What does the somatosensory system do?
transmits & analyzes touch or tactile information from external & internal locations on the body & head
What are the 6 somatic sensations submodalities?
- discriminative touch
- flutter-vibration
- proprioception (position sense)
- crude (nondiscriminative) touch
- thermal (hot/cold) sensation
- nociception (pain/tissue damage)
What are the 4 pathways that transmit the 6 somatic sensation submodalities?
- posterior column-medial lemniscal pathway
- trigeminothalamic pathway
- spinocerebellar pathway
- anterolateral system
What is the primary pathway that transmits discriminative touch, flutter-vibration, & proprioceptive information?
posterior column-medial lemniscal pathway (PCMLS)
What is the posterior column-medial lemniscal system (PCMLS) involved in?
perception & appreciation of mechanical stimuli
What is stereognosis?
recognition of 3-D shape; occurs in the PCMLs
What kind of proprioception do the PCMLs carry?
conscious awareness of body position in space
What is kinesthesia? What afferent tract carries this information?
limb movement in space; PCMLs
What are characteristic features of the PCMLS?
- afferent fibers with fast conduction velocities & limited number of synaptic relays
- precise somatotopic organization
What is somatotopic organization? (PCMLS)
point-for-point correspondence of an area of the body to a specific point on the central nervous system. Think sensory homunculus.
What is PCMLS frequency coding?
cell’s firing rate signals stimulus intensity or temporal aspects of the tactile stimulus
What is PCMLS population coding?
distribution in time/space of the number of activated cells signals location of the stimulus as well as its motion/direction
How is the PCMLS have such a high degree of resolution?
due to inhibitory mechanisms such as feed-forward, feedback, and lateral (surround) inhibition
- basically negative selection/Darwinism for neuronal signal
How does PCMLS play a role regarding two-point discrimination?
It sharpens discrimination between separate points on the skin
- it has the ability to discriminate between two stimuli simultaneously
- it also varies widely over different parts of the body & is related to density of peripheral nerve endings
What can activation of peripheral mechanoreceptors evoke?
somatic sensations of touch
What is the mechanism of action of peripheral mechanoreceptors activation?
mechanical pressure is transduced into an electrical signal by primary afferent neuron —> if this depolarizes the neuron to threshold, an action potential is produced and related to the CNS via PCMLS
What plays a role regarding accuracy with which a tactile stimulus is localized? (2)
depends on receptor density & receptive field size
What is a receptor density gradient?
various body parts have various density of tactile receptors
What is the receptor density gradient of digits and the perioral region?
they have increased density of tactile receptors
What is the receptor density gradient of other regions, like the back?
a decreased density of tactile receptors
What is a receptive field?
an area of skin innervated by branches of a somatic afferent fiber
Where are small receptive fields found?
areas such as fingertips, where receptor density is high!
Where are large receptive fields found?
areas with low receptor density, like the back!
What is special about small receptive fields vs. low receptive fields?
Small receptive fields provide an INCREASE in discrimination
What is special about low receptive fields vs. small receptive fields?
low receptive fields have a DECREASE in discrimination
What happens to densely innervated body parts?
they are represented by greater numbers of neurons —> this takes a disproportionately larger part of the somatosensory cortex
What type of relationship is there between size of the receptive field and the representation of that body part in the somatosensory cortex?
an INVERSE relationship!
EX: the trunk (large receptive fields) = small representation fingers (small receptive fields) = large representation
What’s special about fingertips & lips and the information they provide to the CNS?
They provide the CNS w/ most specific & detailed information about a tactile stimulus
What are the 3 things that a primary afferent fiber consists of?
- *peripheral process* extending from the DRG (mechanoreceptor or free nerve ending)
- *central process* extending from DRG into CNS
- *pseudounipolar cell body* in the DRG
What does the peripheral distribution of the afferent nerves arising from each spinal level delineate?
the segmental pattern of dermatomes
Clinically, what do primary afferent fiber distribution of dermatomes associate with?
They associate with fibers/pathways that convey pain/thermal information
What is the standard categorization of primary sensory fibers? (2)
regarding cell size and fiber diameter, they are categorized as:
- large
- small
What do large-diameter primary sensory fibers do? (3)
relay
- discriminative touch
- flutter-fibration
- proprioception
Where do large-diameter primary sensory fibers enter the spinal cord via?
**medial division of the posterior root** & then branch
The largest set of branches of primary sensory fibers exit in what direction and are called what?
Largest set of branches ascends cranially & contributes to the formation of the **fasciculus gracilus & fasciculus cuneatus** –> collectively termed the **posterior columns**
Primary sensory fibers within the __________ columns are organized topographically.
posterior
Sacral level fibers (primary sensory) are positioned ______ & fibers from progressively more rostral levels (up to thoracic level T6) are added ________ to form the fasciculus gracilus
- medially
- laterally
In what way is the fasciculus cuneatus is organized?
thoracic fibers above T6 & cervical fibers placed laterally
Spinal cord lesions result in _________ reduction or loss of discriminative, positional, & vibratory tactile sensations _____ the segmental level of injury.
- ipsilateral
- at & below
What can happen due to a lack of sensory input?
**sensory ataxia**, loss of muscle stretch (tendon) reflexes, & proprioceptive losses from the extremities
How can a patient compromise a lack of sensory input, in an effort to create the missing proprioceptive input?
the patient may also have a wide-based stance & may place the feet to the floor with force
What nuclei are contained in the posterior medulla? [posterior column nuclei] (2)
- nucleus gracilus
- nucleus cuneatus
What do nucleus gracilus & nucleus cuneatus contain and receive input from?
contain second-order neurons of the PCMLS and receive input from first-order neurons (primary afferents) from the ipsilateral DRG
Where do each posterior column nuclei receive input from? [nucelus gracilus & nucleus cuneatus]
receives inputs from its corresponding fasciculus
Segregation of tactile inputs occurs within the posterior column nuclei. Core “clusters” receive inputs from what?
rapidly- & slowly adapting afferents
Segregation of tactile inputs occurs within the posterior column nuclei. Outer “shells” receive inputs from what?
muscle spindles, joints, & Pacinian corpuscles
Second-order neurons of the posterior column nuclei send axons where?
to contralateral thalamus [thalamic relay]
Internal arcuate fibers do what regarding thalamic relays?
loop anteromedially in medulla
Internal arcuate fibers cross the midline as what regarding thalamic relays?
sensory decussation
Internal arcuate fibers ascend as what regarding thalamic relays?
ascend as the medial lemniscus (ML) on the opposite side
As the medial lemniscus extends rostrally, it rotates laterally in the pons. This makes upper extremity fibers lie what and lower extremity fibers lie what?
Upper extremity fibers lie *medially* and lower extremity fibers lie *laterally*
Where does the medial lemniscus terminate in while somatotopic organization [think homunculus] is maintained?
ML terminates in ventral posterolateral nucleus (VPL) of the thalamus
What does damage at brainstem levels lead to? [midbrain posterior column lesions]
deficits in discriminative touch, vibratory, & positional sensibilities over the **contralateral** side of the body
What does a right-side midbrain lesion produce? [midbrain posterior column lesions]
a left-sided loss of proprioception & discriminative touch they do not experience the loss of any other modality
What is the wedge-shaped cell group located in the caudal thalamus?
The ventral posterior nuclei
What comprises the ventral posterior nuclei? (2)
- ventral posterolateral nucleus (VPL)
- ventral posteromedial nucleus (VPM)
What separates the ventral posterolateral (VPL) and posteromedial (VPM) nuclei?
fibers of the **arcuate lamina**
Is somatotopic arrangement [think sensory homunculus] of the body maintained in the VPL?
YES!!!
What are VPL & VPM supplied by?
branches of the thalamogeniculate of the posterior cerebral artery
What can happen if there is compromise of the thalamogeniculate branches of the posterior cerebral artery?
can result in loss of all tactile sensation over the contralateral body & head
What does the VPM receive?
head proprioceptive information AND **will receive trigeminal input**
Where does the VPL receive ascending input from?
medial lemniscus
What does the VPL receive?
Arms, trunks and legs!
Medial lemniscal fibers terminating in VPL are functionally segregated. Where do rapidly & slowly adapting inputs target the VPL?
The VPL core
Medial lemniscal fibers terminating in VPL are functionally segregated. Where do Pacinian & joint/muscle inputs target the VPL?
VPL shell
The VPL will be the source of third order neurons that exit the thalamus and travel in what structure?
internal capsule
What are the two populations of identified neurons of the VPL for trunk & extremities?
- third-order neurons
- local circuit interneurons (inhibitory)
Where do the third-order neurons of the VPL terminate in?
primary (SI - S one) & secondary (SII - S two) somatosensory cortices
Local circuit interneurons (inhibitory) of the VPL do what?
receive excitatory corticothalamic inputs & influence the firing rates of third-order neurons
Primary somatosensory (SI - S one) cortex [think homunculus] comprise of which gyrus (location) on the brain?
comprises of postcentral gyrus & posterior paracentral gyrus bordered by central sulcus (anteriorly) & postcentral sulcus (posteriorly)
Blood supply to the SI (S one) cortical areas is provided by the anterior and middle cerebral arteries. MCA lesions produce tactile loss over where?
contralateral upper body & face
Blood supply to the SI (S one) cortical areas is provided by the anterior and middle cerebral arteries. ACA lesions produce tactile loss over where?
contralateral lower limb
What are the four (4) subdivisions of SI (S one)?
Brodmann areas
- 3a
- 3b
- 1
- 2
Brodmann area 3a
located in the depths of the central sulcus, abuts area 4 (primary motor cortex)
Brodmann area 3b & 1
extend up the bank of the sulcus onto the shoulder of the postcentral gyrus
Brodmann area 2
lies on the gyral surface & abuts area 5 (somatosensory association cortex)
Lesions involving area 1 produce a deficit in what? [SI (S one) & Sensory Dissociation]
texture discrimination
Lesions involving area 2 results in what? [SI (S one) & Sensory Dissociation]
loss of size and shape discrimination (astereognosis)
Injury to area 3b has a more profound effect than does damage to either area 1 or 2 alone. Deficits of what occurs? [SI (S one) & Sensory Dissociation]
both texture and size/shape discrimination
What is special about Brodmann area 3b? [SI (S one) & Sensory Dissociation]
Since there is a difference between area 3b versus area 1 and 2, it suggests that there is a hierarchical processing of tactile information in the SI (S one) cortex.
It is thought that area 3b is a relay point, before sending information to area 1 or 2, since 3b has a greater deficit.
“Area 3b performs initial processing & distributes information to areas 1 & 2.”
What typically happens with SI (S one) lesions?
usually include larger areas & frequently results in more global deficits.
i.e. - loss of proprioception, position sense, vibratory sense, and pain & thermal sensations on the *contralateral* side of the body
Define cross sensory syndrome/findings.
brainstem or spinal cord lesions that result in deficits on that differ between each half of the body - dependent upon where the fibers cross
What is special about brainstem lesions? [cross sensory syndrome/findings]
Sometimes you can get:
- sensory deficits of the trunk/extremities contralateral to the lesion
- BUT sensory deficits of face/CN ipsilateral to the lesion
i. e. - right face & left arm/leg lack proprioceptive information
What is special about spinal cord lesions? [cross sensory syndrome/findings]
proprioceptive deficits on the right, but anesthesia on the left (or vice versa)
Where does the secondary somatosensory SII (S two) cortex lie?
lies deep in the inner face of the upper bank of lateral sulcus
What does the secondary somatosensory SII (S two) cortex contain?
contains somatotopically representation of body surface
Where do inputs of secondary somatosensory SII (S two) cortex arise from?
inputs arise from ipsilateral SI (S one) cortex & ventral posterior inferior nucleus (VPI) of the thalamus
Where is the parietal cortical regions found?
posterior to area 2, includes area 5 & area 7 (7b)
What does the parietal cortical region receive?
some medial lemniscal input & inputs from SI (S one)
What can lesions in the parietal cortical region produce?
Agnosia - contralateral body parts are lost from the personal body map (you don’t know its yours) - sensation is not radically altered, but the limb is not recognized as part of your own body
What is the main idea of afferent cerebellar pathways?
It receives sensory afferent information and helps you change minute motor/proprioceptive pathways to allow you hit that golf ball appropriately or sink that 3 pointer bitch.
What is the information transmitted to the cerebellum via afferent cerebellar pathways? [remember main idea with sports]
spinocerebellar pathways transmit proprioceptive & limited cutaneous information to the cerebellum including information about limb position, joint angles, & muscle tension/length
Cerebellar input plays an integral role in guiding control of body muscle tone, movement, & posture.
Where do reflexive branches (your reflexes) of primary sensory fibers terminate at? (this is a small set of fibers)
they terminate on second-order neurons in the spinal cord gray matter at, above, & below the level of entry
