Lab 3 Prelab Flashcards
Define reflex.
A patterned, involuntary response to a stimulus
What does it mean for a response to be “graded”?
The strength of the response is proportional to the strength of the stimulus
Are most reflexes learned or unlearned? Exceptions?
- Unlearned
- Exception: acquired reflexes like muscle memory
What is a monosynaptic reflex?
A single synapse b/t afferent and efferent neurons
What is a polysynaptic reflex?
Multiple synapses b/t afferent and efferent neurons
What are reflex arcs?
A pattern of defined segments which describe the components of a reflex
List the components of a reflex arc.
- Recepter
- Afferent nerve fibers
- Integrator
- Efferent nerve ribers
- Effector
What is the function of the receptor?
Converts stimulus to an electrical signal, activating an AP in the afferent neuron
What is the function of the afferent nerve fibers?
Propagate AP to integrator
What is the function of the integrator?
CNS (spinal cord or brain) areas that determine how a response to afferent should be made
What is the function of the efferent nerve fibers?
Propagate AP from integrator to effector
What is the function of the effector?
Receives the signal and carries out the response
What causes a monosynaptic reflex?
Stretching of the muscle spindle
Describe the mechanism of a stretch reflex.
- Tap deflects tendon, stretching muscle spindle
- Excites 1A afferents
- Dorsal root of spinal cord integrates signal
- 1A axons make a monosynaptic connection to an alpha motor neuron
- Activated alpha motor neuron (efferent) synapses w/ muscle fibers (effector), causing contraction
What is a muscle spindle? Function? Characteristics?
- Special sensory receptors that measure length and rate of stretch of a muscle
- Modified intrafusal muscle fiber
- Encircled by 1A afferents
- Runs parallel to muscle fibers
Typically, are muscle fibers extrafusal or intrafusal?
Extrafusal
What is the Hoffman Reflex? What part of the reflex arc does it lack?
- Electrically induced monosynaptic reflex
- Doesn’t have a receptor component to its reflex arc
In lab, what nerve will we be directly stimulating to produce the Hoffman Reflex? Why?
Tibial nerve b/c:
- bypasses muscle spindle
- contains both 1A afferent and alpha motor neurons, so we’ll see 2 types of waves
What causes an H-wave?
1A afferent activation synonymous w/ normal route of the spindle reflex
What causes the M-wave?
Directly activating alpha motor neurons
Typically, do 1A afferents or alpha motor neurons have a lower threshold to direct stimulation? Why?
- 1A afferents have a lower threshold to direct stimulation b/c of their large diameter
- Alpha motor neurons are smaller in diameter and require strong electrical stimulation to be excited to threshold
Define antridromic.
Reverse direction of AP travel toward the cell body of the neuron
Define orthodromic.
Forward-direction of AP travel toward the axonal terminal
How does increasing voltage magnitude affect the H and M waves?
Increase size of both waves at first, then H-wave decreases b/c antidromic APs start to decrease the voltage
Define latency.
Time period b/t stimulation and response
What is a Renshaw cell?
An inhibitory interneuron utilizing glycine
What is an axon collateral? What does it synapse with?
Branch-off of the alpha motor neuron that synapses w the Renshaw cell
When is the Renshaw cell stimulated?
When its alpha motor neuron is activated at high frequency
What is recurrent inhibition?
Single interneuronal synaptic self-inhibition used during the Renshaw circuit to prevent overstimulation of its own circuit at high frequencies
What happens when the Renshaw cell synapses w its own motor neurons and others nearby?
Induces IPSPs
How do low frequencies affect the H wave? High frequencies?
- Low frequencies: alpha motor neuron recovers from the IPSP before the next stimulus –> a peak is seen
- High frequencies: can’t recover from recurrent inhibition, is less responsive to 1A stimulation –> peak is decreased
How does holding at a moderate voltage and increasing frequency affect the M-wave? Why?
No affect on M-wave b/c all inhibition occurs upstream of its pathway
What is the purpose of having more than one interneuron b/t the afferent and efferent branches for a polysynaptic reflex?
Interneurons allow the integrator to collect multiple sources of info together before sending out a final decision to the efferent
What is the example of a polysynaptic reflex we will be observing in lab?
Vestibular-Ocular Reflex
What is the Vestibular-Ocular Reflex involved in?
Involved in sensing angular acceleration and deceleration, balance, and centering of visual field
What is the primary sensory structure of the VOR?
Semicircular canals
How many semicircular canals do we have? How are they oriented?
3 orthogonally oriented canals in each ear
- X, Y, Z axis = one in each plane
- situated at a 30° posterior angle to the head
What are hair cells?
Mechanoreceptors able to detect angular acceleration, axial movement caused by flow of endolymph
What is endolymph? Function? Location?
Fluid medium in the semicircular canals that detects and measures motion
What branch off of hair cells? Heights?
- Kinocilium (tallest branch)
- Stereocilia (shorter branch)
What fibers do cells synapse w/ in the semicircular canals?
Vestibular nerve fibers
What are cilia?
Bundles of hair cells
What is a cupola?
Cilia embedded in a gelatinous mass
When endolymph moves in the semicircular canals, what does it bend?
Hair cells’ kinocilia and stereocilia
What are tip links? Location? Function?
Mechanoreceptors on stereocilia that open K+ channels in response to bending
What is the K+ content of endolymph? Significance?
High K+ content compared to inside the cells, so opening mechanoreceptor channels cause K+ influx
What happens when stereocilia are bent toward the kinocilium? Away?
- Toward = increased K+ influx –> depolarization
- Away = decreased K+ influx –> hyperpolarization
Describe the bending of cilia and movement of endolymph during acceleration and deceleration.
- Acceleration: endolymph is extremely viscous, so it doesn’t move at first –> cilia bend in opposite direction of spin
- Constant speed: endolymph catches up w/ hair cells and they move together –> cilia no longer being bent
- Deceleration: endolymph moves faster while hair cells start to slow down –> cilia bend toward direction of spin
Describe the components of the VOR reflex arc.
- Hair cells (receptor) sense angular momentum (stimulus)
- APs propagated along the vestibular nerve (afferent) to the brainstem (integrator)
- Motor neurons (efferent) to the lateral and medial eye muscles (effectors) are either activated or inhibited according to the response program
- Nystagmus (response)
Define nystagmus.
Alternating slow and fast pattern of eye movements in response to changing acceleration of the head
Define saccades.
Fast component which occurs in the direction of acceleration or perceived motion
When do slow eye movements occur?
Opposite direction of acceleration or perceived motion
What is the best-known example of a monosynaptic reflex?
Knee jerk or stretch reflex
Why is the latency of the M-wave much shorter than the H-wave?
Motor neuron is directly stimulated and the signaling pathway is much shorter than that of the H-wave
What do defects in the VOR lead to?
- Dizziness
- Loss of balance
- Impaired postural control
Why is direct stimulation less efficient thatn neuronal stimulation?
Neuronal stimulation takes advantage of the structure of motor units
