NPB 101L Lab 3 Human Reflexes

Objective: Study reflexes in human subjects, including:

- Components of a reflex arc

- Monosynaptic Reflex

- Poly-synaptic Reflex: the Vestibulo-ocular reflex

Setup/procedures

Record H and M waves at different voltages and frequencies using EMG recording to examine the Hoffman Reflex

Run 4 trials in spinning chair to examine various effects of Vestibulo-ocular reflex

What is a reflex?

a patterned, involuntary response to a stimulus

Characteristics of reflex

patterned, involuntary, graded, most are unlearned

What does it mean that the response is graded?

the strength of the response is proportional to the stimulus

Most reflexes are learned or unlearned?

unlearned

monosynaptic reflex

Reflex pathway with only one synapse between the sensory and motor neurons (ex: knee-jerk).

Polysynaptic reflex

multiple synapses between afferent and efferent neurons

What are reflex arcs?

reflex arcs are patters of defined segments which describe the components of the reflex

5 components of a reflex arc

receptor, afferent nerve fibers, integrator, efferent nerve fibers, and effector.

Receptor

converts stimulus to an electrical signal, activating an AP in the afferent neuron.

Afferent nerve fibers

propagates AP to the integrator

Integrator

CNS (spinal cord or brain) areas that determine how a response to afferent should be made

Efferent nerve fibers

propagates AP from the integrator

Effector

receives the signal and carries out the response

How is the stretch/monosynaptic reflex activated and carried out?

by stretching of the muscle spindle

The tap deflects tendon, which stretches the muscle spindle.

This excites 1A afferents, the sensory nerve. The dorsal root of the spinal cord integrates the signal. 1A axons make a monosynaptic connection to alpha-motor neuron. The activated alpha-motor neuron (efferent) synapses with muscle fibers (effector), causing contraction.

Muscle spindle function

Measures the length and rate of stretch of the muscle

How is the muscle spindle positioned?

the muscle spindle runs parallel to gastrocnemius muscle fibers.

What are muscle spindles made of?

Muscle spindles are a modified (intrafusal) muscle fiber. Typical muscle fibers are extrafusal. The spindle is encircled by 1A afferents.

What is the Hoffman Reflex (H-Reflex) and how is it stimulated?

The monosynaptic reflex but is electrically induced instead of tapping on the tendon to stretch the muscle spindle.

In lab, we will be directly stimulating the tibial nerve, and therefore bypassing the muscle spindle (receptor).

Components of the tibial nerve

1A afferent and alpha motor neurons

H-wave

caused by 1A afferent activation (synonymous with normal route of the spindle reflex)

M-wave

caused by directly activating alpha motor neurons

Why do 1A afferents typically have a lower threshold to direct stimulation than alpha-motor neurons? Why might this be?

Maybe because 1a afferents have a larger axon diameter than alpha motor neurons.

Antidromic Action Potential

Conduction of AP in direction opposing normal physiological direction, if it is large enough it will cancel out the AP. This is only seen in direct stimulation.

Orthodromic Action Potential

Conduction of AP in normal physiological direction (forward-direction)

Increasing voltage magnitude should cause the H-wave and M-wave to appear, usually in that order. Physiology varies, so many people see the M-wave first. Continuing to increase the voltage should increase the size of both waves at first. then H-wave decreases, why?

Maybe because the M wave is likely not seen at a moderate intensity?

latency

Time period between stimulation and response.

The time it takes for the stimulus to reach

Renshaw cell and its mechanics

an inhibitory interneuron utilizing Glycine.

An axon collateral or branch off of the alpha-motor neuron synapses with the renshaw cell. The RC is stimulated when its alpha-motor neuron is activated at high frequency. The Renshaw cell then synapses with its own motor neuron and others nearby to induce IPSPs.

When is the renshaw cell stimulated?

The renshaw cell is stimulated when the a-motor neuron that synapses with the renshaw cell is activated at high frequency. The Renshaw cell then synapses with its own motor neuron and others nearby to produce IPSP's.

Explain the Renshaw Circuit

A Renshaw cell is an inhibitory interneuron utilizing Glycine. An axon collateral or branch-off of the a-motor neuron synapses with the renshaw cell (RC). The RC is stimulated when its a-motor neuron is activated at high frequency. The RC then synapses with its own motor neuron and others nearby to induce IPSP's. Recurrent inhibition is a single interneuronal synaptic self-inhibition.

H-wave

Caused by 1A afferent activation. This is synonymous with normal route of the spindle reflex.

M-wave

caused by directly activating a-motor neurons. Basically starts the second half of the reflex.

Which wave would appear first, M or H?

Usually the H wave would appear first became the 1A afferents have a lower threshold to direct stimulation than a-motor neurons. However, since physiology varies, some people will see the M wave first.

Holding at a moderate voltage and increasing frequency should cause H-wave to decrease. Why?

At low frequencies, the a-motor neuron recovers from the IPSP before the next stimulus so a peak is seen. At higher frequencies, it can't recover from recurrent inhibition, is less responsive to 1A stimulation, so the peak is decreased. This should not affect the M-wave (if it is still visible at the voltage you use). Why?

polysnaptic reflex

one or more interneurons are between the afferent and efferent branches. In this case, interneurons serve to allow the integrator to collect multiple sources of information together before sending out a final "decision" to the efferent. Involved in sensing angular acceleration and deceleration, balance, and centering of visual field.

The semicircular canals

primary structure of the VOP

3 orthogonally oriented canals in each ear. X, Y, Z axis, one in each plane. Are situated at a 30-degree posterior angle to the head. X-axis becomes horizontal when head is tilted forward

Hair cells

the sensory receptors for angular acceleration

Contain mechanoreceptors able to detect axial movement caused by flow of endolymph

kinocilium

This is a large hair cell important in the detection of the head's position. Whether the displacement of the stereocilia is towards or away from this determines whether it is excitatory or inhibitory.

Endolymph

the fluid medium that detects/measures motion

stereocilia

shorter hair cells that branch off membrane

Kinocilium & stereocilia are interlinked by

mechanoreceptors

Hair cell synapses with

vestibular nerve fibers

cupola

Hair bundles (cilia) are embedded in a gelatinous mass called the cupola

Mechanotransduction

changes in acceleration of the head cause movements of the endolymph filling the semicircular canals. hair cells' kinocilia and stereocilia are bent by the endolymph. Tip links on stereocilia are mechanoreceptors that open potassium channels in response to bending. Bending stereocilia toward kinocilium causes increased potassium influx. Bending stereo away from kino causes decreased potassium influx

Endolymph has a very high potassium content compared to inside the cells so opening mechanoreceptor channels causes potassium influx

Hair cells (receptor) sense

angular momentum (stimulus)

AP's are propagated along the __________ nerve (afferent) to the brain stem (integrator)

vestibular nerve

Motor neurons (efferents) to the lateral and medial eye muscles (effectors) are either activated or inhibited according to

response program

nystagmus

involuntary, jerking movements of the eyes,

Alternating slow and fast pattern of eye movements in response to changing acceleration of the head.

Saccade

the fast component of eye movement which occurs in the direction of acceleration or perceived motion.

Slow eye movement occurs in the opposite direction of acceleration or perceived motion