The Somatosensory system and Sensory Tracts Flashcards
Sensory Systems
- Somatosensory: skin and muscles
- Special: bipolar neurons (smell, hearing, equililbium, taste
Peripheral somatosensory neurons
describe the types of receptors
- Location
- specialized:
- mechanoreceptors: mechanical deformation like pressure or vibration
- chemoreceptors: chnages in chemicals
- Thermoreceptors: heat/cold
- Nociceptors: subset of all
Use of somatosensation
does all sensory information reach conscious awareness?
- some give conscious awareness of sensory information
- most: automatic adjustments (so you dont sense EVERY stimulus)
~ selectively prevented from reaching consciousness - inhibitory connections prevent these from reaching awareness
Describe a sensory neuron
- pseudounipolar
- peripheral axon
- central axon
Describe the dorsal root and dorsal root ganglion
Dorsal root:
- carries sensory information into SC
- afferent axons
Dorsal root ganglion
- contains cell bodies of somatosensory neurons
Describe how most sensory information is conveyed from body receptor to CNS
- Pseudounipolar neuron has soma in DRG
- distal axon terminates in skin, muscle, etc
- proximal axon conveys input into CNS
Process of Action potentials in somatosensory system
receptors:
- if receptor potential >threshold = AP
- AP travels along distal axon to DRG
- AP travles along proximal axon to spinal cord
Describe
distal axons
Of sensory system
- classified according to axon diameter
- larger diameter = faster transmission
- 1a = stretch reflex/muscle spindle
- 1B = w/i GTO and ligament receptors
- II/Ab = muscle spindles, pacinian, and ruffini receptors
- A delta = fast pain/free nerve endings
- C = free nerve endings and diffuse pain/ache
Cutaneous innervation
- Receptive field = area of skin innervated by a single receptor
- small receptor fields going distally = better 2 point descrimination
Types of cutaneous sensation
- Discriminative touch/light: localization of touch and vibration and ability to disciminate between two closely spaced points touching the skin
- crude touch: pleasant touch, pressure, tickle, ich - free nerve endings
- pain = nociceptors
- temperature: thermoreceptors
Light Touch receptor types: superficial
1 meissner’s Corpuscles
- dermis
- light touch
- play a role in discriminitive touch and movement of objects over skin
2 Merkel’s Disc
- Localization/2 point discrimination
- below epidermis
- sensitive to pressure
- high density in finger tips
Light touch receptors
Subcutaneous
1 pacinian corpuscles:
- located in subcutaneous layer of skin
- deep tissues of body also
- percieve deep touch and vibration
2 ruffini endings
- deep layers of the dermis
- perception of touch and pressure
- play a role in joint position sense
Free nerve endings
- Crude touch: undefined/diffuse (tickle/itch)
- nociception
- thermoreception
types of pathways carrying sensory info to the brain
- Conscious relay
- divergent
- nonconscious relay
Conscious relay
The information that is carried on these pathyways all…
- high fidelity: accurate/discrimintive
- reaches the brain and conscious
- info about the stimulus location and type is transmitted with high accuracy
Divergent pathway
general info about all these
- conscious and nonconscious
- information to numerous locations to the brain
nonconscious relay
- cerebellum
- autonomic adjustments to sensation
- ex adjusting your body without thinking
What do conscious relay pathways carry
- touch
- proprioception
- pain
- temperature
carries by ALST or DCML
How do conscious relay pathways travel in the spinal cord
- dosal columns
- anterolateral tracts
What is a first order neuron
- brings information from sensory receptors into spinal cord
- pseudounipolar
What is a second order neuron
- conveys information between the spinal cord or brainstem to thalamus
What is a third order neuron
- conveys information from the thalamus to the cerebral cortex
What does the DCML carry
- light touch
- discriminitve touch
- proprioception
- stereognosis: identify objects via touch
- vibration
DCML
1st order neuron
- peripheral axon gets sensory information and carries to soma (DRG)
- central axon enters SC => fasciculus gracillis (travels medially with leg information) and fasciculus cuneatus ( Travels laterally with arm information)
- synpases with 2nd order neuron
travels in dosral columns
DCML
2nd order neuron
- cell bodies in medial nuleus gracillis (medial leg) and Nucleus cuneatus (lateral arm)
- fibers cross in lower medulla (interal acuate fibers)
- travels in VPL of thalamus to 3rd order neuron
DCML
3rd order neuron
- thalamus
- to nerve endings in somatosensory cortex
Fasciculus gracilis
- medial in the dorsal column
- carries infomation coming from the legs
- in medulla becomes the nucleus gracilis
Fasciculus cutneatus
- info coming from the arm
- runs more laterally in dorsal columns
Nucleus gracilis and nucleus cuneatus
- sensory neurons
- relay information to thalamus on opposite side of brain
Trigeminal Lemniscus
Discrimintive touch in the face
pathway
- CN V: trigeminal
- Cross in pons
- First order neuron: trigeminal neuron
- second order neuron: main sensory nucleus in pons
- third order neuron: thalamus
Somatotopic organization
- homunculus
- primary somatosensory cortex: receives somatotopically organized information
Anterolateral tracts carries what type of information
- pain (fast)
- temperature
ALST: first order neuron
- starts in peripheral
- cell body in DRG
ALST: 2nd order neuron
- starts in dorsal horn of spinal cord
- crosses midline
- travels in anterolateral column to thalamus
ALST: 3rd order neuron
- starts in thalamus (VPL)
- terminates in primary somatosensory cortex
Dorsolateral tract
- axons carrying pain information enter dorsal horn of spinal cord
- central axon branches
- ascend, descend several segments before synpasing on 2nd order neurons (in dorsal horn)
- fibers travel in dorsolateral tract
- create a sensory redundacy
Part of ALST - reason why you will lose pain and temp a few levels below
Pain: face-trigeminothalamic
1st order neuron
- from the face
- trigeminal ganglion (outside PONs)
- then descends to medulla and upper cervical SC
Pain: face-trigeminothalamic
2nd order neuron
- spinal trigeminal nucleus (in lower medulla)
- then crosses midline to ascend thalamus
Pain: face-trigeminothalamic
3rd order neuron
- thalamus (VPM)
- terminates in primary somatosensory cortex
Fast pain pathways
Face
- spinothalamic tract
- trigmeinal system
Cross analgesia
- when there is a lesion in the dorsal lateral medulla it results in a pain sensation loss in the opposite side of the body and the same side of the face
- DCML is fine due to being medial to the lesion site
Divergent pathways
Describe what they carry and how they carry the information
- slow pain
- three parallel ascending tracts
- interneuron system
three tracts of divergent pathways
- spinomesencephalic
- spinoreticular
- spinolimbic
Interneuron system of the divergent pathways
- no 1st/2nd/3rd order neuron = 3 relay system
- only spinolimbic pathway = perceive pains (goes to consious)
Spinomesencephalic tract
- DRG to interneuon to dorsal horn of SC
- then cross midline
- termines in midbrain (to superior colliculus nad periaqueductal gray matter -PAG)
- look at painful stimulus
- PAG involved in pain control
- superior colliculus = makes you look at the pain
Spinoreticular tract
- DRG to interneuron to Dorsal horn of spinal cord
- then cross midline
- pain information travels to reticular formation on midbrain, pons and medulla
- interacts with arousal, attention and sleep/wake cycles
- conveys slow, aching pain
reticular formation
- nucle throughout the whole brainstem
- regulate sleep and wake cycles
- sleep wake cycles also interact with slow aching pain so you wake up
Spinolimbic tract
Pathyway, type of stimuli carried
- DRG to dorsal horn of the spinal cord, then crosses midline
- slow pain information to thalamus
- then sends info to amygdala, the insular cortex and the ventral striatum in basal ganglia
- then sends info to the cortex
- reaches consious awareness
Trigeminoreticulolimbic pathway
- slow pain info from face
- path: trigeminal nerve to ascending projections in reticular formation to thalamus
- terminates in cerebral cortex
- projections to retincular formation and thalamus
