Reflexes and Spinal Circuits

reflex

• highly predictable responses to specific stimuli generated by circuits in spinal cord and brain stem

• highly adaptable to changing circumstances in the environment

• best thought of in our terminology as a deep attractor well

monosynaptic myotatic stretch reflex

• most famous reflex

stretch reflex

Step 1:
Muscle is stretched → activates IA afferent (in muscle spindle)

• Cell body in dorsal root ganglion

Step 2:
Signal enters spinal cord

Step 3:
Ia afferent synapses DIRECTLY on alpha motor neuron

• No interneuron → monosynaptic

Step 4:
Alpha motor neuron fires
➡ Muscle contracts

At the same time (IMPORTANT)

• IA afferent also activates an inhibitory interneuron

• This inhibits the antagonist muscle

➡ Called reciprocal inhibition

➡ Only monosynaptic reflex = stretch reflex

Stretch → same muscle contracts, opposite relaxes

reflex grade 0

absent reflexes or A-reflexia, associated with flaccidity and paralysis

reflex grade 1+

hypo reflexia

reflex grade 2+

normal [best determined by cmparison between one side of the body and another]

reflex grade 3+

hyper-reflexia

reflex grade 4+

hyper-reflexia with clonus [unsustained]

reciprocal innervation/inhibition

• Alpha motor neurons of agonist and antagonist muscles are linked

• When the alpta motor neuron of thte agonist is acting, an inter-neuron inhibits the antagonist known as reciprocal inhibition

• This is actually part of a larger pattern of circuitry whereby this agonist/antagonist link can establish the stifness of the limb

flexile, pliant limb

lots or reciprocal inhibiliton yields a _

stiff limb

reciprocal facilitation yields co-contraction or a _

long propriospinals

• Run in anterior funiculus

• Coordinate motor pools of axial muscles (trunk)

• Act across many spinal segments

propriospinal pathways

• Connect different segments of the spinal cord

• Coordinate motor activity across levels

short propriospinals

• Run in lateral funiculus

• Coordinate motor pools of distal muscles (limbs)

• Act across fewer segments

clonus

a hyperactive stretch reflex loop, usually occurs distally i.e. wrist or ankle but can see in proximal muscles

Step 1:
Rapid stretch (e.g., quick dorsiflexion at ankle)

Step 2:
Muscle spindle in plantar flexors is activated

Step 3:
Ia afferents → spinal cord → activate α-motor neurons

Step 4:
Plantar flexors contract

Step 5:
That contraction stretches the dorsiflexors

Step 6:
Dorsiflexor spindles activate → reflex contraction occurs

Step 7:
Cycle repeats → alternating contraction

---

Result

• Repetitive oscillating contractions

• “Beats” of rhythmic movement

• Can be sustained or unsustained

flexor withdrawal reflex

a polysynaptic spinal reflex that produces ipsilateral limb flexion and contralateral extension in response to a painful stimulus

• Multi-segmental:
  Multiple spinal cord levels are activated to coordinate a full limb response

• Cross-cord communication:
  One side of the spinal cord communicates with the other side to maintain balance (crossed extensor reflex)

flexor withdrawl pattern

Step 1:
Painful stimulus (e.g., stepping on something sharp)

Step 2:
Pain receptors activate A-delta and C fibers

Step 3:
Signal enters spinal cord

Step 4:
Interneurons activate:

• Ipsilateral flexors contract

• Ipsilateral extensors are inhibited

👉 This pulls the limb AWAY from the stimulus

At the same time:

Opposite side of body:

• Extensors contract

• Flexors relax

👉 This keeps you balanced while you withdraw one limb

central pattern generators

• Flexible networks of local neural control

• Typically connected up and down and across thte cord or brainstem

• Involved in rhtythtmic movements thtat involve switching or alternatingbetween antagonistic movements

- Inspiraton expiraton

- Stance and swing in the gait cycle

- Swimming

• Often initated as “bursts” of actvity