Lecture 19 Flashcards
What is the nature of reflexes?
A reflex is 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
Can be learned (Pavlovian)
- but here we are considering only unlearned reflexes and specifically somatic reflexes i.e. those involving the somatic nervous system (visceral reflexes will be considered wlsewhere)
Somatic reflexes are also called spinal reflexes since they involve spinal cord circuitary
What do spinal reflexes require?
Stimulation i.e. not spontaneous
- therefore need sensory input
Why are spinal reflexes quick?
Suggests few synapses involved
Spinal reflexes are involuntary and automatic (you often are aware of them only as they happen and they are difficult to suppress). What does this suggest?
Suggests little input from higher centers (in fact, are intact even when spinal cord severed)
What does it mean when spinal reflexes are stereotyped?
Occur the same way each time - suggests underlying circuitary is very simple
Is the Patellar Reflex monosynaptic or polysynaptic?
Monosynaptic
Is the knee jerk a response to pain in the knee
No!
- the tap on the knee stretches the thigh extensor muscle and associated tendon and sets in motion a process to correct the stretching
- This is important in maintaining body posture (part of the proprioceptive system)
- better illustrated by alcohol…
What happens during the stretch (myotactic) reflex?
- Sensory fibers sense muscle stretch and send signals to spinal cord
- Direct (monosynaptic) connection to motor neuron fires action potential which contents the biceps muscle
- Simultaneously, a distinct connection to an inhibitory interneuron inhibits the firing of motor neurons connected to the triceps, thus relaxing the antagonistic muscle. This is known as reciprocal inhibition
What is the special sensory receptor that detects muscle stretch?
Muscle spindle
What are muscle spindles?
Proprioceptors
- sense organs that monitor the position and movement of body
Where is the muscle spindle found in?
Most striated muscle and are particularly abundant in muscles involved in fine motor control (e.g. hand)
What are the muscle spindles innervated by?
Ia sensory fibers
What do muscle spindles provide?
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)
How do the muscle spindles have muscles?
Muscle spindles are also innervated by axons from gamma motor neurons
These stimulate the intrafusal (muscle) fibers to adjust the tension in the spindle as the extrafusal (muscle) fibers of the surrounding muscle contract (so that the muscle spindle is never slack)
- gamma motor neurons are constantly firing to maintain the tone of the fibers
- extrafusal fibers are constantly being contracted
Why can’t Ian Waterman feel whether or not he is standing up?
In 1972, 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 bedbound, unable to get up
Water had lost his proprioception, the sense of the position of parts of the body, relative to other neighboring parts of the body
Through trial and error over three years, Waterman taught himself and how to move again by consciously controlling and visually monitoring every action
To this day, if the lights go out unannounced, he crumbles to the floor, unable to budge until they come back on
ALong with proprioception, goes a sense of yours body moving in space. This is called kinesthesia
