Term
| A reflex is defined as... |
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Definition
| An involuntary, stereotyped response to a sensory input. |
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Term
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Definition
A. a receptor,
B. an afferent neuron (with cell body in a dorsal root ganglion or other sensory ganglion) and
C. an efferent neuron. |
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Term
| Most reflexes involve one or more interneurons, except which reflex? |
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Definition
| the stretch reflex, which is monosynaptic |
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Term
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Definition
1. Represents the simplest segmental reflex since it only involves two neurons (the afferent and efferent neuron).
2. Other terms for this reflex include:
A. knee jerk reflex (patellar tendon reflex) -- can be tested in joints other than the knee
B. deep tendon reflex (DTR)
C. myotatic reflex
D. Ia monosynaptic reflex |
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Term
| How does the Stretch Reflex work? |
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Definition
muscle stretch activates Ia (and Group II) afferents of the muscle spindle which monosynaptically excite alpha motor neurons to cause contraction of the muscle in which the muscle spindle lies (i.e., the homonymous muscle).
4. The stretch reflex is commonly used for clinical testing purposes. Tapping the patellar tendon causes a slight stretch in the quadriceps muscle. Ia endings in the muscle spindle, in particular, are excited, and send receptor potentials up the afferent axon through the DRG neuron to the spinal cord to activate alpha motor neurons of the quadriceps to cause contraction of the muscle. |
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Term
| Does the stretch reflex involve the tendons? |
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Definition
| Doen not involve the tendon organs, even though the stimulus is applied at the tendon. |
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Term
| How is the stretch reflex excitability reduced? |
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Definition
Reflex excitability can be reduced when either the dorsal or the ventral roots are compressed or damaged (e.g., herniated intervertebral disk), or when descending excitatory input is reduced.
Lower motor neuron injuries, those affecting the ventral horn motor neurons and their axons will present with reduced or absent deep tendon reflexes at the level of injury. |
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Term
| How is stretch reflex excitability enhanced? |
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Definition
By increasing excitatory input to alpha and gamma motor neurons or by decreasing inhibitory input to alpha and gamma motor neurons (Upper motor neuron injuries).
Reflex excitability (amplitude of the reflex response) can also be enhanced experimentally and in the clinic by increasing the afferent input to the spinal cord as, for example, by performing the Jendrassik maneuver (clasping fingers together and pulling hands apart). This reflex is mediated by propriospinal (intersegmental) pathways. |
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Term
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Definition
| clasping fingers together and pulling hands apart |
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Term
| H-Reflex (Hoffmann Reflex) |
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Definition
Reflex is a means of assessing spinal reflex excitability in humans.
The reflex is elicited by electrical stimulation of a mixed peripheral nerve. The response is measured by surface EMG electrodes placed over the muscle that nerve innervates. |
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Term
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Definition
| Stimulation of the axons of alpha motor neurons will produce a small EMG signal in the muscle with only a short delay. |
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Term
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Definition
| Stimulation of the large Ia afferents will invoke the stretch reflex and produce a delayed EMG response in the muscle. |
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Term
| Which reflex can be used to test and quantify the efficacy of therapeutic interventions in the rehabilitation of patients with CNS pathology? |
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Definition
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Term
| If the H wave is too large compared to the M wave, it indicates... |
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Definition
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Term
| If the H wave is too small, then it indicates... |
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Definition
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Term
| Tonic Vibratory Reflex (TVR) |
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Definition
| High frequency vibration (100-300 Hz) applied to a tendon or muscle belly causes high frequency stretching of muscle. This stretching stimulates Ia afferents of the muscle spindle which then monosynaptically activate alpha motor neurons causing contraction of the homonymous muscle. |
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Term
| Therapeutic uses of Tonic Vibratory Reflex |
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Definition
A. Application of a vibratory stimulus to a weak but actively contracting muscle to assist the contraction.
B. Application of a vibratory stimulus to a flaccid muscle to lower the threshold for excitation of alpha motor neurons.
C. Application of a vibratory stimulus to the antagonist of a spastic muscle to reduce hypertonicity in the spastic muscle.
D. Application of a vibratory stimulus to synergists to assist contraction of prime movers. |
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Term
| The Tendon Organ Reflex: Autogenic Inhibition |
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Definition
Muscle contraction creates force in the tendon. This force activates the Ib afferent from the Golgi tendon organ.
Ib Afferent feedback to the spinal cord activates an inhibitory interneuron, which inhibits motor neurons from the muscle in which the tendon organ lies. This process is called autogenic inhibition. |
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Term
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Definition
. Ib Afferent feedback to the spinal cord activates an inhibitory interneuron, which inhibits motor neurons from the muscle in which the tendon organ lies.
The function of this reflex in normal movement is thought to be a protective mechanism to prevent muscles from creating forces that will cause injury to the tendon or muscle itself. For example, have you ever carried a heavy box the put a lot of strain on your finger flexor muscles to the point where they are abut the “give way”? The Tendon organ reflex would cause those muscle to shut down before the weight of the box and force of the muscles caused damage to the tendon or muscle belly. |
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Term
| Cutaneous (Flexor Withdrawal) Reflexes |
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Definition
Mediated by cutaneous receptors and A-beta, A-delta, and C fiber afferents.
It requires synapses at multiple levels of the spinal cord to elicit movement of the entire limb.
An example of a cutaneous flexion reflex is withdrawal of a limb from a painful stimulus.
i. The painfull stimulus excites afferents that enter the spinal cord at one or two spinal levels. The afferents, however must excite ventral horn motorneurons at multiple levels of the spinal cord to produce a whole limb movement.
ii. At the same time that agonist muscle groups are causing limb flexion, antagonistic muscle groups are undergoing reciprocal inhibition |
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Term
| phase-dependent reversals |
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Definition
means that the same stimulus could cause flexion or extension of the limb, depending upon which phase of the step cycle the limb is in.
EXAMPLE: Burning your finger with your arm extended will cause you to flex your limb away from the stimulus; while burning your finger with your arm flexed will cause you to extend your arm away from the stimulus. |
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Term
| propriospinal pathway neurons |
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Definition
| Neurons that coordinate flexor responses over several spinal segments |
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Term
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Definition
The flexion withdrawal reflex described above, also includes a contralateral component by which extensor muscles are activated contralaterally via interneurons and motor neurons while flexors are activated ipsilaterally.
Motor neurons and interneurons that take part in ipsilateral and crossed reflexes are also thought to participate in more complex activities such as reciprocal stepping. |
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Term
| Effects of Disruption of Descending Pathways on Segmental Reflex Activity |
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Definition
| Damage to descending pathways can alter spinal cord reflex mechanisms. Complete spinal cord transection initially results in areflexia or spinal shock. This phenomenon is due to sudden absence of descending influences. With time, reflex activity begins to return, but becomes progressively abnormal and exaggerated, developing into hyperreflexia or spasticity. |
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Term
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Definition
- Interneurons project axons that synapse on other neurons within the CNS.
- Interneurons in the spinal cord are involved in descending pathways, segmental reflexes, and ascending pathways.
- These interneurons can be both excitatory and inhibitory.
- They form circuits (simple and complex) that
o integrate and modulate sensory and motor information and
o contribute to the production of coordinated movement.
-Some are involved in networks that produce rhythmic alternating patterns called central pattern generators. |
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Term
| Central Pattern Generators: |
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Definition
Networks of neurons that can endogenously generate rhythmic activity in the absence of sensory feedback.
Are thought to exist in a variety of species, including humans. They control diverse behaviors such as feeding, respiration and locomotion (walking, swimming and flying). |
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Term
| Respiration occurs because... |
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Definition
| The medulla of the brainstem contains networks of neurons and interneurons called the dorsal respiratory group neurons that discharge rhythmically without input from higher centers. The dorsal respiratory group interneurons constitute a CPG. Their output is to motorneurons in the ventral horn of C3, C4 and C5 that innervate the diaphragm (this group of motorneurons is sometimes called the phrenic motorneurons). The dorsal respiratory group neurons are responsible for the automatic control of rhythmic breathing. |
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Term
| Locomotion occurs because... |
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Definition
ventral horn motor neurons (lower motor neurons) must alternately excite extensor and flexor muscles in the leg. In addition, the activity of muscles in one leg must be coordinated with the activity of muscles in the other leg. So that when one leg is flexed, the other is extended and supporting your weight.
In the lumbar spinal cord (SC) intermediate region gray matter (around lamina VII) there are clusters of interneurons that project to the lumbar ventral horn motor neurons and are responsible for the rhythmic control of locomotion. These clusters are central pattern generator called the spinal pattern generators for locomotion. |
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Term
| In the intact central nervous system, spinal pattern generators for locomotion are normally strongly regulated by _______ input from upper motor neurons and sensory feedback from both cutaneous and propriospinal afferents. |
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Definition
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Term
| The motor cortex is essential in what part of walking? |
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Definition
| to control precise stepping that must be guided by visual input (e.g., walking across river stones). |
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Term
| The mesencephalic locomotor region (MLR) of the midbrain is in charge of what during walking? |
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Definition
| walking and controls walking speed. It is connected to the spinal cord via the reticulospinal pathways. |
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Term
| The cerebellum is involved in what for walking? |
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Definition
| In the coordination of stepping movements. |
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Term
| The basic rhythmic pattern for stepping is produced by... |
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Definition
Neuronal circuits (CPGs) contained entirely within the SC. The walking pattern must, however, be taught (trained, or re-trained) after injury to the CNS.
a. With training, some animals can walk on a treadmill following complete spinal cord transection. Supraspinal (brainstem and cerebral) structures are not needed to produce the basic motor pattern for stepping. |
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Term
| Local SC circuits can modify their output to produce a faster walking speed or altered gait pattern based on altered input from... |
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Definition
| proprioceptors and cutaneous receptors. |
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