Spinal Cord Motor Activity
Classification of Reflexes "
1- Somatic reflexes : that you are aware of them
2- Autonomic Reflexes : that control visceral organs.
Examples of spinal reflexes, involving spinal nerves and the spinal cord, include: 1- extensor reflex: leg proprioceptors trigger limb extension 2- myotatic (stretch) reflex: muscle stretch is resisted by reflex contraction of the muscle 3- withdrawal (flexion) reflex: limb flexes to withdraw from a noxious stimulus
A spinal reflex is a stereotyped, automatic motor reaction to an input signal. The monosynaptic myotatic stretch reflex is the most crucial reflex for the maintenance of the erect body posture in humans. For the well-known knee-jerk response, there is latency of around 30 ms between striking (stretching) the tendon of the quadriceps and the muscle contraction. The reflex involves a number of segments in the SC.
Factors that contribute to this latency in the reflex arc are:
1- Speed of transduction at the sensory receptor. This is most rapid when the receptor is spontaneously active and is tuned to the dynamic range. 2- Conduction speed in afferents to the CNS. Speed depends on fiber size and myelination. Larger fibers conduct more rapidly, but at the expense of space. 3- Synaptic delay and number of synapses involved in the pathway. While the interval between arrival of the presynaptic AP and start of EPSP is typically 0.5 ms, it takes a few ms before an AP is evoked in the postsynaptic neuron. 4- Central integration.
a. Spatial summation. 90% of the synapses are on the MN dendrites. The remaining 10% are on the soma, which have the highest priority. The unitary EPSP at a single synapse is about 0.2 mV in amplitude. Depending on the MN, sufficient of these must be activated to cause a depolarization of 5-10 mV at the axon hillock and thereby evoke an AP. b. Temporal summation results from activity arriving at different latencies. Temporal summation is very dependent upon the passive membrane properties (time constant etc.) of the MN. 5- Speed of conduction in the motor axon efferents (size and myelination).
1- Reflex responses are determined by interneurons which “hard-wire” afferent input to efferent output. Interneurons organize efferent neurons (motor units) into meaningful movement components, which can be utilized by either spinal input or descending pathways.
2- Since "voluntary movement" and "involuntary reflex/reaction" compete for control of the same interneurons circuits, they cannot be independent on one another. Thus, brain activity will influence spinal reflex responses, making reflex evaluation an interpretive art. (see the end of this lecture)
Diagram of A reflex Arc
Withdrawal Reflex (Flexion Reflex) and Crossed Extensor Reflex
The po1ysynaptic flexor reflex serves important protective functions. One of its purposes is to achieve a rapid withdrawal of a limb in response to painful cutaneous stimuli. To maintain position the flexor withdrawal reflex is usually accompanied by extension of the opposite limb through action of the crossed extensor reflex. Receptors : free nerve endings in the skin. Afferent arch : Adelta and C fibers which terminate in the marginal zone (Lissauer) and in the dorsal part of the central gray matter. Central mechanism: the central processes of the primary sensory neurons synapse with interneurons and funicular neurons that in turn innervate ipsilateral flexor and crossed extensor muscles.
Like the other reflex pathways, interneurons in the flexion reflex pathway receive converging inputs from several different sources, including cutaneous receptors, other spinal cord interneurons and descending pathways. Although the functional significance of this complex pattern of connectivity is uncertain, changes in the character of the reflex following damage to descending pathways...
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