Exam 2: Somatosensory (17 and 18) Flashcards
what are the 3 sensory systems?
- somatosensory
- visceral sensory
- special sensations
somatosensory sensations
touch, nociception (pain/tissue damage), vibration, proprioception (muscle load, movement, and joint angles), pruriception (itch), and thermoception
visceral sensations
distention (gassiness), hypoxia (lack of oxygen)
special sensations
Vision
Auditory
Vestibular
Gustatory (taste)
Olfactory (smell)
primary afferent neurons
neurons that are doing the sensing in the PNS
- cell body is in the DRG and central terminal is in the dorsal horn
- The peripheral end of its axon generates an action potential in response to the stimulus its tuned to
what is the main somatosensory nerve for the head?
trigeminal nerve (CN 5)
where are the somata of the primary afferent neurons?
- in the dorsal root ganglia where their cell body is and axon is in the peripheral area for sensing
- in the cranial nerve sensory ganglia
peripheral processes
include primary afferent neurons which remain outside the CNS
- processes of dorsal root ganglion neurons are distributed to the body via spinal nerves
central processes
enter the CNS as dorsal roots which contain only sensory axons
what 2 things the spinal nerves contain?
sensory and motor axons which make them “mixed”
dermatome
innervation of specific parts of the body from the spinal nerves
- each nerve innervates its own dermatome
what makes a primary afferent neuron specific?
depends on the sensory receptors at the end of the nerve fiber and their sensitivity
what 5 groups of sense do primary afferent neurons respond to?
touch, thermoception, puriception (itch), proprioception, and nociception
touch (5)
Fine touch
Pressure
Vibration
Movement against the skin
Hair movement
proprioception (4)
Limb and trunk position
Muscle length
Movement
Muscle load
nociception (4)
–> pain and tissue damage
Heat
Cold
Chemical pain
Mechanical pain
mechanoreception
sensing mechanical force ⇒ texture, pressure, vibration, movement, stretch
- Receptors are broadly distributed through the body ⇒ many are in the skin
filterning (touch receptors)
the type of encapsulation, depth in the skin, and the sensory receptor proteins expressed by the neuron determine the stimulus
- there are many different touch receptors
density (touch receptors)
Determines how well we sense different kinds of touch
resolution
(two point discrimination) measures the distance in our sense of touch
proprioception definition
the sense of the position, movement, and load on the limbs and trunk
- mechanoreceptors but are specialized to sense muscle length, load
muscle spindles
- organs that sense muscle length
- Nerve endings are wrapped around a special intrafusal muscle fiber embedded
- Fire when a muscle is stretched ⇒ provide information required to adjust muscle contraction in response to external forces
Golgi tendon organs
- sense muscle load
- Embedded in collagen fibers of tendons
- Compressed by tension
what do Golgi tendon organs prevent?
Help prevent muscles from exerting more force than they can safely bear
- Sensitive to the force exerted by tendon
Piezo2 and how it works
receptor protein expressed in mechanoreceptor neurons
- form a trimer as a mechanically gated Na+ channel which depolarizes the nerve ending and causes the afferent nerve to fire an action potential
- the tiny hole in the middle acts as the sodium channel but only if the arms are distorted a little bit
- Sodium comes in and triggers an action potential
how does a muscle cell sense the amount of load?
the longer the depolarization and channels opened, the cell communicates that there is more load to the CNS
- The frequency of action potentials encodes the strength and duration of the stimulus
Local (spinal) reflexes
input to the brain from proprioceptors or cutaneous receptors causing reflexive response
- Direct pathway to cerebellum
and somatosensory cortex via thalamus
what do reflexes require and where does this happen?
few (1-2) synapses ⇒ all synapses occur in spinal cord
which pathway to the brain is ipsilateral?
Spinocerebellar tract
- can also be contralateral if it crosses over and then crosses again later
which pathway to the brain is contralateral?
Dorsal column pathway and spinothalamic tract
spinocerebellar tract
carrying proprioception in the lateral funiculus of the spinal cord ⇒ cerebellum gets input from cerebral cortex
dorsal column track
carrying touch, vibration (does not synapse before going up to the spinal cord)
- goes to the cerebral cortex
Somatotopically organized
the receptive fields of the axons in the dorsal columns form a map of the body
Fasciculus cuneatus
carries axons from the upper body
Fasciculus gracilis
carries axons from the lower body
medial lemniscus
output axons from FG and FC nuclei ascend here and cross the midline at the sensory decussation and ascend to the thalamus
- synapse in the ventral posterior lateral nucleus of the thalamus
spinothalamic trac (STT)
- carrying temperature, pain, itch, some touch
- ventral lateral funiculi of the contralateral spinal cord
- projects from the ventral posterolateral nucleus (VPL) of the thalamus to the primary somatosensory cortex (S1)
- Pain information also projects to limbic cortex ⇒ motivation to the avoid
what happens if spinothalamic axons get cut as they cross the midline or as they ascend to the thalamus?
you cannot detect pain that bothers you
where does the primary somatosensory cortex receive somatosensory information from? (2)
- The periphery ⇒ the most direct route for information
- Densely of all cortical regions
where do the dorsal column and spinothalamic tract both project?
primary somatosensory cortex S1
where does Somatosensory information from the trigeminal nerve go?
to the ventral posteromedial nucleus (VPM) of the thalamus to primary somatosensory cortex
homunculus
the motor or sensory distribution along the cerebral cortex
- lower leg and foot at the top, knee, trunk, hand, face, mouth
Stroke on the right somatosensory cortex does what?
affects sensations on the left side of the body
- Contralateral to cerebral cortex
Stroke in the right side of the cerebellum does what?
affects motor control of the right side of the body
- Ipsilateral to cerebellum ⇒ not aware of these
what are the similarities between the dorsal column pathway and spinothalamic tract pathway
the relay neurons synapse at the ventral posterolateral nucleus of the thalamus and the thalamocortical neurons synapse on S1 post central gyrus
differences between the dorsal column and spinal thalamic tract pathways?
- dorsal senses touch, vibration, and proprioception while STT is pain, temperature, and itch
- dorsal relays across the midline at the medulla sensory decussation and STT crosses at the spinal cord (anterior commissure)
- dorsal tract axons ascend at the medial meniscus and STT ascends on the STT tract
where does the trigeminothalamic tract synapse?
at the ventral posterior medial nucleus of thalamus to S1 cortex
where is the primary somatosensory cortex for the body and the trigeminal system?
in the S1 primary somatosensory cortex
where does pain information go aside from S1?
it also goes to the limbic cortex as motivation to avoid pain for good/bad feelings
pain
an unpleasant sensory and emotional experience associated with actual or potential tissue damage
what is different about pain neurons compared to touch neurons?
Carried into CNS by thin primary afferent axons with slow conduction velocities
- often unmyelinated (2m/s) or with small diameter myelinated axons (10m/s)
- carried via the STT neurons
nociceptors
signal tissue damage or threat of tissue damage with free nerve endings ⇒ no encapsulsation
- Mechanical injury ⇒ cutting/scraping
- Heat injury ⇒ burning
- Cold ⇒ frost bite
- gut distention ⇒ gas pain
- Chemical injury ⇒ acid
mechanical nociceptors
high threshold, increased force and increased firing, small point like receptive fields
heat nociceptors
respond to heat and capsaicin (form red peppers)
Thermal thresholds 45 degree celsius (~115 F)
TPV1 receptor (David Julius)
transmembrane receptor for painful heat and capsaicin (protein)
cold nociceptors
threshold is 0 degrees celsius ⇒ freezing
- Their thresholds don’t overlap much with those of cooling receptors ⇒ cooling receptors are different (25 deg C)
polymodal receptors
respond to many things ⇒ mechanical, heat, and chemical stimuli
- Thermal thresholds 43-45 degrees C
- High mechanical thresholds
- Respond to chemical agents
where do nociceptor nerve endings go?
in the superficial skin as well as epidermis and dermis and axons extending into deeper layers
- terminals have transduction proteins sensitive to heat, cod, pressure, acid
acute pain
normal pain in response to injury or threat of injury and lasts as long as the stimulus
persistent pain
outlasts the injury or threat of injury and related to healing
- Protective during healing process
chronic pain
outlasts duration of healing and the injury itself
inflammatory pain and its paradox
most common type of persistent pain
- inflammation promotes healing
- Inflammation causes more pain
- Inflammation reduces likelihood of further injury
non steroidal anti-inflammatory drugs
(NSAIDS: aspirin, ibuprofen) block production of prostaglandins ⇒ reduce inflammatory pain and effect on healing is uncertain
sensitization of nociceptors occurs during what kind of pain?
persistent pain
- causes decreased threshold and larger response
- Contributes to increased pain after injury
prostaglandins
inflammatory agents
what % of people have chronic pain? how long is the threshold?
50% and >3 months
- 600 billion annually
neuropathic pain
chronic pain caused by nerve injury ⇒ nerve compression, amputation, chemotherapy, diabetes, etc.
what 2 things does neuropathic pain lead to?
- Loss of innervation ⇒ anethesia in affected area
- Loss of growth factors released by cut nerves ⇒ adjacent nerves behave abnormally and respond to their usual stimuli but encode pain
what does stimulation of the region around the cerebral aqueduct (central/periaqueductal gray) produce in rats?
analgesia aka it kills pain
stimulation of parts of the rostral portion of the ventromedial medulla can facilitate what?
enhanced pain
neurons release what for the analgesic effects of pain morph like stuff in inhibitory circuits?
release endorphins
what types of stimulation can be activated to treat pain? (3)
- Spinal cord stimulation
- Transcutaneous electrical stimulation
- Placebo analgesia
anesthesia
loss of all sensation
general anesthesia
- causes loss of consciousness with no reaction to pain
- Used mainly for major surgery
- Isofluorane, propofol
local anesthesia
used for minor and sometimes major surgeries
- Injectable ⇒ novacaaine
- Blocks action potential generation/propagation by blocking voltage gated sodium channels
analgesia
specific loss of pain sensation
- Non steroidal anti inflammatory drugs (NSAIDS) ⇒ ibuprofen or aspirin
- Opiods: drugs derived from or related to opium
