Lecture 6 - Spinal Reflexes and General Features of Reflex Pathways

Question 1

what is the reflex arc

Answer 1

is a simple neuronal circuit in which a sensory neuron enters the CNS and after one or more synapses leads to an AP in a motor neuron and a detectable response in an effector organ such as a muscle

Question 2

how many synapses do reflexes involve

Answer 2

at least one, often more

Question 3

where do somatic reflexes start and end, where are the synapses

Answer 3

start and end in the PNS but the synapses are in the CNS

Question 4

the reflex arc - relevant anatomy; where are spinal reflexes present and what does their presence reflect

Answer 4

are present on both sides of the animal and occur along the length of the spinal cord. their presence reflects the activity in the PNS and or in the CNS at the level they enter and leave the spinal cord

Question 5

what are the 5 components of most reflex arcs? what is sometimes not present

Answer 5

sensory receptor
sensory neuron
interneuron (not always present)
motor neuron
effector organ

Question 6

reflexes - what does the effector system involve

Answer 6

somatic (skeletal muscle)
autonomic (smooth muscle)

Question 7

reflexes - integration site; where in the CNS are neurons and synapses involved

Answer 7

spinal cord = spinal reflexes
brain = cranial nerve reflexes

Question 8

reflexes - number of synapses/neurons (pathways)

Answer 8

monosynaptic (synapse directly on motor nerve)
polysynaptic (1 or more)

Question 9

types of relfex arcs and details

Answer 9

monosynaptic ipsilateral (same side)

multisynaptic ipsilateral (same side)

multisynaptic contralateral (opposite sides)

Question 10

myotatic (stretch) reflex how to do it, what type of relfex arc, example

Answer 10

tap patellar tendon, observe rapid extension of knee

monosynaptic ipsilateral

ex. patellar (knee jerk) reflex = muscle stretch stimulates muscle spindle –> sensory neuron synapses with motor neuron to same muscle

Question 11

flexor reflex (withdrawal reflex); how to do it, type of reflex arc, response

Answer 11

pinch toe and observe flexion in leg

multisynaptic ipsilateral

response to pain in limb = flexors in same limb contract

Question 12

crossed extensor reflex - how to do it, type of reflex arc, response

Answer 12

pinch toe and observe extension of the opposite leg

multisnaptic contralateral

response to pain in limb –> extensors in opposite limb contract and extend leg

Question 13

muscle spindle concept

Answer 13

Just know that your brain needs to know how tense your muscles are and muscle spindles (intrafusal fibre) measures that tension

• Sends signal from primary neuron into dorsal root ganglion / dorsal horn, synapse to alpha motor neuron
• At same time, secondary motor neuron picks that up and sends gamma motor signal to reset the process
• Alpha motor neurons are called lower motor neuron

Interestingly brain can inhibit lower motor neurons (alpha motor neuron) to stop the patellar reflex

Question 14

reflex arc in clinical context

Answer 14

reflex arcs form the basis of a very significant portion of the clinical neurologic examination of our patients. evaluating reflexes helps us determine where a lesion is in the nervous system and obtain some sense of how severe the problem is