7 reflexes

pgy 452 exam2

what can a spinal cord do?

spinal reflexes

unlearned motor response to sensory stimulus

spinal transection

lose sensation and voluntary movement below the cut

spinal reflexes components

• muscles

• motoneurons

• sensory input from the body

• spinal interneurons

abnormal reflex

a reflection of the dysfunction of some component of the motor system

because circuitry电路 for a reflex may be present in the spinal cord, but reflex strength or gain may be modulated by descending pathways - stronger, weaker, suppresses

stimulus

stretch of a muscle

response

contraction of the same muscle, clinically tested by tendon tap which stretches the muscles

afferent (sensory input)

1A fiber from muscle spindle

efferent

α motoneuron to same muscle, short latency延迟 reflex

monosynaptic reflex

the stretch reflex underlies muscle tone, resistance to passive stretch of a muscle

reciprocal innervation

as the agonist contracts, its antagonist relaxes, circuit branch of 1A synapses on inhibitory interneuron in spinal cord that inhibits motoneuron innervating the antagonist muscle

flexion reflex

stimulusL noxious (damaging, painful) stimulus to skin

response: contraction of flexor and relaxation of extensors at every joint in the limb

not get into the stimulus but to spinal cord, still feel the pain

flexion crossed-extension reflex

leg on the other side extends to increase postural support

double reciprocal innervation

pattern of innervation

head up

forelimbs extend, hindlimbs flex

head down

forelimbs flex, hindlimbs extend

head to side

ipsilateral limb extend, contraclateral limb flexes

descending control of reflexes

under the control of higher levels of the motor system

ex: flexion flex modulation屈区反射调节 - abnormal reflexes

spinal shock

• absence of reflexes followed by spinal transection

• reflexes gradually return

• more complex the CNS the longer period of spinal shock

• endpoint may be hyperreflexia反射亢进

• unknown mechanism

Babinski sign巴宾斯基征

damage to motor cortex or PT

normal in newborns

low descending control

• flaccidity松弛

• hypotonnia肌张力低下

• down syndrome

too high descending control

• rigidity刚性

• hypertonia肌张力亢进

• cerebral palsy (CP)脑瘫

solution: cut dorsal roots, to eliminate excess sensory input

decorticate rigidity-seen

lesions of motor cortex:

1. increase tone - flexors of arms and extensors of legs

2. clonus阵挛 - more than one contraction for a single stretch

rigidity in cerebral palsy

results from damage to any component or combination of components of the motor system, and it reflects the degree of involvement of cortex or cerebellum or basal ganglia

shortening of tendons so the limbs become immobile, and lead to arthritis关节炎

treatment for cerebral palsy

• surgical: selective dorsal rhizotomy

• medical: injection of baclofen into spinal cord to inhibit the stretch reflex