Spinal Reflexes Flashcards
What is a reflex?
A rapid, involuntary, yet stereotyped and co-ordinated response to a sensory stimulus
What do spinal reflexes require?
Stimulation
The stretch (myotactic) reflex
- Knee jerk isn’t a response to pain in knee
- The tap stretches the thigh extensor muscle and associated tendon sets in motion to correct the stretching
- Maintains body posture
The myotactic reflex steps
- Sensory fibres sense muscle stretch and send signals to spinal cord
- Direct connection to motor neuron fires action potential which contracts 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 are muscle spindles innervated by?
Ia sensory fibres
What does the Golgi Tendon Organ do?
Detects muscle tension due to muscle contraction not stretch
Golgi Tendon Organ steps
- GTO sensory (Ib) afferents activated
- Leads to activation of inhibitory interneurons which inhibit alpha motor neurons that innervate the same muscle
- Negative feedback circuit that regulates muscle tension and protects the muscle from damage
Flexor (withdrawal) reflex
- Quick contraction of flexor muscles to withdraw a limb from an injurious stimulus
- Activates nociceptive sensory receptors
Polysynaptic reflex
One or more interneurons connect afferent (sensory) and efferent (motor) signals
Parallel after-discharge circuit
- Time taken to cross each synapse is the same
- Stimulus initiated by a point will take different times to reach output neuron if there are many pathways with different numbers of synapses to cross
- Results in initial signal being sustained over extended period of time
Because rapid withdrawal of limb may lead to imbalance, ____ reflexes often include a ______ element.
- Flexor
- Contralateral
Crossed extensor reflex
- Provides postural support during limb removal
- Due to contralateral motor neurons to extensor being excited
If the spinal cord connections to the brain are severed what happens to the legs?
Basic alternation of activity, and the co-ordination between legs, continues
Central pattern generators
Presence of local circuits that can generate the pattern of alternating flexion and extension
Monosynaptic neuron
When a reflex arc consists of only two neurons in an animal (one sensory neuron, and one motor neuron)
