ICL 6.2: General Sensory Principles & Somatosensory Tracts Flashcards
what are sensory receptors and what are the 2 types?
specialized cells to detect specific stimuli
- interoceptors
- exteroceptors
what are interoceptors?
detect stimuli inside body
include receptors for blood pressure, blood volume, and blood pH
directly involved in homeostasis, regulated by negative feedback
what are exteroceptors?
detect stimuli outside body
include receptors for taste, smell, vision, hearing, and equilibrium
function to inform CNS about environmental state
how does a feeling occur?
- detection
occurs when environmental changes, such as pressure to the fingertips or light to the eye, stimulate sensory receptors
- sensation
occurs when nerve impulses arrive at the cerebral cortex of the brain
- perception
occurs when the brain interprets the meaning of stimuli
what is a generator potential?
external stimulus that results in a graded depolarizing potential
aka something that gets the axon to depolarize and get more than -70 mV
what is transduction?
Converting sensory input into a form interpretable by the nervous system
like how the ear takes a soundwave and converts it into an action potential
what is sensory integration?
occurs before sensory receptors initiate nerve impulses
you can have facilitation, inhibition, disfacilitation or disinhibition
what is spatial summation?
several small action potentials from multiple pre-synaptic neurons are added together to cause an action potential in the post-synaptic neuron
what is temporal summation?
one pre-synaptic neuron repeatedly releases neuro-transmitter over a period of time until the post-synaptic neuron action potential threshold is met
what is sensory adaption?
Sensory receptor change/reduce their sensitivity to a continuous unchanging stimulus
two possible explanations for this:
- sensory receptors have stopped sending impulses; they’ve become numb to the impulses
- the thalamus has filtered out the ongoing stimulus
ex. adapting to hot/cold water after a brief time in it
ex. eyes adjusting to a dark room
ex. the smell of your own house
what is sensory habituation?
a pattern of decreased response to a stimulus after frequently repeated exposures
reduced response to something that used to elicit a stronger response
2 theories = habituation theory and behavioral theory
ex. decreased response to a drug – desensitization
ex. reduced response to an old ring tone
ex. reduced response to a “favorite food” as when we first “loved it
what are the functional classifications of the primary sensory neuroreceptors?
there receptors recieve information from the environment
1) photoreceptors
2) chemoreceptors
3) mechanoreceptors
4) thermo-receptors
5) nociceptors
what are photoreceptors?
Transduce light energy (photons) to action potentials through the use of rods and cones found in the retina
what are rods?
vision at dim light levels
no color acuity (scotopic vision)
what are cones?
vision in brighter light
color vision
what are chemoreceptors?
they transduce chemical changes into action potentials
what are the types of chemoreceptors?
- olfactory receptor cells
- taste receptor cells
- hypothalamus receptor cells
- aortic sinus receptor cells
- carotid sinus/body receptor cells
what is the function of hypothalamus receptor cells?
- sense low blood glucose levels
- sense low oxygen levels
- sense changes in blood pH
what is the fnuction of aortic sinus chemoreceptors?
sense blood pressure changes
what is the function of the carotid sinus chemoreceptors?
sense BP changes
sense O2 and CO2 changes
what are mechanoreceptors?
transduce physical force into action potentials
- vestibular
- auditory
- somatosensory
what are thermoreceptors?
Transduce temperature changes from the skin and viscera into action potentials
what are the different types of thermoreceptors?
- A-δ fibers (fast transmission) – thinly myelinated
- C fibers (slow transmission) - unmyelinated
cold = mostly A-δ fibers (also some C-fibers) –> cold receptors increase their firing rate during cooling and decrease it during warming
hot = mostly C-fibers –> warmreceptorswill turn up their signal rate when they feel warmth—orheattransfer into the body
cooling—orheattransfer out of the body—results in a decreased signal rate
what are nociceptors?
Transduce noxious stimuli (potentially harmful) from the skin and viscera into action potentials
- A-δ fibers (fast transmission) – thinly myelinated
Acute pain - A-δ fibers - C fibers (slow transmission)- unmyelinated
chronic pain – C fibers
spinothalamic tract is responsible for pain!
what are free nerve endings?
a type of nociceptor that registers pain and temperature –> cutaneousnociceptorsused to detectpain, temperature, and itch
they are found in the epidermis and at the edge of the dermis –> so if you have a burn that goes into the deep dermis it won’t really hurt because the free nerve endings have all been destroyed
Aδ fibers (fast transmission) = sharp pain, itch and cold
C fibers (slow transmission) = dull pain, itch and cold/heat
where are free nerve endings found in the body?
they’re found everywhere in the skin but have higher concentrations in:
- finger pads – highest concentration (dermis and epidermis)
- cornea (eye)
- mucosa (oral/nasal, GI)
- periosteum of the bone –> this is what helps you feel things like stress fractures even when you can’t see them on an x-ray
what are Merkel discs?
mechanoreceptors that detect shapes and edges
they detect light touch and information about sustained mechanical pressure/position
ex. when you reach into your pants pocket, how can you tell which coin you are pulling out? Stereognosis!
what are Merkel discs composed of?
- merkel cells
epidermal cells
- tactile discs
Aβ nerve endings that transmit nerve impulses from activated Merkel cells
where are Merkle discs mostly found?
they’re found everywhere in the skin but have higher concentrations in:
- finger pads
- lips/face
what are Krause end bulbs?
a type of thermoreceptors that detects cold temperature
they’re activated by tmperatures less than 20 C (68 F)
what are Krause end bulbs composed of?
Consist of: minute cylindrical or oval bodies
- capsule (contains a soft semifluid core) formed by the expansion of the connective-tissue sheath of a neuron
- Aδ neuron terminates either in a bulbous extremity or in a coiled-up plexiform mass
where are Kause end bulbs located?
- eye conjunctiva
- mucus membrane of lips and tongue
- epineurium of nerve trunks
- genitalia – penis/clitoris (aka. genital corpuscles)
- synovial membranes - esp. fingers (aka. articular end bulbs
what is a root hair plexus?
a type of mechanoreceptor that is sensitive for touch –> we don’t have whiskers so instead we use our hair
the root hair plexus conveys crude touch and pressure sensation through
theanterior spinothalamic tract
they consist of:
1. Aβ fibers form hair plexus (network) around ahair follicle
- sends nerve impulses when the hair moves
they are located at hair follicles
what are Meissner corpuscles?
a mechanoreceptor that is responsible for sensitivity to light touch
they convey light touch and vibration through the DCML tract
highest sensitivity (lowest threshold) when sensing vibrations between 10 and 50Hertz
they consist of Aβ nerve endings surrounded by Schwann cells
where are Meissner corpuscles found?
thick hairless skin
ex. finger pads
ex. soles of the feet
what are Pacinician corpuscles?
mechnoreceptors that are sensitive to vibration and pressure
they respond only to sudden disturbances and are especially sensitive to vibration like detecting surface texutre
groups of corpuscles respond to pressure changes like grasping or releasing an object
what are Pacinician corpuscles composed of?
they contain Aβ nerve endings - unmyelinated nerve ending
capsule (with a soft semifluid core) formed by the expansion of the connective-tissue sheath of a neuron
where are Pacincian corpuscles found?
hairy and non-hairy skin
ex. finger pads and hands
ex. bone periosteum
ex. viscera
ex. joint capsules
ex. breast
ex. genitals
what are Rufinni endings?
slowly adapting mechanoreceptor located between the dermal papilla and the subcutaneous tissue that are sensitive to skin stretch
they are responsible for kinesthetic sense of/control of finger position and movement
they also respond to sustained pressure- show very little adaptation
what are Rufinni endings composed of?
enlarged Aβ dendritic endings
spindle-shaped connective tissue capsule
where are Rufinni endings found in the body?
fingers/limbs - highest density around the fingernails
monitor slippage of objects along the surface of the skin allowing modulation of grip on an object
which receptors have large receptive fields?
- Pacinian
2. Ruffini
which receptors have small receptive fields?
- Merkel
- Meissner
- free nerve endings
fingertips and lips!
which major ascending tract is involved with two-point touch discrimination?
DCML
what is stereognosis?
the recognition of 3-dimensional shape/form
the ability to perceive and recognize the form of an object in the absence of visual and auditory information, by using tactile information to provide cues from texture, size, spatial properties, and temperature, etc.
what is proprioception?
conscious and unconscious awareness of body position
the ability to sense stimuli arising within the body regarding position, motion, and equilibrium
even if a person is blindfolded, he or she knows through proprioception if an arm is above the head or hanging by the side of the body
the sense of proprioception is disturbed in many neurological disorders
what is kinesthesia?
the conscious awareness of the dynamic movement of body
thekinesthetic senseis sometimes called “muscle memory,” and is the awareness of our own movement –> for example when we walk, eat, write, or brush our teeth.
thekinesthetic senseis based on proprioception, which is awareness of the position of our joints
where are proprioceptors located?
- muscles receptors called muscle spindles
- tendons receptors called Golgi tendon organs (GTO’s)
- joint capsule receptors called multiple joint receptors
what do proprioceptors respond to?
- static limb and joint position
- dynamic movement of limb (kinesthesia)
they are important sources for:
1. balance
- posture
- coordination
- stereognosis
what is a muscle spindle?
a mechnoceptor that helps with proprioception
it’s a sense organ that receives information from muscle that senses STRETCH and the SPEED of the stretch
when you stretch and feel the message that you are at the ENDPOINT of your stretch the spindle is sending a reflex arc signal to your spinal column telling you not to stretch any further
this sense organ protects you from overstretching or stretching too fast and hurting yourself.
what is the muscle spindle reflex arc?
2 types of of muscle fibers connected in parallel = nuclear bag (type 1 fibers) and nuclear chain fibers (type 2 fibers)
2 types of sensory fibers: Ia is the central region of all intrafusal fibers and II is the adjacent to the central region of static nuclear bag fibers and nuclear chain fibers
stretching the muscles activates the muscle spindle (intrafusal) and this leads to an increased rate of action potential in Ia fibers –> this causes a contraction of the extrafusal skeletal muscles which is the jerk you see due to a reduced tension on the muscle spindle
it also leads to a decreased rate of action potential in Ia fibers which allows the skeletal muscle to relax
what is the golgi tendon organ?
a mechnoreceptor in the tendons that helps with proprioception
they’re located at the junction between muscle fibers and tendon
ex. when you lift weights, the golgi tendon organ is the sense organ that tells you how much tension the muscle is exerting
if there is too much muscle tension, the golgi tendon organ will inhibit the muscle from creating any force (via a reflex arc), thus protecting the you from injuring itself
what is the golgi tendon reflex arc?
the tendon is innervated by a single Ib axon that goes and synapses in the spinal cord to tell the muscle to stop contracting so you don’t tear it (demyelinates when entering the capsule)
many small endings - intertwine with the collagen fibers
stretching of the tendon organ (when the muscle contracts) straightens the collagen fibers, compressing the Ib branches and causing the cell to fire
synapses with an inhibitory interneuron in the spinal gray (Lamina VII and VIII) – inhibit the motor neuron to same muscle → excitatory interneuron to the antagonistic
muscle
the average level of activity in a population of tendon organs in a muscle gives a good idea of the force done by the muscle
which receptors are rapidly adapting joint kinesthetic receptors?
they’re located in the capsule of the joint or in the ligaments of the joint
- Pancinian corpuscles
- Golgi-Mazzoni corpsuscle
which receptors are slowly adapting joint kinesthetic receptors?
they’re located in the capsule of the joint or in the ligaments of the joint
- golgi tendon organ
- ruffini endings
- free nerve endings
what do golgi-mazzoni corpuscles do?
rapidly adapting joint kinesthetic receptors
- location: ligaments adjacent to its bony attachment
- sensitivity: tension or stretch on ligaments
primary distribution: majority of joints
what are the 3 main sensory ascending pathways?
1) spinothalamic – anterior and lateral
2. DCML
3. spinocerebellar – ventral, dorsal/cuneo, rostral
what information do the ascending pathways transmit?
- Touch – fine and crude/coarse
- Pressure – superficial or deep
- Vibration
- Limb position (stereognosis)
- Heat/cold
- Pain – sharp and dull
- Proprioception
what do the two parts of the spinothalamictract transmit?
anterior tract = coarse touch and pressure
lateral = temperature and pain
what is the pathway of the spinothalamic tract?
1ST ORDER
it enters dorsal horn and ascend/descend 1-2 segments
marginal zone/substantia gelatinosa – from lower body
dorsolateral Tract (Lissauer’s) – from upper body
2ND ORDER
fibers synapse in substantia gelatinosa
then they decussate in anterior white commissure of SAME level of cord
ascend in the anterolateral fasciculus contralaterally
3RD ORDER
fibers synapse in ventral posterolateral (VPL) thalamic nucleus
ascend in the posterior limb of internal capsule
end in “1° somatosensory cortex” (post-central gyrus)
what is the pathway of the spinoreticular and spinomesencepahlic tract? these are part of the spinothalamic tract!
1ST ORDER
fibers enter dorsal horn and ascend/descend 1-2 segments
marginal Zone/Substantia Gelatinosa – from lower body
dorsolateral Tract (Lissauer’s) – from upper body
2ND ORDER
fibers synapse in substantia gelatinosa – decussate in anterior white commissure of cord and ascend in the anterolateral fasciculus contralaterally
3RD ORDER
1. Spinoreticular fibers – reticular formation fibers ascend to Interlaminar thalamic nuclei – then to Limbic System“ which alert response to painful stimuli”
- Spinomesencephalic fibers in the periaqueductal gray and tectum (Superior Colliculus)
fibers descends to spinal cord (through Nucleus Raphe-Magnus) to the
dorsal horn to inhibit pain transmission (Seritonergic/Noradrenergic)
“Gate Control Theory” for pain control
slide 37
what is the pathway of the DCML?
1ST ORDER
fibers enter cord –> some go to the nucleus proprius (Rexed Lamina IV) (spinal reflex) but most go to Dorsal Columns
gracile = from lower body
cuneate = from upper body
2ND ORDER
fibers ascend ipsilaterally to gracile tubercle and cuneate tubercle
decussation level: cross over in the internal arcuate fibers of the caudal medulla
the fibers then ascend contralaterally at this point within Medial Lemniscus
then they get to the thalamic nucleus, specifically the ventral Posterolateral (VPL) nucleus
3RD ORDER
Fibers ascend through posterior limb of Internal capsule and end in “1° Somatosensory cortex” (Post-central gyrus)
what is the pathway of the spinocerebellar tract?
Ipsilateral Lower Body
Dorsal
1st order – 1° from muscles spindles
2nd order – Synapse in Clarke’s nucleus (Lamina VII –T1-L2/3)
Ipsilateral Upper Body
Cuneocerebellar
1st order – 1° from muscle spindles
2nd Order – Synapse in Accessory Cuneate nucleus (lateral)
Contralateral Lower Body
Ventral
1st order – from golgi tendon organs
2nd order – Spinal border cells in anterior horn – decussate in anterior white matter to contralateral ventral tract
Contralateral Upper Body
Rostral
1st order – from golgi tendon organs
2nd order – Spinal border cells in anterior horn – decussate in anterior white matter to contralateral ventral tract
what is the pathway of the dorsal spinocerebellar/cubeocerebellar tracts?
2nd order -through INFERIOR cerebellar - synapse on Cerebellar cortex
3rd order – project to deep cerebellar nuclei
4th order – project to Red Nucleus/Ventrolateral Thalamic nucleus
what is the pathway of the Ventral/Rostral Spinocerebellar tracts?
2nd order – through SUPERIOR cerebellar peduncle
cross over and go to cerebellar cortex (ipsilateral now)
3rd order – project to deep cerebellar nuclei
4th order – project to Red Nucleus/Ventrolateral Thalamic nucleus
what happens during a deep tendon reflex?
it’s actually myotactic reflex
sensory receptor = muscle spindle
afferent neuron = Ia and II
efferent neuron = alpha motor neuron
responding to what stimulus = stretch of the muscle and the sudden change in tension
so you tap on the tendon which quickly stretches the muscle including the nuclear bag and chain which causes an action potential i Ia and II which go to the spinal cord and synapse with the same muscle’s alpha motor neurons of the same muscle
it also inhibits the opposite muscle like flex the bicep and inhibit the triceps or with the patellar reflex the motor neurons will activate the quadriceps and inactivate the hamstrings
what happens during a tension reflex?
aka reverse myotactic reflex
sensory receptor = golgi tendon organ
afferent neuron type = Ib neuron fibers that will send signals to dorsal horn
efferent neuron type = inhibitory to the same muscle and activating to the opposite muscle
responding to what stimulus = excessive stretch and pressure
what is the flexion/avoidance reflex?
response to a hot object and the signal gets sent to the dorsal horn by A-delta fibers
then the efferent signal gets sent to the biceps so that it contracts and pulls your finger away from the hot object
spinoreticular portion of the spinothalamic tract is what’s alerting you intitially that it really hurts and then the spinomesencephalic is what tells you it’s still hurting afterways
what is joint position sense?
grab someones finger from the sides to minimize cues from the touch and pressure sensations in the nailbed
then move the fingerand test if they can tell if you’re flexing or extending the finger
do it with the eyes closed
how does vibration sensation work?
Testvibration senseby placing a vibrating tuning fork on the ball of the patient’s right or left large toe or fingers and asking him to report when the vibration stops
don’t place the tuning fork on the bone initially because bones conduct the vibration to much more proximal sites, where they can be detected by nerves far from the location being tested
sensory receptor = Pancinian
afferent neuron type = A-beta
ascending pathway is the DCML pathway
what is touch testing?
Light touch is best tested with a cotton-tipped swab, but a light finger touch will often suffice, as long as care is taken to make the stimulus fairly reproducible
you can test the relativesharpness of painby randomly alternating stimuli with the sharp or dull end of a safety pin (always use a new pin for each patient)
this would be the spinothalamic tract for the sharp pain and DCML for the light touch
