sensory contributions 3a Flashcards
why do we need sensory info
sensory receptors provide input about the body and environment essential for interacting in a complex world
sensory systems are important for control of movement: visual, vestibular and somatosensory
sensory feedback
the info (input) provided by the receptors of the different sensory systems
sensory feedback integration
1 sensory receptor (eyes, muscle spindles)
2 feedback
3 integration (decision-making) - ie/ brain, spinal cord circuit
Integrated within the central nervous system
Integrated overtime - update bodies model of the world around us
The neuron
Info in the PNS and CNS travel along neuro s
Cell body, dendrites and axons
Pre synaptic and post synaptic terminals
integrators of info
Cell body
Also called the soma
Dendrites
Processes branch off and resemble a tree
Other neurons connect to sites on the dendrite - know. As dendritic spines for communication
axon
propagates electrical signal
most neurons have their axons surrounded by myelin interupted by gaps called nodes of Ranvier
myelin
insulates axon, speeds up transmission of the electrical signal and reduces current leakage
pre-synaptic terminals
house vesicles containing neurotransmitters, which are released into synaptic cleft bc of action potentials
neurotransmitters cross the cleft to post synaptic neuron
synaptic cleft
gap between neurons
post synaptic neuron
receptors on dendrites or cell body recieving neurotransmitters generate electrical chemical signals that sometimes lead to an action potential
APs are most likely when postsynaptic neurons recieve simultaneous inputs from multiple presynaptic neurons
what are the four functional components of a neuron that generate signals to transmit information
local input (receptive component)
trigger (summing or integrative) component
long-range conducting (signaling component)
output (secretory) component
local input (receptive) component
a sensory receptor ending or dendrite of a non-receptor neuron
trigger (summing or integrative) component
sensory neurons = first node of ranvier; motor neurons and interneurons = axon hillock
long-range conducting (signaling) component
the axon that conducts an AP
output (secretory) component
pre-synaptic terminal where neurotransmitters are released
afferent neurons
carry information towards the spinal cord and brain; often associated with sensory neurons
efferent neurons
carry info down the spinal cord and out to the periphery; often associated with motor neurons
interneurons
neurons that connect other neurons, like an afferent and efferent neuron
abundant in the brain
two features of the signal transmitted by a neuron
- number of action potentials
- time of intervals between action potentials
what determines the intensity of sensation or speed of movement
frequency
they can increase or decrease frequency as a change in baseline
what important info does the nervous system extract from its receptors
modality
intensity
duration
location
modality
sight, smell, taste - also within like a sweet taste
intensity
strength of stimulus
duration
length of stimulus percieved, can be disensitized by stimulus (ie/ feeling of clothes on body)
sensory transduction
converting a form of energy into changes in membrane potential (leading to receptor potentials)
what is intensity encoded by
- frequency of action potentials (frequency coding)
- number of sensory receptors activated (population coding)
threshold
a certain intensity which a stimulus can be perceived
sensory threshold
stimulus detected on 50% of trials
psychometric function
plots the percentage of stimuli detected by a human observer as a function of the stimulus magnitude
used to measure the just noticeable difference between stimuli that differ in intensity, frequency or other parametric properties
desensitized
adapting to a persistant stimulus
off response
rapidly adapting receptors sometimes also fire briefly when a stimulus decreases
static response
slowly adapting receptors represent static stimuli
dynamic receptors
rapidly adapting receptors represent time varying
receptive field
region of sensory space in which a stimulus activates that neuron causing the receptor potentials and possibly action potentials
greater spatial resolution
can discriminate smaller stimuli
labelled lines
sensory afferents carry info regarding a single type of receptor from a specific part of the body
somatosensory system
this system conveys information about the body and its interaction with the environment
includes proprioception and touch
receptors of this system are muscle spindles, golgi tendon organs, joint receptors and cutaneous mechanoreceptors
proprioception
the sensation and perception of limb, trunk, and head position
- where they are in space and in relation to your other limbs/body
receptors involved in this send info about characteristics such as limb movement direction, location in space and velocity to the CNS
the most prominent sources of this info are muscle spindles, golgi tendon organs and joint receptors
vision, cutaneous mechanoreceptors and vestibular organs can give info but are not proprioception
muscle spindles
encapsulated spindle-shaped sensory receptors located in the muscle belly of skeletal muscles
detect static muscle length or position
detect changes in muscle length or limb/muscle movement
for voluntary contractions spindle and muscle fibres are activated the same amount
better at detecting muscle lengthening
brain uses input from multiple muscle spindles to sense limb position and movement
- spindle input from different muscles are combined to provide limb state information.
- thus, population of spindle activity is integrated
what are muscle made of
intrafusal muscle fibres
sensory neuron endings
motor neuron endings (efferent control)
intrafusal muscle fibres
nuclear bag (dynamic bag1 and static bag2) and chain fibres
sensory neuron endings
group 1a and group 2 afferents
wrap around central regions of intrafusal fibres
carry sensory input from spindle to the spinal cord
motor neuron endings
efferent control
make intrafusal fibres tighter
activate polar contractile regions of intrafusal fibres
spindles are unique as somatosensory receptors because they have this efferent part
two types of motor nerve endings
static and dynamic gamma motoneurons
active movement
person moves their own limb via alpha motoneuron activated muscle
muscle spindles detect static muscle length
via static bag 2 and chain fibres
sensed mostly by group 2 afferents
alpha motoneuron
innervates the muscle (not the muscle spindle) and contracts the extrafusal muscle fibres
how is muscle stretch detected
unstretched muscle: APs are generated at a constant rate in the associated sensory fiber
stretched muscle: stretching activates the muscle spindle, increasing the rate of APs. Spindle being stretch
muscle spindles detect changes in muscle length
via dynamic bag 1 fibres
sensed by group 1a afferents
dynamic gamma motorneurons increase sensitivity to detect muscle length changes (which is signalled by group 1a afferents) and static gamma motorneurons increase sensitivity to detect static muscle length ( which is signalled by group 2 afferents)
highest spindle density muscles: extraocular (ie. eye muscles), hand and neck
what would happen if only alpha motor neurons were activated instead of coactivation
only the extrafusal muscle fibres contract. the muscle spindle becomes slack and no APs are fired. It is unable to signal further length changes
passive movement
someone or something other than person moves limb
what is the purpose of alpha gamma coactivation
gamma motoneuron activity contracts spindle to maintain sensitivity of group 1a and 2 afferents to muscle length changes
so spindles can send spindles
both extrafusal and intrafusal muscle fibers contract. Tension is maintained in the muscle spindle and it can still signal changes in length
eccentric contractions
generate very strong 1a afferent activity because lengthening is paired with gamma drive; both by themselves increase 1a activity, so combined the result is even stronger firing
increased spindle feedback accompanies
shortening contraction only when the contractions are relatively slow, or when the muscle is working against a load
agonist
contracts rapidly
group 2 afferents
increase porportionally with amount of stretch
- instaneous muscle snapshots of static muscle length
group 1a afferents
show dynamic response to muscle stretch (ie/ changes in muscle length)
also show changes in firing rate with amount of static stretch similar to group 2 afferents, thus can signal muscle length a bit too
antagonist
lengthens passively
dynamic responses
fire to the slope or derivative of the stretch
muscle spindle feedback
provide feedback about the state of the muscle and the position of the limb
voluntary muscle commands and cortical commands muscle spindle feedback
activate alpha motoneurons to contract the muscle and gamma motoneurons to pull the spindle tight
passive limb movement cortical commands
no cortical commands
no alpha or gamma motoneuron activation
spindle can still detect muscle length changes
what is feedback from muscle spindles used to do
regulate muscle activity via the alpha motoneuron
- feedback from 1a afferents can trigger the activity of an alpha motoneuron (which is normally activated by descending commands from the brain)
~ elicits the stretch reflex
- nervous system can use info about muscle length to adjust how active a muscle is at a given moment in time (which is important for precise muscle control like buttoning up your shirt)
inform higher centres (ie/ cortex, brainstem, cerebellum) about muscle length (and thus limb position)
- this info can help the brain to make decisions about how to move
monosynaptic stretch reflex
used to regulate muscle length (maintain desired muscle length OR joint position)
the muscle is stretched (by your doctor tapping a hammer against a muscle tendon, for instance, when doing a routine exam of your reflexes)
the muscle spindles sense this change in muscle length. The muscle contracts in response to the stretch
the circuit uses a monosynaptic pathway to cause contraction
it is monosynaptic because there is only one pathway (Ia afferent connects directly to the alpha motoneuron controlling the agonist, or homonymous, muscle)
disynaptic pathway
used to inhibit the antagonist muscle
there are two synapses
Ia afferent connects to an inhibitory interneuron in the spinal cord, which then connects to an alpha motoneuron
golgi tendon organs
tiny receptors located at muscle-tendon junction
- GTO is in series with the muscle and tendon (as opposed to in parallel like in the muscle spindle)
sensory info relayed via group Ib afferents
sensitive to tension/force changes in muscle and body (weightbearing) load info
they have no efferent connections and are not under CNS modulation like muscle spindles - sense stretch - how much effort the muscle contraction is doing
mechanism of action: under force/load, collagen fibrils pinch the axon of Ib afferent (thereby causing a graded receptor potentials to the point of eliciting an action potential)
role depends on the state (or task) and the limb
feeback can either lead to inhibition or excitation of muscles
effects are complex because Ib afferents connect to a complex neuronal circuit filled with neurons arising from different areas
GTOs stationary situations
when too much force is generated, may act in an inhibitory role to decrease force (or muscle activity)
it can modulate muscle output to prevent (or control) fatigue though a similar inhibitory role
where are joint receptors found
within connective tissue, capsule, and ligaments of joints
joint receptors
depending on the type, they sense joint pressure and angle, direction and velocity or twisting force
some only appear sensitive only at extreme ranges of motion
some groups only respond to limited ranges of joint motion
contribute to the perception of our position is space at some joints more than others
how do joint receptors relate to the concept of range fractionalization
some groups only respond to limited ranges of joint motion
because it is about having multiple receptors activated in overlapping ranges
this is important because it provides better resolution about joint angles
what are the techniques to study proprioception
deafferentation (surgical or temporary) - results in proprioception being unavailable (blocking information)
sensory neuropathy
muscle/tendon vibration
surgical deafferentation
surgically cut or remove afferent neural pathways
in animals this results in less precision of well learned motor skills such as climbing and reaching in monkeys
temporary deafferentation
blood pressure cuff inflated around a part of a limb until person can’t feel anything below
- portion of the limb “falls asleep” (lose sensation)
- efferent paths still intact
can also give injection around nerve with anesthetic to eliminate feedback (ie/ nerve block)
Sensory neuropathy patients
Diabetes is one of the number of causes of neuropathy - increase
sensory neuropathy patients
diabetes is the number one cause of neuropathy - increase sugar levels are toxic for cells and neurons (cells start dying) - can not feel getting hurt properly cause they can’t sense it therefore they may keep walking on their foot and keep hurting it
in these patients, peripheral afferent nerves in various body parts are not functioning properly
- efferent pathways intact (strength is normal)
unless these patients can see their limbs, they cannot sense their position nor detect motion of joints, because these sensations are mediated primarily by receptors in muscles and joints supplied by large-diameter fibres
tactile sensation is also impaired
-manual dexterity is severely impaired in these patients even in habitual tasks such as writing and buttoning clothes
they can perform a surprising range of pre-programmed finger movements that do not require somatosensory feedback with remarkable accuracy
- eg discrete movements that happen rapidly and/or are very short in duration
muscle/tendon vibration
high speed vibration applied to a muscle/tendon
distorts muscle spindle firing patterns and hence distorts proprioceptive feedback
- preferentially affects group Ia afferents
gives illusion of muscle lengthening
- cause compensatory movements
proprioception facilitates movement accuracy
provides kinematic (position/speed) and kinetic (force) feedback
- helps your brain know where and how your limbs are moving, which allows it to correct trajectory of a movement and ensure distance accuracy
deafferentation causes several movement deficits
proprioception facilitates the co-ordination of body and limb segments
postural control
- neuropathy causes increased postural sway
spatial-temporal coupling between limbs and segments
- knowing the joint angle and how fast a limb is moving is important because it allows the nervous system to adjust the timing/onset of different muscles that act across different joints to ensure smooth muscle
touch relies on cutaneous receptors
tactile information of texture, composition, and shape of surfaces and objects
relies on receptors in the skin (ie/ cutaneous mechanoreceptors
concentrated more around the lateral edges, heel, and forefoot/toes on the bottom of the feet. this allows the nervous system to detect the edges of the BOS to better regulate the COP and hence body
important for:
object manipulation
precision
sensing body position
types of cutaneous receptors
meissner corpuscle
pacinian corpuscle
ruffini’s corpuscles
merkel’s disks
free nerve endings
meissner corpuscle
cutaneous receptor
stroking and vibration
pacinian corpuscle
cutaneous receptor
vibration
ruffini’s corpuscle
cutaneous receptor
skin stretch
merkel’s disc
cutaneous receptor
pressure
free nerve endings
cutaneous receptor
pain
FA-I (fast-adapting type I)
meissner endings
sensitive to dynamic skin deformation or relatively high frequency
insensitive to static force
transmit enhanced representations of local spatial discontinuities
SA-I (slowly-adapting type 1)
merkel endings
sensitive to low-frequency dynamic skin deformations
sensitive to static force
transmit enhanced representations of local spatial discontinuities
FA-11 (fast-adapting type 2)
pacini ending
extremely sensitive to mechanical transients and high-frequency vibrations propagating through tissues
insensitive to static force
respond to distant events acting on hand-held objects
SA 2 (slowly adapting type 2)
ruffini-like endings
low dynamic sensitivity
sensitive to static force
sense tension in dermal and subcutaneous collagenous fibre strands
can fire in the absence of externally applied stimulation and respond to remotely applied stretching of the skin
how do cutaneous receptors contribute to proprioception
skin stretch and muscle vibration each produce the illusion of movement
when skin stretch is applied in the same direction as muscle stretch via vibration, there is an increase in perceived sensation of movement above and beyond that produced when each is applied alone
this shows that inout from skin stretch contributes to proprioception
how somatosensory feedback reaches the brain
sensory info from muscle spindles, GTOs, joint receptors and cutaneous receptor is carried to the spinal cord via afferent neurons, where it ascends via other neurons to the brain
somatosensory info from the peripheral receptors enters spinal cord via dorsal roots
- cutting the dorsal roots is a means to create surgical deafferentation
what are the ascending sensory tracts
dorsal column - medial lemniscus
spinocerebllar tracts
dorsal column - medial lemniscus tract
transmits touch, vibration, and conscious proprioceptive info to supraspinal centres
- sent to somatosensory cortex
- provides conscious awareness of body position
spinocerebellar tracts
transmits unconscious proprioceptive info to cerebellum
- transmits muscle spindle and GTO input
divided into ventral and dorsal spinocerebellar tracts
somatosensory cortex
includes Brodmann areas 3,1 and 2
contains a map of sensory space
brodmann area
based on cortex’s cellular composition and structure
primary somatosensory cortex is S1 = BA 3b
somatotopy
correspondence of the body area to a specific part of the brain such that adjacent body parts are represented near each other in the brain
somatotopic map can be visualized as a sensory homunuclus
different body parts have different size representations in the somatosensory cortex
- representation size is proportional tp tje number of receptors in the skin rather than the area of the skin
