to spinal cord

Answer 13

Static response If central region of a spindle is slowly stretched --> impulses in both primary and secondary endings ↑ ∝ degree of stretch Dynamic response If length of the spindle is suddenly increased, the primary sensory fibre exhibits a vigorous response --> signals the rate of change in length of the fibre Important function of this response is dampening effect on jerky or oscillatory movements

Answer 14

spindles are the starting point of the stretch reflex Passive muscle stretching produces reflex contraction by exciting spindle primary afferents, which synapse directly onto motor neurons in the anterior grey horn of the spinal cord the basis of the knee jerk, the only monosynaptic reflex in the body Type Ia sensory fibres enter the spinal cord through the dorsal roots, and their branches terminate in the spinal cord near the entry level or ascend to the brain. The fibres (from the primary sensory fibres) that terminate in the spinal cord synapse directly with motor neurons in the ventral horn, which innervate the extrafusal fibres of the same muscle. Two components to stretch reflex dynamic component activated while the spindle is being stretched dampening effect on jerky or oscillatory movements. Clonus or unusual repetitive contractions of muscles occur when muscle spindle sensory function is abnormal. static component occurs when the muscle reaches a new static length. Antagonistic muscle groups are inhibited by spinal stretch reflexes e.g. knee-jerk flexor muscles are relaxed by muscle spindle afferents acting via spinal inhibitory interneurons transmitter released by the inhibitory interneuron is glycine, which hyperpolarizes the motor neuron membrane by opening chloride channels, producing an IPSP.

Answer 15

encapsulated receptor found near the muscle-tendon junction responds to changes in tension in its muscle fascicle. more sensitive to active muscle tension than passive stretching of the muscle. Covered by a capsule which is continuous with the connective tissue of the muscle or tendon. The afferent nerves are group Ib axons which branch over several fascicles. large, myelinated, ↑ tension --> ↑ frequency of impulse inhibits the motor neurons innervating the muscle associated with the tendon organ. lie in series with the muscle If muscle tension becomes excessive, high-threshold tendon organs are stimulated and reflexly inhibit the muscle and excite antagonists. Negative feedback prevents injury to the muscle from excessive tension.

BT_PO 1.98a - Skeletal Muscle Phys Flashcards

(40 cards)