Somatosensory Tracts: Proprioception and Tactile Paths Dr. Dennis T#2 Flashcards
What is the somatosensory system responsible for?
- Discriminative touch such as flutter-vibration and proprioception
- Crude touch, thermal sensation, nocioception
- on body and head
How is the somatosensory system information transmitted?
- Posterior column medial leminscal pathway (D/PCML)
- Trigeminothalamic pathway specific to face
- Spinocerebellar pathway
- Anterolateral system specific to pain and temp processing for body
What is involved with the PCMLS?
- size shape and texture discrimination and 3-D shape
- Proprioception
- Fast conduction velocities and precise somatopic organization
In the PCMLS what does two point discrimination mean and receptive field?
- Ability to discriminate between two stimuli simultaneously
- Distance before two points becomes one on hands or face is much bigger than other parts of the body bc these areas have high amounts of sensory neurons
- Areas of skin innervated by afferent fibers
- small receptive fields have high receptor density
- large receptive fields have low receptror density
What forms the posterior colums?
The primary afferent fibers enter spinal cord on the medial side of the dorsal horn and form the posterior columns as they travel upwards.
- Fasciculus gracilis (sacral to T6)
- Fasciculus cuneatus (T6 and above)
Damage to spinal cord results in ___ reduction or loss of discriminative positional and vibratory tactile sensations at and below segmental level of injury.
Ipsilateral, hasn’t crossed over yet!
What is sensory ataxia?
Loss of muscle stretch reflexes and proprioceptive loss from the extremities due to alck of sensory input.
Patient will have wide based stance and places feet to floor with force to create missing proprioceptive input
Where do axons from the gracile and cuneate fasciculus go? Where are they located?
- Gracile fasciculus goes to the Gracile nucleus in the medulla and this contains second order neurons
- Cuneate fasciculus goes to cuneate nucleus also in the posterior medulla and dcdontains second order neurons
If damage occurs in the bains stem or medulla or above it results in ___ damage.
Contralateral
Before the medial leminscus reaches the VPL of thalamus waht happens?
- ML rotates!
- ML flattens horizontally in rostral medulla and caudal pons, so the upper extremity fibers are medially and LE are lateral
- Turns vertically and shifts laterally in midbrain so UE fibers lie anterior and LE are posterior
- Somatotopic orientation shifts as fiber tract rotates
VPM recieves info from?
Face
The Primary somatosensory cortex (SI) comprises ____ & ___.
Post central gyrus and posterior paracentral gyrus bordered by the central sulcus anteriorly and posteriorly the post central sulcusw
Blood supply to the Si is from what? What do lesions in them produce?
Anterior and middle crerbeal arteries
- ACA: lesion affects contralateral lower limb
- MCA: lesion produces tactile loss over contralateral upper body and face
What is importance of SII cortex?
- Located on Inner face of upper bank of lateral sulcus
- Recieves inputs from ipsilateral SI cortex and ventral posterior inferior nucleus of the thalamus
Importance of Parietal cortical regions? Lesions result in?
Recieves tactile inputs- referred to as association areas
- Agnosia- you don’t recognize your limbs as part of your body, contralateral body regions lost from body map, but sensation is not radically altered.
Desscribe the afferents from the trigeminal?
- Opthalmic maxillary and Mandibular nerves have facial dermatomes that are very defined
- Pain temp and nondiscriminative touch
- Discriminaive touch and proprioceptive touch
Goes to VPM
What are the central pathways of the CN 5?
- Mesencephalic nucleus
- Principal sensory nucleus
- Trigeminal motor nucleus
- Spinal nucleus
Where does information come from in the Mesencephalic nucleus?
- Proprioceptivve afferents sfrom TMJ masticatory and extraocular muscles
- Located in the midbrain, it is the most superior nucleus of the trigeminal central paths
What is the principal sensory nucleus?
- Located in the midpons
- Discriminative touch and pressure
What is the spinal nuclesu?
- Located at C2/3
- pain temp and nondiscriminative touch
How are V1,2,3 arranged in the principal sensory nucleus
- VI are anterior
- V3 are posteiror
- V2 are in middle
Face is represented upside down
What two tracts can be taken from the principal sensory nucleus? What happens next?
- Anterior trigeminothalmic tracts, which travel contralateral to the VPM
- Posteior trigeminothalmic tracts which targets VPM on the ipsilateral side
- We go from VPM to posterior limb of internal capsule to primary somatosensory cortex
Describe the mesencephalic nucleus.
- Only made of psuedounipolar neurons and only “nucleus” within the CNS, it is displaced trigeminal ganglion cells
- Conveys unconscious proprioveptive and pressure from muslces of oral and extraocular regions
- Reason you know how to chew gum and not chew tongue at same time, or knkow how to chew hot food vs cold vs different shapes
What is the jaw jerk reflex?
- Stretching the masseter by tapping on the chin it causes it to cotract bilaterally
- This will assess function of trigeminal brainstem nuclei and V3
Describe the pathhway for the jaw jerk reflex?
- Afferent limb: from the mesencephalic nucleus dendritic processes within the muscle spinder of masseter send axons that synapse in trigeminal motor nucleus
- Efferent limb activation of trigeminal motor neuron
Not easily seen in person with no damage, youll see an exaggerated reflex in a person with damage
How is CSF formed?
- Passive filtration of serum occurs depending on Hydrostatic pressure which pushes fluid out of the capillary into the ventricle. Oncotic pressure is also present, but it cancels out as the oncotic pressure in the capillaries pulls fluid in and the oncotic pressure surrounding capillary pullls fluid out.
- Modification of ions. Bicarb Cl and K concentrations are controlled by channels on the epithelial cells
- Na, K, Ca, protein and glucose are higher in plasma than CSF
- Mg, Cl and CO2 are greater in CSF than plasma
When pressure goes below 68mmHg what happens with CSF?
No absorption occurs
where does the CSF go from the arachnoid villi?
Goes into the venous sinuses by bulk flow
What are the two components of BBB capillaries?
- Tight junctions btw endothelial cells
- Glial endfeet
How does Glu get in the BBB?
Glut 1 transporter to get the glucose out of blood stream into CSF. From there Glut3 takes glucose up into the neuron.
- 55K on capillary
- 45K on atroglia
NOT insulin dependent
What moves substances out of the BBB back to the blood?
P glycoprotein aka MDR-1
What is the function of the BBB?
- Maintain electrolyte composition related to K and maintainace of Vm
- Protect from toxins
What are the four exceptions to the BBB?
- Posterior pituitary
- Area Postrema
- OVLT
- Subfornical organ
