13. Motor Reflexes and their modification (iBook DSA and lecture) Flashcards
Describe the pre-synaptic junction
lots of vesicles
no NT variability
A lot of mitochondria
Describe the motor end plate
skeletal muscle cell
junctional folds increase the surface area of the muscle cell
Describe synaptic transmission in your own words
The AP comes into the presynaptic terminal and depolarizes the cell with a calcium invasion
The calcium travels to the synaptic proteins and causes the vesicles to dock onto the membrane
ACH is released from the vesicles and into the synaptic space to the nicotinic receptors on the motor plate
ACH allows for a sodium influx which leads to the end plate potential and the depolarization spreads opening more channels and leading to more excitation
Nociceptors and mechanoreceptors are apart of the (AFFERENT/EFFERENT) reflexes
AFFERENT
What are the efferents involved in reflexes?
motoneurons
What are the reflexes that the spinal cord helps to contribute to?
Myotactic reflex
Golgi tendon reflex
Crossed extensor reflex
What are the reflexes that the brainstem/midbrain help to contribute to?
Vestibular reflexes
Righting reflexes
What are the reflexes that the cortical regions help to contribute to?
Placing response
Hopping response
Describe the myotactic reflex.
monosynaptic
The afferent portion (Ia and group II fibers) come in and notice that there is a change in the tension (stretch) of the muscle (via the intrafusal fibers) and then the alpha motor neurons (efferent) causes a muscle contraction with ACH
What are the primary afferents that are involved with the afferent innervation of the myotactic reflex? What do they innervate?
Ia group of fibers (large and myelinated with fast innervation)
They are sensitive to changes in length and the rate of those changes
They innervate nuclear bag and nuclear chain fibers in the muscle spindle
Differentiate between nuclear bag and nuclear chain fibers of the muscle spindle
nuclear bag fibers are a bulge of nuclei and the nuclei chain fibers have nuclei that are more spread out
Describe the secondary afferent nerves that are involved in the myotactic muscle reflex
Group II fibers: they are also myelinated but are not as large and rapid as group Ia fibers
only innervate nuclear chain fibers
Differentiate between the primary and secondary afferent nerves that are involved with the myotactic reflex
the primary afferents are sensitive to changes in length as well as the rate of those changes and innervate nuclear bag fibers as well as nuclear chain fibers
The secondary afferents are sensitive only to changes in length and innervate nuclear chain fibers only
What is the static information that the CNS receives in regards to muscles?
Length of the muscle
What is the dynamic information that the CNS receives in regards to muscles?
rate of change
Describe the overall circuitry of the myotactic reflex
The muscle spindle has afferent fibers that enter into the SC via the dorsal horn. EAA is released here and binds to the non-NMDA receptors that a re present on the dendrites of the alpha motoneurons.
The alpha motoneurons release ACH and illicit a muscle contraction
What is reciprocal inhibition and why is it necessary in a reflex?
In order for a muscle to stretch adequately, the actions of the antagonistic muscle must be inhibited, so that muscles do not act against each other
What is the purpose of the secondary efferent limb involved in the myotactic reflex?
To take care of the reciprocal inhibition and work on the antagonist via Ia fibers
Describe the Golgi tendon reflex (briefly)
Inverse myotactic reflex
abrupt relaxation of a strongly contracted muscle (usually to prevent damage to the muscle)
What are the afferent fibers that are involved in the Golgi tendon reflex?
Ib fibers that contain golgi tendon organs
Describe the two synapses that are involved in the golgi tendon reflex
- EAA is released by the afferent branch and binds to the non-NMDA receptors that are present on the interneurons which causes an AP to occur
- The interneuron then releases GABA to the alpha motoneuron and Cl- enters into the motoneuron and hyper polarizes it
In the golgi tendon reflex, what is the purpose of the efferent limb?
To relax the muscle, so that no further damage occurs
Describe the reflex threshold of the Golgi Tendon reflex
It is HIGH because it only happens if the muscle is about to tear
True/False: For the most part, the cortex inhibits the spinal reflexes
True
What is spinal shock?
period of areflexemia after a transection of the SC and loss of excitatory bias from the CNS
Describe the gradual recovery from a spinal shock syndrome
The gradual recovery is due to the formation of new synapses of the axons to nearby cells
serotonergic receptors become self activating after the injury in an attempt to increase the amount of calcium into the cell which can help to bring the motoneuron close to threshold
Why do muscle spasms occur in only SOME patient with spinal shock?
The rate of muscle spasms is dependent on whether or not the connection of the muscle with the brain is re-estblished; if this does NOT occur, then the muscle starts to die, resulting in spasm
Extensor rigidity and spasticity are components of ________ _______
Decerebrate posturing
Why does decerebrate posturing occur?
Because there is a loss of structures rostral to the pons, and there is a lesion caudal to the red nucleus
some of the tonic reflexes against gravity are uncovered
Why does extensor rigidity occur in the decerebrate posturing?
Because there is an increase in alpha motoneuron excitability
What is spasticity and why does it occur in decerebrate posturing?
muscle contraction that is resistant to passive motion and it is due to an increased rate of gamma neurons firing
_______ _______ is independent of afferent nerves in decerebrate posturing and ________ will be abolished if the afferent innervation is cut in decerebrate posturing
Extensor rigidity is independent
Spasticity will be abolished if the afferents are cut (dependent)
Why does clonus happen in cases of decerebrate posturing?
There is increased gamma motoneuron firing which lengthens the muscle spindle, increases sensitivity to stretch, which causes the muscle to want to resist passive motion at all costs, resulting in clonus
What kind of posturing will result from the damage to the internal capsule?
Decorticate posturing
If there is bilateral damage to the red nuclei, describe the position of the patient if the
- head is straight
- head is turned to the side
- both legs are extended and both of the arms are flexed
- the legs are extended and the arm that the head is turned to is increased in flexion and the arm opposite to the head turn is extended a bit
In unilateral red nucleus damage, what happens with head movement
When the patient turns their head to the affected side, the arm on that side will flex more, and when they turn away, it will extend
Why are a variety of reflexes uncovered in decorticate rigidity?
Because the damage to the red nucleus causes an increase of activity in the rubrospinal tract. Add this to the fact that the corticospinal tract is disrupted, and those reflexes start to come to the surface
What is the purpose of reflexes?
Protection
FAST
Compare the level of organization of reflexes versus volitional responses
reflexes are in the CNS level only while the volitional response requires cortical and subcortical activation
Differentiate between rigidity and spasticity
Rigidity:
resists all motions
Maintained muscle contraction
continuous alpha motoneuron activation
Spasticity:
resists all motion in a given direction
Myotactic reflex is hyperactive
Continual activation of gamma motorneurons
What is the purpose of the brainstem facilitatory region?
activate the gamma motoneurons and makes the muscle spindle more sensitive
