Lecture 3 ; NS Physio Part 2 / Special Senses Flashcards
When the action potential is propagated down the axon, where does it next arrive ?
The axon terminal
Aka
Synaptic end bulb
Transmission of a neuronal impulse occurs across a
Synapse
Aka neuronal junction between the presynaptic neuron to the post synaptic neuron
After the action potential reaches axon terminal on presynaptic membrane, what is the next step?
Voltage gated calcium channels open and Calcium enters the axon terminal
After calcium enters the presynaptic end bulb after having propagated action potential what happens?
Calcium entry causes neurotransmitter containing vesicles to release their contents via exocytosis
After action potential has reached synaptic end bulb and calcium has caused vesicles to release their neurotransmitters , what happens?
Neurotransmitter diffuses across synaptic cleft and binds to specific receptors on the post synaptic membrane.
Other than Na + what other ions are higher on the outside of a cell?
Calcium and chloride
After neurotransmitter is released from post synaptic membrane and has bound to receptors on post synaptic membrane, what happens ?
Chemically gated channels open on post synaptic membrane giving rise to a graded potential known as a POST SYNAPTIC POTENTIAL or PSP
What is an excitatory post synaptic potential ? EPSP
Graded potential
Cause depolarization of post synaptic neuron
What channels open to produce a EPSP ? Which close ?
Na + or Ca2+ open
Closing of k+
What neurotransmitter most often released to produce EPSP?
Acetylcholine (Ach)
There can be more than one synapse on a cell body. Let’s say there are 3 and all 3 are excitatory , what do you think may happen?
Summation of the 3 EPSPs will create an action potential at axon hillock
The EPSP is on the dendrites or cell body of post synaptic neuron.
What is the threshold potential of this portion of the neuron?
Very high +25mV
Get clarification
T or F Na + influx is greater than K+ efflux
Why
Because the gradient for sodium is much steeper than potassium
What are inhibitory post synaptic potentials ?
When the graded potential on the PS membrane yields hyperpolarization
This is caused by opening of Cl- and K+ gated channels. Cl- moves jn and K moves out.
AP is more difficult
What neurotransmitter most generates IPSPs?
Glycine / GABA
What determines if there will be an action potential on a post synaptic membrane when there are EPSPs and IPSPSs arriving at the post ?
The sum of the EPSPs and IPSPs.
If more EPSPs etc..
What are upper motor neurons ?
The pyramidal cells of the motor cortex and the neurons of the subcortical motor nuclei (part of descending pathway for motor impulse)
What are lower motor neurons?
The ventral horn motor neurons. These directly inner ate the skeletal muscles (the effectors)
At the neuromuscular junction what nT is released ?
Acetylcholine
What is an end plate potential ? What causes it?
An end plate potential is a graded potential on the muscle fiber (end plate)
This occurs after acetylcholine is released into the synaptic cleft and Na+ chemical gates have opened on the end plate of the muscle
Explain why an EPP always generates an action potential on the adjacent sarcolemma.
The depolarization at the end plate is Always enough to generate an Action potential on the adjacent sarcolemma because a lot of acetylcholine is released into synaptic cleft.
Stimuli are detected by receptors which may either be:
A)
B)
Dendrites on unipolar neurons
☀️——–<
Or
Individual cells that synapse on to neurons
What is a receptor potential?
A graded potential that occurs at a sensory receptor membrane
Ie a an individual cell.
What happens when a receptor is stimulated?
1) opening of voltage gated channels, usually Na+ , located on the membrane of the receptor
2) a graded potential occurs on the receptor and the stimulus has now been converted to an electrical stimulus
What is a GENERATOR potential?
A generator potential is a graded potential. A depolarization on the dendrites of a unipolar cell which directly generates action potential.
Describe how AP happens via receptor potential
Ex: hair cell = receptor potential
Neurotransmitter is released onto associated neuron
The associated neuron gets the generator potential and then an action potential occurs on the axon
There are two types of receptors, what are they?
Phasic and tonic
Phasic Receptors are…
Able to show adaptation, this means there will be a decrease in sensitivity.
Therefore a decrease in the frequency of action potentials to the CNS, seen while the stimulus is at constant strength.
What’s an example of a phasic receptor in action ?
Touch… Clothes on the skin
what are tonic receptors, give an example ?
They do not show adaptation, they give continuous information
Example: posture, condition, pain
Can monitor the presence and intensity of stimulus.
How does the brain perceive different types of stimuli?
Mainly by the type of receptor being stimulated.
The receptor activates the axon that that synapses to the portion of the CNS concerned with that particular sense.
The brain is hard wired to know what receptor is being stimulated and where to deliver the information in order to receive the appropriate response.
Give an example of how the brain interprets touch
Meiseners corpuscle in rt index finger is stimulated
Impulses go to post central gyrus of he brain for right index finger
How does the brain perceive different strengths?
By the frequency of action potentials occurring per unit time that are arriving at the CNS
And a stronger stimulus will activate more receptors
Generator cell
The sensory cell that can produce the action potential
Describe what the 1st 2nd and 3rd order neurons would be from the path from touch stimulation to post central gyrus.
1st - meisenners corpuscle to dorsal horn of spinal column
2nd- dorsal horn to the thalamus
3rd- thalamus to post central gyrus
When a generator potential has been reached for touch after the mechanical gates have been stimulated and the information must now reach the general sensory area how then are the action potentials propagated inbetween the order neurons?
When travelling down the first order neuron action potential propagated by first stimulation of mechanical gates then voltage gates. EPSP and chemical gates propagate the next AP and the next till the general sensory area is reached
Describe how the eye reaches graded potential
1) the light image is focused on the retina , it is reduced and inverted
2) this stimulates a chemical reaction on the rods / cones (photo receptors) which produces a receptor potential ( graded potential )
After a receptor potential is reached on the photoreceptors of the eye describe the pathway of information to the visual cortex of the occipital lobe
Receptor Potential >nt >bipolar neurons (graded pot.) >nt >ganglion cells (generator pot.)AP > optic nerve > optic tract > visual cortex of occipital lobe
Where is the only place GABA is released to produce a generator potential ?
The ear
In hearing, describe how hair cells transmit information to the auditory cortex of the temporal lobe.
Hair cells in COCHLEAR DUCTS stimulate receptor potential
> NT released called GABA
>generator potential on associated neuron
> AP on axon of cochlear branch of nerve VIII
> auditory cortex of temporal lobe
Describe the sensory pathway for equilibrium till it reaches the specific branch for nerve VIII.
The hair cells of the semicircular canals, saccule and utricle (vestibule) reach receptor potential.
- NT released ^ from cells
- generator potential triggers AP on vestibular branch of nerve VIII
After a generator potential has produced an AP to the vestibular portion of nerve III (for equilibrium) what happens ?
The AP for nerve VIII arrives at brain stem then cerebellum.
The cerebellum coordinates muscle contractions (cortex controls muscle contractions for balence)
What are reflexes?
Motor function of the spinal cord.
Rapid automatic response to stimulus
Stimulus always causes same motor response. Usually protective and involve 2 or more neurons
Describe the pathway of a reflex from receptor to effector
1) receptor 2) sensory neuron 3) integration center ( sp. cord ) 4) motor neuron 5) effector (muscle or gland)
Sense- dorsal horn Motor - ventral
T of F, in the reflex pathway, when a stimulus reaches a receptor the information must travel to the brain
False, because a reflex must occur quickly, it travels directly to the spinal cord before leaving to create the impulse
The type or reflex generated is dependant on the types of effector cells present at the end of the pathway , what are the two pathways?
1) somatic reflexes : the effector is skeletal muscle
2) visceral or autonomic reflexes: effector is smooth or cardiac muscle and glands
Somatic Spinal Reflexes: what part of CNS, what effector?
Spinal cord
Skeletal muscle
What is an example of a stretch reflex ?
Knee Jerk reflex (extensor muscle contracts)
How does the knee jerk reflex happen?
Stimulated by tapping the patellar ligament
Receptor is MUSCLE SPINDLE
Effector is QUADRICEPS (stretch/contract)
The knee jerk reflex is mono synaptic , what does that mean?
1 sensory neuron
1 motor neuron
Knee jerk reflex is also IPSILATERAL what does that mean?
The motor activity occurs on the same side of the body as the stimulus
What is the role of the interneurons during the knee jerk (patellar) reflex?
Make inhibitory synapses with ventral horn that prevent the antagonist muscle (hamstrings) from resisting the contraction of the quadriceps
All stretch reflexes are
Monosynaptic and Ipsilateral
Give an example of a FLEXOR reflex
Withdrawal reflex
Eg: leg : flexor contacts ( hamstrings )
Describe how a withdrawal reflex would occur if someone stepped on a nail. How many synapses? Same side?
Stimulus: step on a nail Receptor: pain Effector: hamstring (flex / contract) Polysynaptic Ipsilateral
Often the flexor (withdrawal) reflex accompanies the crossed extensor reflex, describe how a CER is involved with a flexor reflex and what it does.
Stimulus: stepping on nail
Receptor; pain
Effector: quadriceps (extend/contract) meanwhile the withdrawal reflex contracts hamstring.
Poly synaptic / contralateral
This reflex keeps you from falling when withdrawal reflex occurs in leg.
What does contralateral mean?
Means that the reflex occurs on the opposite aide of the body as the stimulus
What is reciprocal inhibition?
When a muscle group contracts (Agonist muscle) and at the SAME time
Antagonist muscles are prevented from contracting (inhibitory neurons-interneurons)
What is an example of reciprocal inhibition with regards to a reflex?
Stretch reflex: (patellar)
Quads (agonist) contracts
Hamstrings (antagonist) inhibited
The micturation reflex is autonomic. What are the stimulus, receptors, CNS area and effectors?
Stim: stretch of bladder
Receptor: stretch receptors in wall
CNS: sacral portion of the spinal cord (PSNS)
Effector: detrusor muscle >wall of bladder contracts >internal sphincter opens
The external urethral sphincter is under our control therefore the muscle is this kind of muscle…
Is it apart of micturation reflex?
Skeletal muscle
No
