spinal reflexes Flashcards
(20 cards)
whats a reflex
-a rapid, involuntary, yet stereotyped and co-ordinated response to a sensory stimulus
-Reflexes usually involve muscle contraction (but can include glandular responses, e.g. lactation in response to suckling)
what are reflexes that can be learned called
-Pavlovian
-but here we are considering only unlearned reflexes and specifically somatic reflexes, ie those involving the somatic
what are somatic reflexes also called
-spinal reflexes since they involve spinal cord circuitary
what are some properties spinal reflexes
-Require stimulation (ie not spontaneous)- therefore need sensory input
-Are quick - Suggests few synapses involved
-Are involuntary and automatic; you often are aware of them only as they happen and they are difficult to suppress.- Suggests little input from higher centres
-Stereotyped - occur the same way each time- suggests underlying circuitary very simple
what’s the Patellar Reflex
-knee jerk reflex
-sensory stimulus relayed by the primary afferent (axon from neurone based I the dorsal root ganglion) project into dorsal horn vis the dorsal route- Direct synapse to motor neuron
-is a Monosynaptic circuit: only one synapse involved
-Motor neuron then stimulates extensor muscle contraction
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whats the extensor muscles
-Extensors: muscle causing limb to extend
-Flexors: muscle causing limb to flex (bend)
whats The Stretch (Myotactic) Reflex
-The knee jerk is not a response to pain in the knee!!!
-The tap on the knee stretches the thigh extensor muscle and associated tendon and sets in motion a process to correct the stretching (so we dont fall over when we are standing up)
-This is important in maintaining body posture
-This is part of your proprioceptive system
whats an example of The Stretch or Myotactic Reflex
- Sensory fibres sense muscle stretch and send signals to spinal cord
-The special sensory receptor that detects muscle stretch is called the Muscle Spindle - Direct (monosynaptic) connection to motor neuron fires action potential which contracts the biceps muscle
- Simultaneously, a distinct connection to an inhibitory interneuron (black) inhibits the firing of motor neurons connected to the triceps, thus relaxing the antagonistic muscle. This is known as reciprocal inhibition (so no counteraction happens)
what are The Muscle Spindle
-Muscle spindles are among the body’s proprioceptors - sense organs that monitor the position and movement of body parts
-Muscle spindles are found in most striated muscle and are particularly abundant in muscles involved in fine motor control (e.g. hand).
-The muscle spindles are innervated by Ia sensory fibres
-These provide feedback to the motor neurons innervating the surrounding muscle (properly called alpha motor neurons) on the amount of muscle stretch that is occurring (as we have seen in the ‘knee-jerk’ reflex)
do muscle spindles have muscles
-yes
-Muscle spindles are also innervated by axons from gamma motor neurons
-These stimulate the intrafusal (muscle) fibres to adjust the tension in the spindle as the extrafusal (muscle) fibres of the surrounding muscle contract (so that the muscle spindle is never slack).
whats kinesthesia
-a viral infection destroyed all of the nerves that provided Ian Waterman’s sense of touch and all of the nerves attached to muscles and tendons that provide a sense of joint and limb position.
-He lost the ability to co-ordinate any kind of movement unconsciously and was effectively bed-bound, unable to get up.
-Waterman had lost his proprioception, the sense of the position of parts of the body, relative to other neighbouring parts of the body
-Through trial and error over three years, Waterman taught himself how to move again by consciously controlling and visually monitoring every action
-To this day, if the lights go out unannounced, he crumples to the floor, unable to budge until they come back on.
-Along with proprioception goes a sense of your body moving in space
whats the Golgi Tendon Organ
-The GTO is another kind of proprioceptor
-GTOs detect muscle tension due to muscle contraction, not muscle stretch
-Activation of GTO sensory (Ib) afferents leads to activation of inhibitory interneurons which, in turn, inhibit alpha motor neurons that innervate the same muscle
-the Golgi Tendon Reflex is a negative feedback circuit that regulates muscle tension and protects the muscle (and tendon) from damage when large forces are generated.
-Like the muscle spindle, it also regulates muscle contraction to maintain muscle length, but in this case it prevents the muscle contracting too much (see YouTube video)
-As we will see later, both this and the stretch reflex are under control of higher centres
whats the Flexor (Withdrawal) Reflex
-Flexor reflex is a quick contraction of flexor muscles to withdraw a limb from an injurious stimulus (e.g. heat or cut)
-It results from the activation of nociceptive sensory receptors or nociceptors
-Despite speed of response, this is a polysynaptic reflex:
-Activation of multiple excitatory interneurons sustains the response:
=’Parallel after-discharge circuit’
-polysynaptic circuit- - activation of multiple excitatory neurones
whats the Parallel after-discharge circuit
-Assume time taken to cross each synapse is the same.
-Therefore, stimulus initiated by A will take different times to reach output neuron Z.
-Result is that initial signal is sustained over extended period
-several different routes with many synapses
whats the Flexor (Withdrawal) Reflex II
-Despite speed of response, this is a polysynaptic reflex:
=Activation of multiple excitatory interneurons sustains the response:
=’Parallel after-discharge circuit’
As in the stretch reflex, inhibitory interneurons are also activated to relax extensor muscles (reciprocal inhibition again)
-Because rapid withdrawal of limb (esp. leg) may lead to imbalance, flexor reflexes often include a contralateral element.
-This crossed extensor reflex provides postural support during limb withdrawal
whats the Crossed Extensor Reflex
-Extensor muscles at the front need to relax.
-Flexors at the back need to contract
-Importantly, the contralateral signal activates muscle movements in the opposite leg that will counteract the weight of the other leg being lifted
what does ipsilateral mean
-same side
what does contralertal mean
-opposite side
why may Simple reflexes may underlie repetitive behaviours: locomotion
-In individual limbs, extensors and flexors are alternately active or inhibited as animal walks
-Activity of these muscles in different limbs is also co-ordinated between the limbs so that legs alternate
-Surprisingly, the basic alternation of activity, and the co-ordination between legs, continues even if spinal cord connections to the brain are severed]
-Implies the presence of local circuits that can generate the pattern of alternating flexion and extension.
-these are known as central pattern generators
how To dissect proprioception at the molecular level
-Nematode worm – C. elegans swimming through Petri dish
-Mutagenesis identified a mutant with ‘loopy’ swimming
-Due to loss of TRP-4* ‘stretch’ receptor in DVA neuron
-DVA neuron detects body bending directly using the TRP-4 mechanosensor
-DVA appears to inhibit motor neurons, allowing alternating muscle contraction
