Somatosensation Part 1 Flashcards
What is the somatosensory system responsible for in simple terms?
Input received from skin and musculoskeletal systems, “body sensations”
True or False
The somatosensory system includes specialized senses such as taste, sight, hearing, and smell
False
Specialized senses are not included in somatosensation
Hint: remember this by thinking somatosensation = body sensation and specialized senses come from the head not the body
What is superficial sensation?
Information received from the environment through the skin and subcutaneous tissues via exteroreceptors
What is deep sensation?
Stimuli received from muscles, tendons, ligaments, joints, and fascia via proprioceptors
What is cortical sensation?
A combination of superficial and deep sensory mechanisms, requires information from both exteroreceptors and proprioceptors as well as intact function of the cortical sensory dissociation areas
What is primary somatosensation?
Sensation of touch, pain, temperature, vibration, and proprioception
What are the types of cortical sensation?
Includes grapesthesia, stereognosis, and tactile extinction
This type of sensation may be lost when damage to the cortex occurs even if primary sensation is still intact
What is grapesthesia?
A type of cortical sensation, the ability to decipher a letter or symbol traced onto the skin using only tactile stimuli (no visual stimuli)
What is stereognosis?
A type of cortical sensation, the ability to identify an object in your hand using only tactile stimuli (no visual stimuli)
What is tactile extinction?
A type of cortical sensation, the the ability to identify right-sided and left-sided tactile sensation equally
Ex. A patient has sensory weakness on their right leg, but is still able to identify tactile sensation in the area. When left-sided sensation and right-sided sensation are tested at the same time, the patient loses their ability to identify tactile stimuli on the weaker side, in this case the right leg.
True or False
Sensory dysfunction can occur anywhere along the ascending pathway
True
Sensory dysfunction can occur in the following structures: sensory receptors, peripheral nerves, spinal nerves, spinal cord nuclei and tracts, brain stem, thalamus, sensory cortex
How will injury to a peripheral nerve present?
Will have loss of sensation and motor function along the distributions of the nerve, regions of loss may be smaller than expected due to overlap from adjacent nerves
What type of injury can present as dermatomal loss of sensation?
Damage to the dorsal nerve root or dorsal root ganglion
The density of sensory receptors varies based on location in the body, where will sensory receptors be the highest in density?
We will have higher density of receptors in the distal-most areas of our extremities (ex. more density in fingertips that on our elbow)
What are the types of sensory receptors in the skin?
- Mechanoreceptors (superficial and deep)
- Thermoreceptors (hot and cold)
- Nociceptors (pain)
How do mechanoreceptors respond to stimuli?
They respond to mechanical deformation due to touch, pressure, stretch, or vibration
The deep mechanoreceptors respond to proprioception and tension in tendons
How do thermoreceptors respond to stimuli?
They respond to hot or cold temperatures
How do nociceptors respond to stimuli?
They are sensitive to stimuli that damage or threaten to damage tissue
What are the characteristics of type 1a and 1b (A-alpha) axons?
Largest in diameter and fastest in conduction velocity, cooperates with muscle spindles and golgi tendon organs to communicate proprioception and muscle stretch
What are the characteristics of type 2 (A-beta) axons?
Large in diameter and fast in conduction velocity, cooperates with muscle spindles to communicate proprioception, merkel’s receptors and meissner’s corpuscles to communicate superficial touch, and with pacinian corpuscles, ruffini endings, and hair receptors to communicate vibration and deep touch
What are the characteristics of type 3 (A-delta) axons?
Small in diameter and slow in conduction velocity, cooperates with free bare nerve endings to communicate pain, cool temperature, and itch
What are the characteristics of type 4 (C) axons?
Smallest in diameter and slowest in conduction velocity, axons are NOT myelinated unlike all other sensory axon types, cooperates with free bare nerve endings to communicate pain, warm temperature, and itch
What is the general somatosensory pathway?
- First-order neuron
- Second-order neuron
- Third-order neuron
What is the general route of a first-order neuron in the body?
From sensory receptors in skin or musculoskeletal systems to the spinal cord, travels from distal to proximal
Where are the cell bodies of first order neurons located?
In the dorsal root ganglion
First-order neurons are pseudounipolar, what does it mean for a neuron to be pseudounipolar?
Long axon stretching from dendrites to axon terminal with the cell body in the center, the distal axon conducts signal from the receptor to the cell body and the proximal axon conducts signal from the cell body to the spinal cord
What are the 3 main ascending sensory pathways?
- Dorsal column-medial lemniscal pathway
- Anterolateral pathways (including spinothalamic tract)
- Spinocerebellar pathways
What is the DCML pathway responsible for?
- Conscious pathway
- Conveys proprioception, vibration, and fine discriminative touch
What is the Anterolateral pathway responsible for?
- Conscious pathway
- Conveys pain, temperature, and crude touch
What is the Spinocerebellar pathway responsible for?
- Unconscious pathway
- Conveys information from proprioceptors and interneurons to the cerebellum
Describe a typical conscious somatosensory pathway
(Hint: use a picture and describe it like you are talking about directions on a map)
The pathway will include three neurons and two synapses. The first order neuron is the sensory neuron with its cell body located in the dorsal root ganglion. It will synapse with the second order neuron in either the spinal cord or brainstem. The second order neuron will decussate (cross) over to carry information to the contralateral side of the thalamus where it will synapse with the third order neuron. The third order neuron will carry information from the thalamus to the somatosensory cortex.
First order neuron axons in the DCML pathway enter the ipsilateral dorsal column and ascend to the medulla. This area of the medulla is made up of the fasciculus gracilis and the fasciculus cuneatus, what type of information travels up the FG and what type of information travels up the FC?
Fasciculus gracilis (medial): sensory information from the lower extremities and trunk
Fasciculus cuneatus (lateral): sensory information from the trunk above T6, upper extremities, and neck
True or False
The fasciculus cuneatus runs along the whole spinal cord
False
The fasciculus cuneatus is not found below the cervical spinal cord, it does not begin until the cervical region because it only carries sensory information from the upper trunk, arms, and neck in the DCML pathway
When does the DCML pathway turn into the ML pathway?
Once the axons of the second order neuron decussate as the internal arcuate fibers in the medulla and begins to ascend in the brainstem
Where will the first order neuron synapse with the second order neuron in the DCML pathway?
Axons coming from the fasciculus gracilis will synapse in the caudal medulla at the nucleus gracilis, and axons coming from the fasciculus cuneatus will synapse in the caudal medulla at the nucleus cuneatus
Where will the second order neuron synapse with the third order neuron in the DCML pathway?
At the ventral posterior lateral nucleus inside the thalamus
In the DCML pathway, the third order neuron axon travels through __________ to reach its destination, which is __________.
Travels through: thalamic somatosensory radiations through the posterior limb of the internal capsule
Destination: primary somatosensory cortex in the postcentral gyrus
The somatotopic fiber orientation of the DCML pathway rotates as it ascends, describe how LE and UE information is organized in each of the following regions: spinal cord, medulla, pons/midbrain, and the cortex
Spinal cord: LE medial, UE lateral
Medulla: LE ventral, UE dorsal
Pons/Midbrain: LE lateral, UE medial
Cortex: LE medial, UE lateral
What is the trigeminal lemniscus pathway?
A pathway analogous to the DCML pathway that conveys touch sensory information from the face, it synapses in the ventral posterior medial nucleus of the thalamus
Either type 3 (A-delta) axons and type 4 (C) axons can be found on the first order neuron in the anterolateral pathway, what type of pain is each axon responsible for?
Type 3 (A-delta): Early perception of sharp pain
Type 4 (C): Later sensation of duller, longer-lasting burning pain
Describe a first order neuron in the anterolateral pathway
The cell body will be found in the dorsal root ganglion and the axon will enter the spinal cord via the dorsal root and ascend in Lissauer’s tract to synapse with the second order neuron in the gray matter of the spinal cord
What is Lissauer’s tract?
This tract allows the first order neuron in the anterolateral pathway to synapse with second order neurons at the level of entry and two spinal segments above
(Ex. if the axon enters the dorsal root of the spinal cord at C7, it will synapse with three second order neurons: C7, C6, C5)
After the first order neuron synapses with the second order neuron, where will the second order neuron travel in the anterolateral pathway?
The axon crosses immediately in the anterior white commissure of the spinal cord and ascends in the anterolateral white matter on the contralateral side (fibers from UE medial, fibers from LE lateral)
Three different tracts arise in the anterolateral system after the first order neuron synapses with the second order neuron, what are they?
Spinothalamic tract
Spinoreticular tract
Spinomesenphalic tract
(Which tract the second order neuron belongs to is based on where the synapse occurs which depends on Lissauer’s tract)
What is the spinothalamic tract responsible for?
Discriminative aspects of pain and temperature (location and intensity)
What is the spinoreticular tract responsible for?
Conveys the emotional and arousal aspects of pain
What is the spinomesenphalic tract responsible for?
Central pain modulation
Where does the anterolateral system travel in the medulla?
Between inferior olives an inferior cerebellar peduncles
Where does the anterolateral system travel in the pons and midbrain?
Just lateral to the medial lemniscus
Describe the third order neuron in the spinothalamic tract
The second order neuron will synapse with the third order neuron in the ventral posterior lateral nucleus in the thalamus. The third order neuron will then travel alongside the DCML pathway neurons through the thalamic somatosensory radiations in the posterior limb of the internal capsule to the primary somatosensory cortex on the postcentral gyrus.
What is the somatotopic organization of the spinothalamic tract in the spinal cord, brainstem, and somatosensory cortex?
Spinal cord: LE lateral, UE medial
Brainstem: LE lateral, UE medial
Somatosensory cortex: LE medial, UE lateral
What is the trigeminothalamic tract?
Analogous pathway to the spinothalamic tract that conveys pain and temperature sensation for the face. It synapses in the ventral posterior medial nucleus in the thalamus
Which pathways/tracts synapse at the ventral posterior lateral nucleus in the thalamus?
DCML and spinothalamic tract
(somatosensory information from the spinal cord) (hint: Lateral = L = Limbs)
Which pathways/tracts synapse at the ventral posterior medial nucleus in the thalamus?
Trigeminal lemniscus and trigeminothalamic tract (somatosensory information from cranial nerves) (hint: Medial = M = Mouth on face)
Describe the spinoreticular tract
Second order neuron in the anterolateral pathway that terminates in the medullary-pontine formation, from there it projects to the intralaminar thalamic nuclei (AKA centromedian nucleus) and information is relay diffusely across the entire cortex and is involved in behavioral arousal
Describe the spinomesenphalic tract
Tract in the anterolateral pathway that projects to the periaqueductal gray matter and the superior colliculi of the midbrain and participates in central pain modulation
