What is a reflex?
an involuntary, stereotyped response to sensory stimuli
What are the three things a reflex must have?
1) receptor structure
2) afferent neuron
3) efferent neuron
What are interneurons?
they reside entirely within the CNS
What are three principal spinal (segmental) reflexes involving the limbs?
They all originate from different sensory receptor and have different intraspinal circuitry
1) muscle stretch reflex (deep tendon reflex)
2) golgi tendon organ reflex
3) flexor withdrawal reflex
What kind of reflex causes a muscle to contract in response to being stretched?
muscle stretch reflex or deep tendon reflex (DTR)
What is the name of the sensory receptor in the muscle?
the muscle spindle
What is a monosynaptic reflex?
when only 2 neurons and 1 synapse is involved.
This is what a muscle stretch reflex is.
How does a knee-jerk reflex (a DTR) work?
the gamma loop
1) patellar ligament is tapped
2) muscle spindles are stretched
3) dorsal root ganglion are excited
4) alpha motor neuron excited (in the ventral horn)
5) quadraceps contracted
6) sensory fibers also innervate inhibitory muscles of antagonist muscles
What often happens to a DTR when a peripheral nerve injury occurs?
hyporeflexia or areflexia
What happens to a reflex when descending pathways are damaged?
they become hyperactive;
also called hyperreflexia;
this is because the DESCENDING neurons INHIBIT spinal reflexes via inhibitory interneurons
Which spinal cord segments are involved with the tricep reflex?
C6 and C7, radial nerve
Which spinal cord segments are involved with the bicep reflex?
C5 and C6, musculocutaneous nerve
Which spinal cord segments are involved with the knee-jerk reflex?
L3 and L4, femoral nerve
Which spinal cord segments are involved with the the ankle jerk reflex?
S1 and S2, tibial nerve
What is responsible for the muscle stretch reflex and normal muscle function?
the gamma loop
What controls intrafusal muscle fibers within the muscle spindle?
What controls the degree of activation of the alpha motor neuron?
afferents from the muscle spindle
What is the clinical test to assess muscle tone?
passively extending and flexing the limbs
How is abnormal activity of the gamma system manifested in a patient?
stiffness or hypertonia (increased muscle tone);
this is called spasticity
What might a patient with a spinal cord injury or stroke present with?
2) hyperactive reflexes
What does a patient with an upper motor neuron lesion present with?
spastic paralysis with hyperreflexia
2) hyperactive reflexes
What does hyperactivity of the stretch reflex result in?
clonus- a prolonged oscillation of contraction and relaxation
What is flexor withdrawal reflex?
cutaneous skin receptors elicit a withdrawal due to a painful stimulus
e.g. a hot stove, broken glass
How does a flexor withdrawal reflex work?
1) sensory neurons excite interneurons
2) interneurons synapse with flexor muscles to withdraw
3) antagonistic muscles are inhibited by inhibitory interneurons
What occurs as musculature of the opposite side of the body is activated in a flexor withdrawal reflex?
crossed extension reflex, which occurs via excitation of contralateral extensor muscles and inhibition of flexor muscles
e.g. stepping on broken glass
What is the golgi tendon reflex?
It's the inverse of the muscle stretch reflex in that it causes relaxation, not contraction
1) localized between muscle and tendon
2) provides info about the tension in the muscle
3) the afferent fiber of the golgi tendon innervates inhibitory interneurons
4) activation of afferents causes relaxation
How is the muscle spindle put together?
1) a sheath of CT surrounding 10-12 intrafusal muscle fibers
2) the main skeletal muscle fibers are called extrafusal fibers
What affects the sensitivity of the stretch receptor within he sheath of the muscle spindle?
the amount of tension(tone) of the intrafusal muscle fibers;
the tone is regulated by input from gamma motor neurons