Reflexs
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
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