Spinal Reflexes Flashcards
Myotatic reflex
stretching any striated muscle will evoke a contraction of that muscle
longitudinal structure embedded within muscle, oriented parallel with extrafusal muscle fibers
muscle spindle
What do the ends of the muscle spindles attach to?
extrafusal fibers (outside the spindle)
The number of spindles is directly related to what?
The amount of control required of the muscle (finger muscle will have more spindles than back muscle per unit)
What are the two types of receptors within a muscle spindle–how many of each?
1) nuclear bag, 2
2) nuclear chain fibers, 5
What are nuclear bag and nuclear chain fibers collectively called?
Intrafusal fibers (within the spindle)
Two important qualities of nuclear bag and nuclear chain fibers
- both have contractile elements at their distal ends
2. both are oriented along the long axis of the spindle
The intrafusal fibers are afferently innervated by these nerve fibers
Ia and II (one each per spindle)
These nerve fibers have annulospiral endings which coil around the central region of all nuclear bag and all nuclear chain fibers in a spindle
Ia fibers
There is no synapse between Ia fibers and nuclear bag and nuclear chain fibers. How does it work?
Mechanical perturbation caused by stretch opens Na channels in the annulospiral endings (mechanoreceptors)
These nerve fibers have flower spray endings that innervate the pericentral regions of only one type of nerve intrafusal. Which one?
Type II fibers, only innervate nuclear chain fibers
Why are Ia deemed primary over II?
Greater innervation and conduction velocity
The strong, short lasting, phasic response to a brief stretch (tapping patellar ligament), is mediated by which fiber type?
Nuclear bag fibers
The weaker, longer lasting tonic response is mediated by what type of fiber?
Nuclear chain fibers
This type of fiber is important in correcting abrupt changes in muscle length
Nuclear bag fibers
This fiber type is important for maintaining muscle length, such as posture
Nuclear chains
How can the stretch reflex be evoked at different muscle lengths?
Primary afferents synapse on alpha motor neurons and on smaller gamma neurons (via interneurons)
How do the axons of the gamma motor neurons work?
They innervate the contractile elements on the distal poles of the intrafusal fibers-has the effect of stretching the central region of intrafusal fibers, thus activating Ia and II afferents.
Alpha motor neurons innervate what type of muscle
Extrafusal muscle fibers
Spindle receptors transmit information about”
The length of the spindle relative to muscle length
Knee-jerk reflex demonstrate a myotatic reflex, but what is an example where the reflex is always occurring? Explain.
Standing; gravity stretches quadriceps, activates Ia afferents, activates quads via monosynaptic connection to alpha motor neurons, keeps repeating, providing continuous tone to quads
The myotatic reflex is a mechanism for ________, a necessary condition for standing
muscle tone
Gamma motor neurons innervate what type of fibers?
Intrafusal
Consequence of spinal cord injuries that results in inactivation of gamma motor neurons
intrafusal fibers can no longer contract, resulting in hypotonia
-changes in muscle length can no longer adequately stretch flaccid muscle spindle to activate primary and secondary endings
Why might hypertonia occur in spinal cord injuries?
removal of inhibitory influences of gamma motor neurons, muscle is continuously activated. Muscle spindle is no very sensitive to stretch, taping patellar ligament results in exaggerated reflex
Clonus
repetitive triggering of myotatic reflex in hypertonic muscles (inertia on way down of leg swinging causes muscle to stretch again, resulting in another myotatic reflex)
These fibers carry information about dynamic (phasic) and static (tonic) changes in muscle length
Ia afferent fibers
Function of efferents to intrafusal fibers
To adjust the length of the muscle spindles relative to the length of the muscle
The number of golgi tendon organs is somewhat (lower/higher) than muscle spindles in a given muscle
lower
While muscle spindle detects muscle length, the golgi tendon organ detects
Muscle tension/force
The GTO (other major muscle receptor), is embedded in
the musculotendinous sheath
During passive stretch, the muscles fibers are (more/less) elastic than the tendinous sheath
More. Thus, muscle spindles are stretched, but there is miminal stretch applied to the GTO
When the muscle contracts, it becomes ____ and stretches the _____ thus activing ____
inelastic, tendon, GTO
Each golgi tendon organ is innervated by a single __ fiber
Ib
Ib have their cell bodies located in _________ and ______
Dorsal root ganglion and in the cord synapse via an interneuron (disynaptic) onto alpha motor neurons
Activation of a signle Ib results in inhibition of ______ and excitation of __________
homonymous motor neurons; motor neurons of antagonistic muscles
Golgi tendon organ is exquisitely sensitive (low threshold) to what? It has a high threshold for what?
Sensitive to muscle contraction; high threshold to passive stretch
True or false: More than one single motor unit is required to activate the golgi tendon organ receptor.
FALSE. Activity of a SINGLE MOTOR UNIT is sufficient.
The function of muscle spindle and GTO afferents is not solely for mediating spinal cord reflexes. For example, Ia and Ib afferents participating in reflexes also synapse in _____
Clarke’s column (then form the dorsal spinocerebellar tract)
Ia also travel in the dorsal column and synapse in the lower medulla in nuclei related to _____ and _____
gracilis and cuneatus (then they travel with the medial lemniscus and synapse in thalamus–>cortex)
