Nervous System 2.0 (for midterm 2) Flashcards
What is the main function of the somatosensory system?
to mediate sensations such as touch, pressure, limb position, vibration, temperature and pain
What senses stimulation? Where are they located?
Receptors in the skin, muscle and tendon receive sensations and send information to the CNS
What are the 3 subsystems of the somatosensory system?
cutaneous mechanoreceptors
proprioception
pain and temperature
What kind of sensory information do cutaneous mechanoreceptors receive?
touch, vibration, pressure on the skin
What kind of sensory information do proprioceptors receive? Where are these?
limb positioning
load on joints
located in muscles, tendons and joints
How is sensory information transported from the body (ex. finger tips) to the brain?
via sensory receptors with dorsal root ganglia and cranial nerve ganglia
What are dorsal root ganglia?
the cell bodies of afferent fibers (sensory receptors for the body)
What are cranial nerve ganglia?
the cell bodies of afferent fibers (sensory receptors for the brain)
Explain how a sensory stimulation would be transferred from the finger tip to the brain?
dorsal root ganglia extend axons into the finger and up the spinal cord
information travels from the finger tip through the dorsal root ganglia cell body and up the spinal cord
synapse at the top of the spinal cord
synapse in the thalamus
cranial nerve ganglia axon extends into the somatosensory cortex of the cerebral cortex
Where does the dorsal root ganglia synapse with the cranial nerve ganglia?
in the thalamus
What part of the brain does the cranial nerve ganglia synapse to transmit the somatosensory information to the brain?
the somatosensory cortex of the cerebral cortex
What is a dermatome?
the region of the body that is innervated by a single dorsal root ganglion and its spinal nerve
Give an example of a dermatome
Cervical nerve 2 innervates the back of the head and neck
What type of membrane receptors are mechanoreceptors?
transmembrane cation channels
called PIEZO channels
Explain the transduction of mechanosensory afferents
Mechanoreceptors have PIEZO channels closed to both inside and outside of afferent nerve
Na+ is high extracellularly and low intracellularly
- mechanoreceptors on the afferent nerve membrane sense a stimulus (ex. pressure on skin)
- stretching of the membrane in response to the pressure stimulus opens the PIEZO channels (ie., changes the permeability of the membrane)
- Na+ fluxes into the afferent causing a depolarization current across the afferent membrane (generator/receptor current)
- if stimulus is strong enough, an AP is generated
How is the firing frequency of APs in mechanosensory afferent nerves related to the stimulus?
the frequency of APs is proportional to the magnitude of membrane depolarization or the strength of the stimulus
ex. weak stimulus = weak receptor potential (doesn’t reach threshold)
strong stimulus = receptor potential reaches threshold and causes strong spike potential
What are the cutaneous mechanoreceptors for touch?
Merkel cells
Meissner cells
Pacinian cells
Ruffini cells
What type of afferent axons are involved with touch cutaneous mechanoreceptors?
Abeta
myelinated, fairly thick
What is the axon diameter of afferents involved with touch cutaneous mechanoreceptors?
6-12 um
second largest
What is the conduction velocity of afferents involved with touch cutaneous mechanoreceptors?
35-75 m/s
second fastest
Are afferents associated with cutaneous mechanoreceptors for touch myelinated?
yes
What are the proprioception receptor types?
muscle spindles and Golgi tendon organ
What are the afferent axon types for proprioception?
Aalpha
Ia
II
Ib (Golgi organ)
Are afferent axons for proprioception myelinated?
yes
What is the axon diameter of afferent nerves for proprioception?
13-20 um
largest size
What is the axon conduction velocity of afferent nerves for proprioception?
80-120 m/s
fastest
What are the receptor types for pain and temperature?
myelinated free nerve endings
What are the afferent axon type(s) of pain and temperature afferent nerves?
Adelta
myelinated
What is the axon diameter of afferent nerves for pain and temperature?
1-5 um
second smallest
What is the conduction velocity of afferent nerves for pain and temperature?
5-30 m/s
What receptors are involved in pain, temperature, and itch?
unmyelinated free nerve endings
What is the axon type of afferent nerves for pain, temperature and itch?
C
unmyelinated
smallest
What is the axon diameter of afferent nerves for pain, temperature and itch?
0.2-1.5 um
the smallest
What is the axon conduction velocity of afferent nerves for pain, temperature and itch?
0.5-2 m/s
the slowest
Organize the afferent axon types for the 4 sensory functions in order of fastest-slowest conduction velocities. What contributes to conduction velocity?
proprioception afferents
touch afferents
pain, temperature afferents
pain, temperature, itch afferents
myelination and axon diameter contribute to conduction velocity = larger axon diameter contributes, but myelination contributes significantly
How much do myelination and diameter of axons contribute to axon conduction velocity?
A lot
ex. afferents for pain, temperature, and itch have unmyelinated axons and CV of 0.5-2 m/s and diameter of 0.2-1.5 um
whereas
afferents for pain, temperature have myelinated axons and CV of 5-30 m/s and diameter of 1-5 um
What attributes differ between somatosensory afferents?
axonal diameter
conduction velocity
receptive field sizes
temporal dynamics
What is the main function of sensory receptors?
to convert stimulus signals to APs in an afferent neuron
What are receptive fields in afferent nerves? what kind of information do they provide about the stimulus?
the area of the body that affects the activity of an afferent nerve when it’s been stimulated
they provide information about stimulus location
the smaller the size of a receptive field, the more precise location of stimulus
How are receptive fields in afferent nerves measured?
measured with 2 point discrimination (the smallest distance at which 2 points can still be distinguished)
How do small receptive fields compare to larger ones in terms of precision of stimulus location?
small receptive fields can give very precise location information = improved localization
What parts of the body have smaller receptive fields? larger? why?
fingers have ~3mm receptive fields
shoulders have 50mm receptive fields
it’s more important for fingers to have precise localization of stimuli for activities such as grabbing and holding objects, this is less important in areas like shoulders or back
What are the two types of temporal dynamics in somatosensory afferents?
rapidly adapting afferents
slowly adapting afferents
What kind of stimulus information do rapidly adapting afferents/mechanoreceptors provide?
dynamic information about stimulus movement
What frequency do rapidly adapting afferents/mechanoreceptors generate APs?
APs are increased when there’s a change of stimulus
ie., APs are initiated only when stimulus is applied and when it is removed, ex. when pressure is applied and when it is removed
What kind of stimulus information do slowly adapting afferents/mechanoreceptors provide?
static information about the size and shape of the stimulus
What frequency do slowly adapting afferents/mechanoreceptors initiate APs?
APs are generated continuously throughout the duration of the stimulus
ex. they are generated the entire time pressure is applied
What are the 4 mechanoreceptor cell types in the skin? What’s the 5th one that isn’t technically a mechanoreceptor? what makes it different?
Meissner cells
Merkel cells
Pacinian cells
Ruffini cells
these cells cap the ends of afferent nerves
free nerve endings are afferent fibers that also receive cutaneous stimulation about pain and temperature but lack specialized receptor cells
Where in the layers of the skin do each of the mechanoreceptors exist?
epidermis:
free nerve endings are closest to the surface
Meissner cells
dermis:
Merkel cells are at the interface of epidermis/dermis in pits
Ruffini cells
subcutaneous layer:
Pacinian cells are at the interface of dermis/subcutaneous layer
Where are mechanoreceptor cells located on the afferent nerves?
at the endings like caps on the nerves
What is the function of mechanoreceptor cells that encapsulate afferent nerve endings?
these cells lower the threshold for initiating APs and increase the afferent nerve’s sensitivity to stimulation
Which mechanoreceptor afferent type is most sensitive to stimulation: encapsulated afferents or free nerve endings?
encapsulated afferents because of the specialized mechanoreceptor cells
Describe Meissner afferents (receptive field, temporal dynamics, dermatome, location, sensitivity, sensation type, etc)
small receptive field (spatial resolution) - 3mm
rapidly adapting
high innervation of hands
close to skin surface (epidermis)
4x more sensitive than Merkel
sense textured objects moving across skin, grip, low frequency vibrations
Describe Merkel afferents (receptive field, temporal dynamics, dermatome, location, sensitivity, sensation type, etc)
highest spatial resolution/smallest receptive field size (0.5mm)
located in the epidermis
slowly adapting
highly concentrated in finger tips
gives info about shape, texture, curvature of objects - sensitive to edges, points, curves
Describe Ruffini afferents (receptive field, temporal dynamics, dermatome, location, sensitivity, sensation type, etc)
low spatial resolution/ large receptive field size (>7mm)
deep in the dermis
slowly adapting
sensitive to internally generated stimuli (ex. finger movement)
provides information about finger positions and hand conformation
Describe Pacinian afferents (receptive field, temporal dynamics, dermatome, location, sensitivity, sensation type, etc)
lowest spatial resolution/largest receptive field (>10mm)
located deepest in the skin (deep in the dermis)
rapidly adapting
most sensitive
sensitive to high frequency vibrations through an object - ex. using a tool, writing
Which specialized mechanoreceptor cell type has the highest spatial resolution (receptive field)?
Merkel cells
Which specialized mechanoreceptor cell type has the lowest spatial resolution (receptive field)?
Pacinian
Which specialized mechanoreceptor cell type has the highest sensitivity?
Pacinian
Which specialized mechanoreceptor cell type(s) are rapidly adapting?
Meissner and Pacinian
Which specialized mechanoreceptor cell type(s) are slowly adapting?
Merkel and Ruffini
Describe the cutaneous mechanosensory pathway (how sensory information is transmitted from the body into the brain)
cutaneous information is received by a mechanoreceptor/afferent nerve
info is passed into the dorsal root ganglion
info travels ipsilateral up dorsal spinal cord (gracile or cuneate tracts)
synapse in caudal medulla in gracile or cuneate nuclei
crosses over through arcuate fibers to travel up medial lemiscus tract
synapse onto ventral posterior lateral nucleus of thalamus
projects onto somatosensory cortex
Where are the synapses in the cutaneous mechanosensory pathway?
at the caudal medulla and the thalamus
What side of the spinal cord does cutaneous mechanosensory information travel up and through what tracts?
starts travelling ipsilateral dorsally along either gracile or cuneate tract
when it synapses at the caudal medulla, it crosses to travel up caudally (contralaterally)
travels contralaterally along the medial lemiscus tract to the thalamus where it synapses and projects onto the somatosensory cortex
What are the two receptor types (proprioceptors) for proprioception?
Muscle spindle
Golgi tendon organ
What is the main function of proprioceptors?
to provide continuous information about limb and other body part position
where are muscle spindles located?
in skeletal muscles
What are muscle spindles comprised of?
specialized intrafusal muscle fibers
Describe the structure of muscle spindles
sensory afferents coiled around intrafusal fibers
What type of afferents are involved in muscle spindles? What type of information do these types provide?
Ia and II
Ia is rapidly adapting and provides information about the movement of limbs
II is slowly adapting and provides information about static limb positions
T or F: muscle spindles have only rapidly adapting sensory afferents
false, they also have slowly adapting
both Ia and II
What type of information do muscle spindles encode?
info about muscle length changes
What type of sensory afferents are found in Golgi tendon organs?
group Ib
Where are Golgi tendon organs located?
amongst collagen fibers of tendons
What type of info do GTO provide about the stimulus?
about the tension (load) on the muscle or tendon
What reflex exists to protect tendons/muscles when the load is too high?
reflexes protect the muscles/tendons by forcing the body part (ex. arm) to let go of the weight if the tension (load) is too much on the tendon
Describe the proprioceptive pathway for lower body information
lower body proprioceptors send stimulus information through dorsal root and up dorsal spinal cord ipsilaterally
synapse at the Clarke’s nucleus and project signal up the dorsospinocerebellar tract
collaterals travel ipsilaterally to cerebellum and other collaterals cross the caudal medulla
medial lemiscus synapses to ventral posterior lateral nucleus of thalamus which projects to the somatosensory cortex
describe the proprioception pathway for upper body information
upper body information follows same pathway as mechanosensory but also sends collaterals ipsilaterally to cerebellum
What side of the spinal cord does lower body information enter and travel?
dorsally
Does the lower body proprioceptor information ever cross the spinal cord from dorsal to caudal like the mechanosensory pathway?
hmmm unclear
seems like it travels only ipsilaterally? but does it cross at caudal medulla or is that just ‘collaterals’?
Where are the synapses in lower body proprioceptive pathway?
Clarke’s nucleus
Dorsal spinocerebellar tract
medial lemiscus/ventral posterial lateral nucleus (thalamus)
What are the major pathways along the spinal cord in proprioception?
dorsal column
dorsospinocerebellar
What are the major pathways along the spinal cord in mechanoreception?
dorsal column
medial lemiscus
Where does sensory information from the body enter the brain?
mechanosensitive and proprioceptive info enters the brain at the Ventral Posterior Complex of the Thalamus
What part of the brain does mechanosensitive and proprioceptive information reaching the thalamus project onto?
layer 4 of the primary somatosensory cortex in the cerebral cortex
Which layer of the primary somatosensory cortex is mechanosensitive and proprioceptive information project onto?
layer 4 aka the Granular cortex
Which Brodmann’s areas does the primary somatosensory cortex comprise of? where are these in relation to layer 4?
layers 3a, 3b, 1, and 2
layer 4 is ventral to layer 3a
Describe the somatotopic organization of the cerebral cortex concept
each region of the cerebral cortex corresponds to a specific part of the body which is controlled via motor output or receives sensory input from
When looking at the somatotopic organization of the cerebral cortex diagram, what does the size of the body part drawn represent?
the size of the body part drawn represents the size of the brain region that contributes to the body part
Who determined the somatotopic organization of the cerebral cortex diagram?
Wilder Penfield and Herbert Jasper from Montreal Neurological Institute
What is nociception?
pain detection
Which receptor types are involved in nociception?
free nerve endings
T or F: all free nerve endings have unmyelinated axons
false
the ones for pain and temperature are myelinated
the ones for pain, temp, and itch are not
Do afferents for nociception have large or small axon diameters?
very small
second smallest and smallest
How do the conduction velocities of nociception afferents compare to other mechanoreceptors or proprioceptors? why?
they’re significantly slower because their axons have much smaller diameters and some of them are unmyelinated (slowest)
What afferent types do nociceptors have?
Adelta (myelinated)
C (unmyelinated)
What are the 2 types of pain? what differentiates them?
first and second pain
different velocities of transmission
What type of pain is first pain?
sharp, rapidly transmitted pain
What type of afferent fibers mediate first pain?
Adelta fibers (myelinated)
What type of pain is second pain?
delayed, diffuse and longer-lasting pain sensations
What type of afferent fibers mediate second pain?
C fibers (unmyelinated)
Which of the two pain types is rapid transmission?
first pain
Describe nociceptors
free nerve endings that detect pain
Overall, do the axons of nociceptors have fast or slow velocities (think of the diameters and myelination)?
slow conduction
Adelta are faster than C fibers because slightly larger diameter and myelinated
C fiber is slowest because very small and no myelination
What type of pain stimulation is detected by Adelta fibers in the fast pain pathway?
intense mechanical and heat stimuli
What type of pain stimulation is detected by C fibers in the slow pain pathway?
intense mechanical
warm/cool
chemical
What are the channels for pain and temperature stimuli?
TRP channels
What are TRP channels?
Transient Receptor Potential channels for cations
Are TRPs selective channels?
they’re non-selective cation channels for pain detection
Which TRP is involved in sensing noxious thermal stimuli (>43 deg C)? and is also sensitive to what spicy food?
TRPV1
capsaicin from chilli peppers
Which TRP is involved in sensing mechanical pain?
TRPV4 and TACAN
Which TRP is involved in sensing chemical pain?
TRPA1
Which TRP is involved in sensing innocuous temperatures (warm/cool)?
TRPV3 and 4 = warm
TRPM8 cool
What tract/system does the pain pathway follow in the nervous system?
the Anterolateral System
Describe the anterolateral pathway
pain/temperature information enters spinal cord through dorsal root ganglion on the dorsal horn
information crosses spinal cord at cervical and lumbar spinal cord and travels up spinal cord
synapse to ventral posterior lateral nucleus of the thalamus to project on the somatosensory cortex
What is Rexed Laminae? what happens there?
layers (laminae) within the spinal cord where free nerve ending afferents synapse
Adelta synapses to neurons in laminae 1 and 5
C synapses to laminae 1 and 2
Which layers of Rexed’s laminae do Adelta fibers synapse to?
1 and 5
Which layers of Rexed’s laminae do C fibers synapse to?
1 and 2
What are the 2 divisions of the peripheral nervous system?
efferent and afferent (sensory)
How is the efferent branch of the peripheral nervous system divided?
autonomic system and motor division
How is the autonomic nervous system of the efferent branch of the peripheral nervous system divided?
sympathetic, parasympathetic and enteric systems
Describe the autonomic NS
involuntary
involved in homeostasis
includes 3 subsystems: parasympathetic, sympathetic, enteric
Is the autonomic NS involuntary or voluntary responses?
involuntary
When is the sympathetic NS most activated?
by stress or physical activity
What is another term for the sympathetic nervous system?
fight or flight
When is the parasympathetic NS most activated?
during resting periods
What is another term for the parasympathetic nervous system?
resting and digesting
Describe the enteric autonomic nervous system
functions independently of the other 2
functions in digestion by innervating organs of alimentary canal
How are the para and sympathetic NS involved in homeostasis?
the two systems have to work in balance to maintain homeostasis
What are the 3 mechanisms for regulating autonomic function/maintaining homeostasis?
dual innervation
antagonistic action
basal tone
What is dual innervartion in the regulation of autonomic function?
both systemsare sending information to most organs simultaneously (ie., most organs are innervated by both systems)
What is antagonistic action in the regulation of autonomic function?
the systems are acting antagonistically on the organs - one is stimulating while the other is inhibiting
What is the basal tone in the regulation of autonomic function?
the constant travelling of APs along autonomic neurons even during periods of rest
T or F: the neurons that innervate the same organs through different autonomic systems originate from the same parts of the spinal cord
false
ex. cranial nerves mediate parasympathetic responses in the eyes, whereas, thoracic nerves mediate sympathetic responses in the eye
Where do the sympathetic neurons originate from on the spinal cord?
thoracic and lumbar
Where do the parasympathetic neurons originate from on the spinal cord?
cranial and sacral
T or F: all organs have dual innervation from the P and S autonomic systems
false, not all do (ex. nasal glands)
If the parasympathetic systems stimulates the eye pupil, how might the sympathetic system antagonistically stimulate the pupil?
dilate it
What are preganglionic neurons?
autonomic neurons with cell bodies that originate in the CNS spinal cord and project into the PNS to synapse with postganglionic neurons
What are postganglionic neurons?
autonomic neurons that synapse in the peripheral nervous system with preganglionic neurons to synapse with target organs and cause a response
How many neurons are in series in the sympathetic nervous system? the parasympathetic?
2 in both (pre and post-ganglionic)
Where do the ganglia run in relation to spinal cord or target organ in the SS?
close to the spinal cord
Where do the ganglia run in relation to spinal cord or target organ in the PS?
close to the target organ
Describe the length of the pre- and postganglionic neurons in the SS - why are they like this?
preganglionic is short and post is long because the ganglia runs closer to the spinal cord
Describe the length of the pre- and postganglionic neurons in the PS - why are they like this?
pre are long and post are short because the ganglia run close to the target organ
How many synapses are there between the pre and post ganglionic neurons in the SS? how does this compare to the PS?
SS = many >10
PS = few <3
What type of neurotransmitters are released by the preganglionic neuron at the synapse to the post-ganglionic neuron in the SS?
ACh
What type of neurotransmitters are released by the preganglionic neuron at the synapse to the post-ganglionic neuron in the PS?
ACh
What type of receptor does the postganglionic neuron have at the synapse between it and the preganglionic in the SS?
Nicotinic ACh receptors
What type of receptor does the postganglionic neuron have at the synapse between it and the preganglionic in the PS?
Nicotinic ACh
What type of neurotransmitters are released by the postganglionic neuron at the synapse to the target organ in the SS?
Norepinephrine
What type of neurotransmitters are released by the postganglionic neuron at the synapse to the target organ in the PS?
ACh
What type of receptor does the target organ have to receive the neurotransmitter from the postganglionic neuron in the SS?
Adrenergic
What type of receptor does the target organ have to receive the neurotransmitter from the postganglionic neuron in the PS?
Muscarinic ACh
Are the responses caused by the binding of ACh to the nicotinic ACh receptors on postganglionic neurons stimulatory or inhibitory? for SS and PS?
stimulatory for both and rapid
What kind of response can be triggered in the target organ by the binding of either norepinephrine in the SS or ACh in PS to the receptors on the target organ?
either excitatory or inhibitory
What does the binding of norepinephrine to adrenergic receptors on target organs cause?
a second messenger pathway which triggers multiple responses
What does cholernergic mean?
neurotransmitters released are ACh
both PS and SS are this
What type of movement does the motor pathway of the efferent branch of the peripheral nervous system control?
voluntary/somatic movement
Where does voluntary movement initiate?
in the primary motor cortex of the cerebral cortex
What cells are involved in initiating voluntary motor movement in the primary motor cortex?
Betz pyramidal cells (huge)
non-Betz pyramidal neurons (small)
What is different about the primary motor cortex layers from the somatosensory cortex layers? What does this mean for the primary motor cortex?
somatosensory cortex has layer 4 which is where the sensory info is projected onto from the thalamus
the motor cortex does not have a defined 4th layer = does not receive sensory info
What is the major CNS tract/pathway for voluntary movement called?
Corticospinal tract
Describe the corticospinal pathway
motor cortex in the brain projects information downwards towards the spinal cord
90% of the fibers cross at the caudal medulla and travel the lateral corticospinal tract
the other 10% travel ipsilaterally in ventral corticospinal tract
lateral corticospinal tract can directly or indirectly innervate alpha motor neurons which innervate distal muscles
ventral corticospinal tract indirectly innervates alpha motor neurons that innervate proximal muscles
What are the 2 major tracts within the corticospinal pathway that motor neurons can send information along?
lateral corticospinal (90% of efferents)
ventral corticospinal (10% of efferents)
What % of fibers from the motor cortex cross at the caudal medulla? which tract do they continue along?
90% cross to follow the lateral corticospinal tract
What % of fibers from the motor cortex travel ipsilaterally? which tract do they continue along?
10% along the ventral corticospinal tract
What function do motor neurons along the lateral corticospinal tract have in effecting motor output?
they directly or indirectly innervate alpha motor neurons which innervate muscles distal from the spinal cord
What function do motor neurons along the ventral corticospinal tract have in effecting motor output?
they innervate alpha motor neurons which innervate proximal muscles
Where do most lateral corticospinal tract neurons terminate? ie., how do they innervate alpha motor neurons?
indirectly: on interneurons in lateral parts of the ventral horn and intermediate grey matter before synapsing on alpha motor neurons
directly: synapse directly onto ventral alpha motor neurons
What muscles are innervated by efferent neurons that travel the lateral corticospinal tract?
distal muscles via innervated alpha motor neurons
What muscles are innervated by efferent neurons that travel the ventral corticospinal tract?
proximal muscles via innervated alpha motor neurons
What factors can effect muscle tension?
the frequency and intensity of stimulation
What are the 4 ways muscles can respond to stimuli?
single muscle twitches
temporal summation
unfused tetanus
fused tetanus
Describe single muscle twitch responses to stimulation rate
few, low intensity stimuli cause small contractions at the time stimulus is applied
Describe temporal summation
3 stimuli are applied consecutively not giving the muscle time to relax between stimuli application causing increased responses each time
Describe unfused tetanus
stimuli intensity and frequency is increased
allows some relaxation of muscle between applications, but contraction is high until stimuli removed
Describe fused tetanus
highest stimuli intensity and frequency, no relaxation of muscle at all (contraction is a plateau)
Which is the strongest type of muscle contraction? what exercise would cause this response?
fused tetanus
ex. weight lifting
What exercise would induce unfused tetanus?
running
Which motor neuron cells synapse directly onto alpha motor neurons?
Betz cells
Which neurons are involved in the stretch reflex circuitry?
sensory afferent neurons - muscle spindles
motor efferent neurons
Describe an example of the stretch reflex circuitry
- a hammer taps the knee at the reflex location causing the tendons to stretch = causes sensory receptors in extensor muscle to stretch
2a. the sensory neuron in the extensor muscle synapses to and stimulates motor neurons in the spinal cord
2b. the sensory neuron in the extensor muscle also stimulates a spinal interneuron
2c. the spinal interneuron synapses to and inhibits motor neurons in the flexor muscle
3a. APs travel along the stimulated motor neuron in the spinal cord to synapse on extensor muscle fibers = contraction of extensor
3b. because the flexor motor neurons have been inhibited, the flexor muscle relaxes
- the leg extends
in the example of the knee tap reflex, which is the extensor muscle? is this the sensory or motor neuron? is it stimulated or inhibited?
the extensor is the quadriceps of the thigh, this is where the sensory afferent neuron originates and where a motor efferent neuron terminates
the sensory neuron is stimulated by the knee tap stretching the tendons, and the muscle contraction is stimulated by motor neurons
in the example of the knee tap reflex, which is the flexor muscle? is this the sensory or motor neuron?
the flexor is the hamstrings of the thigh, this is where motor efferent neurons terminate
it is inhibited by interneurons in the spinal cord
Which type of neuron stimulates the muscle contraction in the extensor muscle in the knee tap reflexes?
motor neurons in the spinal cord
Explain how the reflex circuit is an example of antagonistic actions from the motor and sensory systems?
the sensory neuron synapses and triggers the motor neurons in the spinal cord to both stimulate the extensor muscle and to inhibit the flexor muscle
What is flexion?
when a limb bends at a joint (muscle shortens)
What is extension?
when a limb straightens at the joint (muscle lengthens)
What causes flexion and extension?
they are a response to two antagonistic muscles contracting
Explain how biceps and triceps are antagonistic muscles
when the bicep contracts to bend the arm at the elbow (flexion), the triceps relax to lengthen
when the bicep relaxes to extend the arm at the elbow (extension), the triceps contract to shorten
Explain how the biceps and triceps are involved in stretch reflex circuitry when holding a glass at 90 degrees and then the glass is filled with liquid
arm is at 90 degrees holding empty glass
stimulus: liquid added to glass
when the liquid is added, the Ia sensory fibers in the muscle spindles of the bicep send stimulus information to the alpha motor neurons in the spinal cord
the bicep muscle passively stretches in response to the added weight
motor neurons in the spinal cord stimulate the bicep muscle to contract and inhibit the triceps, causing it to relax and allow the bicep to resist the weight of the glass and maintain flexion at 90 degrees
What type of feedback loop is maintaining muscle length in the bicep/tricep glass reflex example?
negative feedback
How is reciprocal innervation connected to the actions of the biceps and triceps when weight is added to the arm?
these two muscles are working antagonistically
the bicep is contracting while the tricep is relaxing to maintain flexion
What afferent neuron regulates muscle tension?
Golgi tendon organ
What is the function of regulating muscle tension?
to protect muscles and tendons when the load is too heavy by decreasing muscle activity
Describe how golgi tendon organs are involved in regulating muscle tension
Ib GTO afferents receive information when there is strain on the tendons
send information to the spinal cord where it synapses with motor neurons and inhibitory interneurons
motor neurons inhibit flexor muscle (bicep) and stimulate the extensor muscle (tricep) causing the arm to extend and release weight
What is the central pattern generator?
a network of neurons in the spinal cord that receive a signal from the brainstem to coordinate motor output signals to generate coordinated movement of limbs
Where is locomotion initiated?
the motor cortex and mescencephalic locomotor region
T or F: studies (such as the one with cats) have shown that sensory feedback is not necessary to cause the rhythmic locomotion observed in animals
true
How does a lack of sensory input affect the central pattern generator of locomotion?
it doesn’t prevent it, it just changes the output of the pattern
What did the study on cats with damage to the premotor cortex show?
cats can still walk on smooth surfaces but cannot step over objects
How do animals move over obstacles while running or walking through dynamic environments? what does this mean for the role of reflexes?
stretch receptors (mechanoreceptors) and proprioceptors in limbs receive sensory information about limb position and impact and send information to the pattern generator which can alter the motor outputs
reflexes, and autonomic nervous system, are important in coordinating locomotion
T or F: afferent inputs are critical for rhythmic locomotion
false
What happens if the central pattern generator is stimulated while muscles are paralyzed but there’s no damage to nervous system?
if the central pattern generator is stimulated, motor neurons will still be stimulated and produce rhythmic locomotion
What 3 parts of the brain have key roles in regulating locomotion?
brainstem
cortex
cerebellum
What function does the cerebral cortex have in locomotion?
it coordinates locomotion in complex environments and uses visual signals
What happens to locomotion if there’s damage to the premotor cortex? why?
the cat can still walk along even terrain, even if there’s an incline, but it cannot step over obstructions or obstacles
sensory information enters the brain through the thalamus which projects information onto the somatosensory cortex, so if there’s damage to the cerebral cortex, changes to the environment causing sensory input will not be received by the cerebral cortex and motor neurons will not be triggered to move limbs in space
What neural structures control movement?
in the brain: motor cortex controlled by the basal ganglia and brainstem centers such as the cerebellum
afferent and efferent neurons in the spinal cord
What is the basal ganglia?
in the motor cortex, it causes initiation of wanted movement and suppression of unwanted movement
basically filters the motor outputs generated by the motor cortex to decide what is appropriate
What is the overall function of the motor cortex?
to plan, initiate and direct voluntary movement
What is the function of brainstem centers in movement?
to coordinate (via the cerebellum) rhythmic movements and posture
What is the dorsal striatum?
the brain region that includes the caudate and putamen
What projects onto the dorsal striatum?
the substantia nigra pars compacta
How is the thalamus affected by the dorsal striatum?
the cortex sends information to the dorsal striatum which stimulates medium spiny neurons (MSNs) to disinhibit thalamic neurons through the globus pallidus internal
What type of receptor types are on the MSNs and globus pallidal neurons?
GABAergic
What type of receptors are on cortical neurons?
glutamatergic
What is Parkinson’s Disease? What causes it? What are it’s symptoms?
a neurodegenerative movement disorder that affects 1-2% of 65+ yr olds
progressive loss of dopaminergic neurons in the substantia nigra causes motor symptoms (ex. bradykinesia, resting tremor and muscular rigidity) when >60% of dopaminergic neurons are lost
What type of neurons are affected by Parkinson’s Disease?
loss of dopaminergic neurons cause motor symptoms
When looking at a cross section of a Parkinson’s brain v. non Parkinson’s brain, what is the evident difference?
a non-Parkinson’s brain has evident very dark Substantia Nigra regions in the midbrain filled with dopaminergic neurons
a Parkinson’s brain does not have this dark band, it’s very faded due to the loss of dopaminergic neurons
What is Huntington’s Disease? What causes it? What are the symptom?
a neurodegenerative disease that affects 5-10 people in midlife per 100,000 caused by the HTT gene mutation = huge loss of MSNs in caudate putamen
symptoms:
- cognitive deterioration
- psychiatric impairment
- chorea (abnormal involuntary movement)
- dystonia (sustained muscle contractions = twisting, abnormal posture)
- difficulty initiating movement
What gene mutation causes Huntington’s Disease?
an autosomal dominant mutation to the HTT gene has 50% transmission
if there’s more than 35 repeats of CAG (polyglutamine), the person will get Huntington’s Disease
What is the neural affect of Huntington’s Disease?
huge loss of medium spiny neurons in the dorsal striatum (caudate putamen)
When looking at a cross section of a Huntington’s Disease brain v. a non Huntington’s Disease brain, what is the evident difference?
the Huntington’s Disease brain is severely degraded and has a very reduced dorsal striatum
what is cerebellar ataxia?
coordination and accuracy of motor activity
