Lecture 12: Neural Control and Reflexes (Hayward) Flashcards
where are motor neurons located?*
ventral horn of spinal cord
2 types of motor neurons that have control over muscle system*
1) alpha motor neurons
2) gramma motor neurons
contrast alpha motor neurons with gamma motor neurons*
alpha motor neurons are larger and send axons down to muscle fibers and activate muscles in general. Gamma motor neurons are smaller and innervate muscle fibers assoc. with intricate sensory endings in the muscle spindles to regulate the sensitivity of the muscle spindle
extrafusal vs. intrafusal muscle fibers*
extrafusal muscle fibers are fast/slow twitch muscle fibers and innervated by alpha motor neurons. Intrafusal muscle fibers are muscle fibers assoc. with the muscle spindles and innervated by gamma motor neurons
kinds of orientations/locations of receptors in a muscle*
1) in parallel with muscle (muscle spindles; provide info on muscle stretch)
2) in series with muscle (sensitive to force production)
3) proprioceptors (provide info on where limb is in space)
4) receptors in the skin
what are considered upper motor neurons?*
cortex, subcortical nuclei, and cerebellum
what are considered lower motor neurons?*
spinal cord (alpha and gamma motor neurons)
what could be the cause of lower motor neuron disease?*
dysfunction in alpha/gamma motor neurons, how the motor neurons attach to a muscle, or the muscle fibers themselves
Describe a typical reflex mechanism*
1) input from muscle enters spinal cord via afferent nerve
2) synapses on alpha motor neuron either monosynapticaly or polysynaptically (if polysynaptic, must synapse through interneurons before reaching motor neurons, and signal can be stopped. Monosynaptic synapses directly on alpha MN and cannot be stopped)
3) alpha MN sends output to muscle fiber via efferent nerve to cause effect
general characteristics of reflex arcs*
- genetically programmed and fully developed at birth
- may be segmental or intersegmental (involves one or many segments of spinal cord)
- monosynaptic or polysynaptic
which is more common in the nervous system: monosynaptic or polysynaptic reflexes? Give an example of system that uses monosynaptic reflex*
polysynaptic. Motor system
what happens to central region of muscle spindle when outside region contracts? *
stretches
What kind of neural control is outer region of muscle spindle under?*
gamma motor neuron system
In which layer of muscle do muscle spindles lie?*
perimysium. They are functionally connected to the tendons by the connective tissue of the muscle
Describe monosynaptic stretch reflex*
muscle stretched, which causes muscle spindle to send signal to spinal cord and cause excitation of the motor neurons from where spindle was stretched to contract the muscle. Contraction of opposing muscle is inhibited to allow for muscle contraction to be unopposed
are intrafusal fibers part of the stretch reflex?*
NO, because the gamma MN system has to be activated during descending command, not via the stretch reflex
what does lack of stretch reflex indicate?*
problem in lower motor neuron
what does exaggerated stretch reflex indicate?*
problem in descending command (upper motor neuron)
what does alpha-gamma MN co-activation allow for?*
muscle length detection during shortening contractions
golgi tendon organ*
has free nerve endings intertwined in tendons that sense force production in muscle. During high force production, inhibits alpha MN from whence input came. Is fast conducting and lies in series with the muscle fibers. Discharge increases with increasing force production
which is faster conducting: type 1 or 2 afferents?*
type 1
golgi tendon organ reflex*
a protective mechanism that inhibits capacity to produce so much muscle force that the bone is damaged. An polysynaptic inhibitory reflex.
when do muscle spindles “unload”?*
when they are contracted. Discharge when stretched.
which types of muscle receptors modulate length and force production?*
type Ia, II. They are large diameter, myelinated and fast-conducting
which types of muscle receptors sense muscle fatigue?*
type III, IV. They are small diameter, unmyelinated and slow-conducting.
flexion withdrawal reflex*
A polysynaptic body-crossed extension reflex. Extensor muscle inhibition and flexor muscle excitation.
where is problem if one limb is pinched and the opposite limb extends?*
in descending control pathway (upper MN)
where is problem if no flexion withdrawal reflex occurs?*
between pre and postsynaptic site (mostl likely in MN)
4 conditions leading to muscle contraction*
1) agonist stretch
2) muscle spindle activation
3) antagonist contraction
4) gamma MN system stimulation
how does agonist stretch cause muscle contraction?*
reflex response is to cause excitation of surrounding agonists, and inhibition of surrounding antagonists.
how does muscle spindle activation cause muscle contraction?*
causes AMN activation, which sends signal to contract muscle
how does gamma MN system stimulation cause muscle contraction?*
activates end region of the muscle spindle and stretches the central sensory region
pyramidal system*
component of descending control responsible for voluntary actions
extra-pyramidal system*
component of descending control responsible for postural system
symptoms of upper MN lesions*
inappropriate movements, inability to know limb position, abnormal segmental reflexes
symptoms of lower MN lesions*
paralysis, atrophy, loss of segmental reflexes
doe extrafusal fibers contract spontaneously?**
NO! They only contract when signaled to contract
fx of sensory/afferent n.
carries sensory signals to the CNS, entering via the dorsal roots of the spinal cord
fx of a receptor
converts environmental energy into action potential
fx of motor/efferent n.
transports the action potentials from the CNS to the target organ. Leaves via the ventral roots of the spinal cord.
effector organ
organ which causes the reflex response
is the flexion withdrawal reflex intersegmental or segmental?
intersegmental
what do type III, IV muscle fibers respond to?
mechanical forces (compression and muscle contraction) and chemical changes within the muscle that are assoc. with prolonged muscle activation (fatigue)
what reflex do type III, IV muscle fibers activate?
exercise pressor reflex (a cardiovascular response to exercise). Has an inhibitory effect on alpha MN
How do flexion withdrawal and crossed extension reflexes work together?
their combined action provides a quick shift of body support away from a painful stimulus. Both reflexes are POLYSYNAPTIC.
what is the “final common pathway” through which the CNS regulates muscle?
the alpha motoneuron. Therefore, clinical signs of lower MN disease can be explained by the loss of the alpha MN or associated motor nerve.
