Term
| where is the "real control" of muscle movement? |
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Definition
| "real control" of muscle movement occurs in the spinal cord (meaning muscle movement is impossible w/o the spinal cord). the higher centers are in place to regulate, modify, change the frequency, change the intensity of the mechanisms that lie within the spinal cord. |
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Term
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Definition
| the ability to maintain the body's center of mass over its base of support (which is small for bipeds) |
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Term
| what does balance depend on? |
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Definition
| 1) the *posture/position of the body and limbs in space. 2) the *length/rate of contraction of each muscle. 3) the *tension/force at which each muscle is applying at its point of insertion. |
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Term
| what is the sense of position? how is it measured clinically? |
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Definition
| proprioception, which is clinically measurable by a subject's ability to detect an externally imposed passive movement of a limb or the whole body (awareness of body's position). |
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Term
| what is the sense of movement? how is it measured clinically? |
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Definition
| kinesthesia, which is clinically measurable by the subjects ability to reposition a joint or their entire body to a predetermined position (awareness of body's movements). |
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Term
| what do motor units allow due to their arrangement and innervation? where are the pathways for their activation? |
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Definition
| the mechanism for maintaining posture/balance through summation and recruitment (b/c normal skeletal muscle cells cannot independently depolarize - they require an alpha motor neuron). the pathways for motor unit activation are located in the spinal cord. |
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Term
| can part of a motor unit be depolarized? |
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Definition
| no - it is an all or nothing thing |
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Term
| what is the sequence of motor unit recruitment (depolarization)? what determines this sequence? |
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Definition
| motor units are recruited from smallest to largest depending on the strength of the signal from the spinal cord. the smaller motor units are easier to depolarize first, so if a signal is weak only they will be recruited. a strong signal guarantees the larger and harder to depolarize motor units will also be recruited. |
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Term
| how are motor units arranged in relation to each other? |
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Definition
| the motor units are diffusely interspersed amongst each other in order to maintain smooth contractions |
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Term
| what allows a motor unit to have finer control? |
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Definition
| a smaller number of muscle cells to command (seen in the eye and hand) |
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Term
| what are the 3 kinds of metabolism motor units may employ? how does this affect their function? |
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Definition
| fast glycolytic (have thickest alpha motor neuron axons = strongest contraction, but require lots of ATP = fatigue quickest), fast oxidative glycolytic, and slow oxidative (have the smallest alpha motor neuron axons = relatively weak contraction, but can sustain long periods of repeated contractions without fatiguing). all the muscle cells of a specific motor unit are of the same metabolic type - that being whatever type the alpha motor neuron is. |
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Term
| what are the 2 ways of increasing the force of muscle contraction? |
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Definition
| 1) temporal summation: increasing the frequency of stimulation (as long as the next contraction occurs after the refractory period). 2) spatial summation: recruitment of more (stronger) motor units by increasing the strength of stimulation (fast glycolytic motor units require a strong stimulus for their contraction). |
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Term
| what is the strength of contraction directly proportional to? |
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Definition
| the number of crossbridges being formed at any particular time |
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Term
| how can learned reflex patterns improve balance? |
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Definition
| by establishing new reflex pathways in the spinal cord (football players train themselves to avoid other players by running between tired on a field) |
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Term
| what is the continuum of spinal reflexes? what determines how learned tasks sit on this continuum? |
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Definition
| from totally autonomically driven tasks such as breathing on one end to voluntarily driven tasks such as riding a bike on the other. the more complex a task (such as playing piano), the more training is required for it to become involuntary. |
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Term
| what is considered a spinal cord reflex? |
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Definition
| a rapid, automatic nerve response which is triggered by specific stimuli and coordinated w/in the spinal cord through interconnected sensory, motor, and interneurons. |
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Term
| what is all movement designed around? |
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Definition
| patterns of reflexes which are only regulated by the higher centers of the brain |
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Term
| what does one spinal reflex produce? |
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Definition
| one motor response (in a stylized and infinitely repeatable manner) |
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Term
| what does every spinal reflex require to start? |
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Definition
| some kind of sensory stimulus |
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Term
| what is the pathway of a spinal reflex? |
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Definition
| the spinal reflex starts w/a sensory receptor (eg muscle spindle) which signals down an afferent pathway to the CNS, synapsing 1-2x, then back down an efferent pathway to the periphery and ending at an effector (a muscle or gland). |
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Term
| what are the 2 types of proprioceptors and what information do they provide? where are they located? (*know for exam*) |
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Definition
| 1) muscle spindles (stretch receptors) provide information about the *velocity of the stretch, the *length of the stretch, and if the stretch is *constant/changing. the muscle spindles are embedded *parallel to regular muscle fibers. 2) golgi tendon organs (force/tension receptors) provide information about the *load/force being applied to the muscle and are located in *series to normal muscle fiber at the ends of the muscle. |
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Term
| what is the shape of the muscle spindles? what are the fibers inside a muscle spindle called? |
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Definition
| shape of the muscle spindles: fusiform (torpedo). the fibers inside a muscle spindle are called: intrafusal. |
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Term
| what does it mean to say that the sensory fibers of muscle spindles are tonically active? what function does this perform? |
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Definition
| they are always sending out signals - which keeps adjacent muscles consistently slightly contracted = muscle tone. |
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Term
| what is the structure of a muscle spindle? that is the function of the components? |
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Definition
| the exterior is composed of extrafusal muscle fiber (which includes an alpha neuromuscular junction). the interior is composed of a capsule w/a fluid filled cavity containing intrafusal muscle fibers, gamma motoneurons (the *only input from the higher brain here - controls intrafusal fibers), nuclear bag fibers (composed of groups of Ia fibers at center of intrafusal fiber = sensitive to stretch acceleration), and nuclear chain fibers (found along the intrafusal fibers = sensitive to muscle length, composed of group Ia and group II fibers). |
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Term
| what does a higher level of muscle spindles in a particular section of muscle tissue allow? |
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Definition
| a higher sensitivity to stretch |
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Term
| what happens when the intrafusal fibers stretch (along w/the rest of the muscle they are embedded in)? |
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Definition
| the sensory fibers increase their firing rate. this occurs b/c the stretching of the spindle lengthens the central region of the intrafusal fiber around which the sensory fibers are entwined. this leads to activation of the stretch sensitive channels which depolarizes the membrane/generates action potentials. these sensory fibers then synapse w/the alpha motor neurons in the spinal cord (via the dorsal root either directly or through an interneuron) and tell the muscle to contract as a protection from over-stretching and to establish a range of motion. |
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Term
| what are the 2 kinds of information/response relayed by the 2 kinds of sensory fibers in the muscle spindle during a *linear* stretch? |
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Definition
| 1) dynamic, mediated by group 1a fibers which increase their rate of firing proportionally to the rate of the stretch's acceleration (therefore they do not fire while the muscle relaxes back to its set point, at which it resumes tonic firing). 2) static, mediated by group II fibers whose rate of firing increases uniformly to mirror the degree of length the muscle is being stretched to (therefore, these fibers are always firing, even when the muscle is relaxing). thus, the brain always has some information of where that muscle is in space during the entire duration of a linear stretch. |
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Term
| what are the 2 kinds of information/response relayed by the 2 kinds of sensory fibers in the muscle spindle during a *tap* stretch? |
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Definition
| 1) in the dynamic response, the group 1a fibers increase their firing rapidly w/the brief acceleration from the tap, then stop firing as the muscle length returns to normal. 2) in the static response, the group II fibers are too slow to respond to the brief stretch involved w/a tap, and simply do not fire during the entire length of the stretch. thus, *there is a moment during a tap stretch when the brain has no information concerning the position of the muscle in space. |
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Term
| what are the 2 kinds of information/response relayed by the 2 kinds of sensory fibers in the muscle spindle during a *sinusoidal* stretch? |
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Definition
| 1) in the dynamic response, the group 1a fibers increase their firing with the rise of the sinusoidal waves of stretching and cease firing as the wave drops back down. 2) in the static response, the group II fibers consistently fire, providing realtime information regarding the changing muscle length during sinusoidal stretching. |
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Term
| what is the most basic function of the muscle spindle stretch receptor? |
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Definition
| an ability to limit the amount of stretching in the whole muscle. |
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Term
| how does the stretch reflex arc occur? |
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Definition
| extensor and flexor muscles are reciprocally arranged on either side of a joint. for example, if the extensor muscle is in danger of being overstretched, afferent information from the muscle spindle goes to the spine, through the dorsal horn and synapses w/an alpha motor neuron that causes contraction of the extensor as well as an inhibitor interneuron which causes relaxation of the opposing flexor via inhibition of its alpha motor neuron. |
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Term
| what problem does the parallel arrangement of intrafusal and extrafusal fibers in a muscle spindle lead to? how is this resolved? |
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Definition
| as a muscle contracts at the end of a reflex arc, the intrafusal fibers slacken (shorten) - which should cause the spindles to be unable to send afferent information (proprioceptive information is sent only in terms of stretch). the gamma motor units however innervate only the intrafusal fibers, and as the rest of the muscle contracts, the gamma motor units keep the correct level of intrafusal tension to allow afferent information to continue to be sent. through trial and error, these pathways can be trained to send just the right amount of signal to get your limb to the place where you’re performing a meaningful activity. |
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Term
| what is the "gamma gain"? |
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Definition
| the *ratio of the extrafusal fibers to the intrafusal fibers*. the more the gain is increased, the more contracted the intrafusal fibers are compared to the extrafusal fibers (a gamma gain of 4 is more contracted than a gain of 2). |
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Term
| what is a clinical application of the reflex arc? |
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Definition
| if there is cramping of an antagonistic muscle, contraction of the agonist muscle necessarily causes a relaxation in that antagonistic muscle. |
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Term
| can the gamma gain be increased even while the muscle is being stretched? |
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Definition
| yes - this leads to an even higher sensory output |
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Term
| what do counterstrain techniques capitalize on? |
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Definition
| decreasing gamma gain, which stops inappropriate proprioceptor activity causing problematic shortening in the muscle containing a malfunctioning muscle spindle. |
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Term
| what characterizes a stretch reflex? |
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Definition
| this is a monosynaptic reflex (only requires 1 sensory and 1 motor neuron), where when the reflex hammer hits the tendon it creates a stretch which the group 1a fibers respond to by signaling through the dorsal horn to an alpha motor neuron = contraction of the muscle attached to the tendon being stretched/tapped and relaxation of the opposing muscle. this occurs too quickly for group II fibers to play a part. the stretch reflex is considered an ipsilateral reflex as it responds on the same side as that from which the initial stimulus came. |
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Term
| what is reciprocal inhibition? |
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Definition
| the term for when the muscle opposing the contracting muscle in a stretch reflex relaxes. |
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Term
| what is the role of the higher brain centers in spinal reflexes? |
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Definition
| regulation of the spinal reflexes so unnecessary/exaggerated movement may be avoided |
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Term
| what is the role of the gamma motor neuron? |
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Definition
| regulation of the muscle spindle gain (gamma gain) so the spindles can operate efficiently at any length of the parent muscle. |
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Term
| what are the only sensory receptors whose output is centrally modifiable? how? |
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Definition
| the muscle spindles - via gamma efferents |
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Term
| what is the renshaw cell inhibitor system? |
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Definition
| renshaw cells are inhibitory neurons surrounding alpha motor neuron bodies in the spinal cord. the axons of afferents, primary motor cortex neurons, and axons looping back off the main efferent axons leaving that alpha motor neuron (*recurrent inhibition*) all synapse on renshaw cells, which then synapse on the alpha motor neuron. thus, the algebraic sum of excitatory/inhibitory signals recieved by the alpha motor neuron determines whether it will fire. the extra modulation provided by the renshaw cells *prevents neurons from discharging at excessive frequency or beyond a useful range*. |
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Term
| what is the function of the golgi tendon organs? |
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Definition
| to keep muscle from contracting to the point where it might detach from bone. |
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Term
| how does the golgi tendon organ function? |
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Definition
| the golgi tendon organs are positioned in tendon at the poles of the muscle. when the tendon is stretched, a conformational change in the golgi tendon organ occurs, leading to a depolarization and a signal being sent down the group 1b afferents proportional to the strength of contraction. |
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Term
| what is the difference between group 1a and 1b fibers? |
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Definition
| the group 1b afferents *always synapse on an inhibitory neuron, which then synapses on the alpha motor neuron - inhibiting further contraction and avoiding detachment of the muscle from bone. these 1b signals will continue as long as the muscle remains contracted. |
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Term
| what characterizes the golgi tendon reflex? how does it compare to the spindle reflex? |
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Definition
| the golgi tendon reflex is polysynaptic, (requires an inhibitory interneuron) and is opposite to the spindle reflex because it’s responding to excessive contraction rather than excessive stretching. the golgi tendon reflex is considered ipsilateral b/c the muscle which is contracting responds to its own sensory information from that side and relaxes. the group 1b fibers can also cause a contraction in the opposing muscle to help stretch the initially over-contracted muscle. |
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Term
| what is the flexor withdrawal reflex? |
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Definition
| a polysynaptic (employs interneurons), crossed extensor, contralateral AND ipsilateral reflex which occurs in response to noxious stimuli (pain/tissue damage). |
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Term
| how does the flexor withdrawal reflex work in the case of stepping on a tack w/1 foot? |
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Definition
| pain receptors in the foot send afferent information to the spinal cord, which synapses on the ipsilateral alpha motor neuron causing flexion (withdrawal) at the ankle, knee, and hip as well as inhibition of the opposing extensors. simultaneously, the contralateral alphamotor neurons do the exact opposite - where the extensors all contract/flexors all relax to provide maximum support for the other withdrawn leg. the same mechanism occurs in the upper body if one hand touches a hot stove. the flexor withdrawal reflex can also be viewed as a version of the stepping reflex. |
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Term
| what is the definition of human posture? |
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Definition
| the relative orientation of the body segments and their orientation in 3d space. upright posture is unstable and a continuous activation of postural muscles is required to avoid falling down. |
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Term
| what is the center of balance? |
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Definition
| the point in whatever the structure is so that if a string were attached to it, all the downward forces would be exactly balanced so it’d be in upright position. this point is hard to maintain in the human form b/c it has a high center of gravity for such a small base. |
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Term
| what do muscle movements required for locomotion have to be superimposed on? |
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Definition
| the basic postural reflexes |
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Term
| where is the neuronal circuitry for purposeful locomotion? |
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Definition
| this exists in the spinal cord as pattern generators |
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Term
| what is a basic example of a pattern generator sequence? |
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Definition
| in the background, there is continuous afferent input from both flexor and extensor. in response to this afferent input, the excitatory interneuron which innervates the flexor fires - but since it is also synapsing w/a inhibitory interneuron for the opposing extensor, the extensor muscle is inhibited from contracting for the duration of flexor contraction. once the flexor has contracted, the extensor then repeats the same sequence, and so on until fatigue or activation of a different pathway. also, the inhibitor interneurons which mediate inhibition of the opposing muscle during contraction of the other can also inhibit itself (*recurrent inhibition*), allowing contraction of both flexors and extensors simultaneously. |
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Term
| what is necessary for the pattern generators in the spine to produce differently rhythmic outputs? |
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Definition
| atonic descending input from the brain |
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