Motor Functions of Spinal Cord and Reflexes Flashcards
What is the neuronal organization of the spinal cord?
Sensory neurons (dorsal root ganglion): transmit info to higher centers
Motor neurons (anterior horn): directly innervates skeletal muscle fibers
- alpha MNs excite extrafusal muscle fibers, muscle contraction
- Gamma MNs excite intrafusal fibers ( muscle sensory receptors)
Interneurons: local neurons that have connections with sensory & motor neurons, they can be inhibitory & excitatory
What are muscle spindles?
are distributed throughout the muscle belly (midportion of intrafusal fibers)
Sense muscle length (stretched, contracted) and rate of change of length
sensory signals are transmitted through
- 1a fiber ( primary sensory ending)
- 11 ( secondary sensory ending) fiber
What are Golgi tendon organs?
Located in the muscle tendon
Sense tendon tension and rate of change of tension
Sensory signals are transmitted through 1b fibers
What is the function of muscle spindles?
To provide info on the position of body parts ( proprioception)
Describe 1a fibers.
Primary sensory endings of the muscle spindle:
terminate on the nuclear bag and nuclear chain fibers
Respond to the rate of change in muscle length (dynamic) and to the degree of stretch of the muscle (static)
Describe type 11 fibers
Secondary sensory endings of the muscle spindle:
terminate on the nuclear chain fibers
Respond mostly to the degree of stretch of the muscle (static response)
What is Alpha-Gamma coactivation?
It causes the tension of the muscle spindles to be maintained as the muscle shortens, thus sensitivity to stretch is maintained
What are Golgi Tendon Organs (GTO)?
They are encapsulated sensory receptors through which tendon fibers pass.
When are GTOs stimulated?
When muscle fibers are ‘tensed’ by the contraction or the stretching of the muscle (sensitive to muscle tension).
They react intensively when muscle tension suddenly increases (dynamic) or settles down rapidly to a lower level of steady state tension (static)
Which fibers transmit the signals produced by GTOs?
1b fibers, they inhibit indirectly alpha motor neurons at the spinal cord to allow the muscle to relax and therefore release the tension on the tendon
What are spinal cord reflexes?
Mechanism by which sensory impulses are automatically converted into a motor response.
Involuntary and rapid response controlled by the spinal cord’s integration centers.
Protective mechanism to the body
What is a stretch reflex?
sudden stretch of the muscle activates the muscle spindles, the associated sensory fibers transmit impulses at the spinal cord level, the fibers synapse directly with alpha motor neurons causing the muscle to contract
Is the stretch reflex a monosynaptic reflex?
Yes
Give an example of the sretch reflex
Patellar reflex, knee jerk reflex
What is the function of the Golgi tendon reflex?
To relieve excess tension on the tendon
What are the roles of the sensory fibers (1b) in the golgi tendon reflex?
sensory neuron synapse with inhibitory interneurons that inhibit alpha motor neurons and cause the contracted muscle to relax
Sensory neurons synapse with excitatory interneurons that excite alpha motor neurons of the antagonist muscle and causes it to contract
When do GTOs respond?
GTOs respond to increased tension during extrafusal muscle contraction
Extrafusal fibers are activated by which type of motoneuron?
Alpha motor neurons
What is the flexion reflex? (also known as the withdrawal reflex)
The flexion is caused by a painful stimuli that causes automatic withdrawal of the threatened body part (touching a hot stove)
Wjat do the sensory fibers do in the flexion reflex?
they synapse with interneurons that excite motor neuron of the flexor muscle to contract it and inhibit motor neurons of the extensor muscle to relax it
What is the flexion and crossed extensor reflex?
Cause by a painful stimuli that make you flex you knee for example and contract the other leg to keep your balance (think nail in foot)
Is the stretch reflex after a spinal cord injury more pronounced than in control groups?
Yes, it is
