NROC64H3 Chapter Notes - Chapter 12: Trigeminal Nerve, Proprioception, Posterior Parietal Cortex

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17 Mar 2012
Chapter 12 Somatic Sensory System
- Somatic sensation: Enables body to feel, ache, chill and
know what parts are doing.
- Sensitive to many kinds of stimuli: Pressure of objects on skin,
position of joints and muscles, distension of bladder,
temperature of limbs and brain
- Somatic sensory system is different from other sensory
1. Receptors distributed everywhere on body touch,
temperature, pain and body position
2. Is catch-all name collective category for all sensations that
aren’t the “5 senses”
- Begins at skin major skin types are hairy and
- Outer layer epidermis- and inner layer dermis
- Finger can feel raised dot of 0.006 mm high, 0.04 mm wide (braille dot’s 167X higher)
Mechanoreceptors Of The Skin
- Most sensory receptors are mechanoreceptors sensitive to physical distortion (i.e. bending and
- Axons have mechano-sensitive ion channels their gating depends on stretching or changes in tension
Best studies and largest receptor is Pacinian corpuscle Deep in dermis largest of 2mm and 1 mm in
diameter 0 (200-300 Hz; hand on vibrating speakers)
Ruffini’s ending – in hairy and glaborous skin are smaller than Pacinian corpuscles (super low
frequencies yields fluttering feeling
Messiner’s corpuscles are 1/10 size of Pacinian’s at ridges of glaborous/hairless skin (50 Hz; finger
hitting bumps of speaker i.e. raised part of fingertip) sense texture
Merkel’s disks - consist of nerve terminal and flattened, non-neural epithelial cell may be the
mechanically sensitive part synapse=like junction with nerve terminalKrause end bulbs: Lie in
border regions of dry
skin and mucous
membrane (i.e. lips and
- Skin can be vibrated,
pressed, pricked, stroke d and
hair can be bent and pulled
- Messiner and Merkel have
small receptive field
- Pacinian and Ruffini’s have
large receptive field (from
fingertip ½ of palm)
- Messner’s and Pacinian -
respond quickly then stop
firing rapidly adapting
- Merkel’s and Ruffinis
slowly adapting generate
sustained response during
long stimulus
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- Hair grows from follicles in skin several types of
follicles and details of innervation differ
- Vibration and Pacinian Corpuscles: Selectivity of
mechano-receptive axon depends on structure of its
special ending
Pacinian football shaped w/ 20-70 layers of
connective tissues and nerve terminal in middle
When capsule compressed energy
transferred to terminal Membrane deformed
Mechano-sensitive channels Generate
receptor potential = depolarization AP
- 2-Point Discrimination: Ability to discriminate
detailed features of stimulus varies
Then bend wire to bring points close together and
touch them to your finger point - feels like single point
Varies throughout the body fingertips has highest resolution, why Braille uses
1. Higher density of mechanoreceptors in the skin of fingertip than other body parts
2. Fingertips are enriched in receptor types that have small receptive fields
3. More brain tissue
4.Special neural mechanisms devoted to high-resolution discrimination
Primary Afferent Axons
- Skin has many axons peripheral nerves to CNS
- Axons bring information form somatic sensory spinal cord/brain stem = Primary
afferent axons
- Have widely varying diameters, and size correlates with type of sensory receptor to
which they are attached
- Different sizes of axons are designated by 2 sets of names, using Arabic and - Greek
letters and Roman numerals
- A-alpha, A-beta C skin sensory receptors
- I, II, III, IV Muscles and tendons
- Sensory nerves from A-alpha lack largest group of axons
- Group C or IV axons - unmyelinated axons, rest are myelinated
- Smallest axons C-fibers have no myelin, < 1 picometer in diameter,
mediate pain and temperature sensation slowest of C fibers
- Touch sensations mediated by cutaneous mechanoreceptors, conveyed by
relatively large A-beta axons, which can conduct 75m/sec
The Spinal Cord
- Most peripheral nerves communicate w/ CNS via SC
- Segmental Organization fo Spinal Cord
Repeated 30 times down Human SC
Each spinal nerve, dorsal and ventral root axons pass through notch b/w
vertebrae (“back bones”)
30 spinal segments divided into 4 groups cervical (1-8), thoracic (1-
12), Lumbar )1-5) and sacral (1-5)
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Area of skin innervated by right and left dorsal roots
of single spinal segment is dermatone 1-1
correspondence b/w dermatones and spinal segments
dermatones delineate
- Bundles of spinal nerves streaming down w/I lumbar and
sacral vertebral column are called cauda equine- horse’s tail
down the spinal column w/I sack of dura, filled with CSF
Lumbar puncture used to collect CSF for medical diagnostic
test needle inserted at midline spinal tap
The Dorsal Column-Medial Lemniscal Pathway
- Pathway serving touch is dorsal column-medial lemniscal
- Ascending branch of large sensory axons A axons enters ipsilateral dorsa column of SC, white matter tract
medial to dorsal horn (carry info about sensation and limb position)
- Composed of primary sensory axons, and second-order axons form neurons in spinal gray matter
- Axons in dorsal column terminate in dorsal column nuclei, which lie junction of SC and medulla
- Info still represented ipsilaterally but will decussate at ventral and medial medulla
- Medial lemniscus: White matter tract rises from medulla, pons and midbrain, and axons synapse on
neurons of ventral posterior/VP nucleus of thalamus
- Thalamic neuron of Vp project to regions of primary somatosensory cortex/S1
- Information is altered every time it passes through a set of synapses in the brain
- A axons enter ipsilateral dorsal column carry info about tactile sensation and limb position terminate in
dorsal column nuclei at junction of spinal cord and medulla and decussate ascend via medial lemniscus through
medulla, pons and midbrain synapse at ventral posterior (VP) nucleus of thalamus project to primary
somatosensory cortex (S1
The Trigeminal Touch Pathway
- Trigeminal nerve (V): (Latin trai=three, geminus=twin) enter brain at pons, twin nerves, one on each
side and break up into 3 peripheral nerves and innervate face, mouth areas, 2/3 of tongue
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