UBC NSCI 311 - Lecture 5 (Sensory Systems)

Advanced Neuroanatomy, Midterm 1 Content

two levels of conscious somatosensation

exteroception and proprioception

exteroception

external stimuli (eg. sensations detected by cutaneous receptors)

enteroception

internal stimuli (eg. visceral sensation related to homeostasis)

conscious proprioception

• awareness of (and ability to move) body parts without visual cues

• pathways to cerebral cortex

nonconscious proprioception

• coordination of complex movements (eg. walking, riding a bike)

• pathways to cerebellum

list of somatosensory modalities

• vibration

• discriminative touch

• proprioception

• pain

• temperature

mechanoreceptors are for…

• external stimuli (discriminative touch, vibration, stretch, pressure on skin)

• internal stimuli (proprioception, stretch in blood vessels)

what receptors are in DCML pathway

mechanoreceptors

nociceptors are for…

damaged tissue and associated chemical changes

thermoreceptors are for…

relative changes in temperature (until it damages, then become nociceptor)

what receptors are in the spinothalamic pathway

nociceptors and thermoreceptors (conscious)

free nerve endings

unencapsulated, pain and temperature cutaneous receptors

other than free nerve ending, what are kind of receptors are the other cutaneous receptors

mechanoreceptors

what do muscle spindles made of intrafusal fibers do 

specialized receptors that respond to changes in muscle length

what do tendon organs near myotendinous junctions do

slow-adapting response to tension (stretching) in tendons

stretch reflex steps

1. stimulus of tendon hammer

2. increase in muscle length sensed by muscle spindles

3. sensory neuron synapsed with spindle goes into spinal cord

4. synapse with motor neurons (interneurons if applicable)

5. response of concentric muscle contraction

reciprocal inhibition

interneuron facilitating inhibitory signalling to antagonist muscles

are reflexes monosynaptic or polysynaptic

either (polysynaptic when interneurons get involved)

are reflexes isolated to a single spinal level

almost never

why aren’t reflexes isolated to a single spinal level

because the sensory information could be coming from somewhere completely different than where the action is being enacted

list of unencapsulated cutaneous receptors

• free nerve ending

• hair follicle receptor

• Type 1 cutaneous mechanoreceptor

list of encapsulated cutaneous receptors

• Tactile/touch corpuscle

• Type 2 cutaneous mechanoreceptor

• Lamellar corpuscle

what is an encapsulated receptor

sensory receptor enclosed in epithelial or connective tissue (eg. muscle spindles, golgi tendons)

primary (first order) neurons of somatosensory pathways

• pseudounipolar

• cell bodies in dorsal root ganglia

• peripheral ends contain receptors

secondary (second order) neurons of somatosensory pathways

• multipolar

• decussate

• synapse in thalamus

tertiary (third order) neurons of somatosensory pathways

• multipolar

• synapse in primary somatosensory cortex

• cell bodies form nucleus in thalamus

• axons contribute to corona radiata and posterior limb of internal capsule

pathway of primary neurons of DCML pathway

• enter dorsal horn

• enter ipsilateral dorsal column

  • if from T6 and above, fasciculus cuneatus

  • if from T7 and below, fasciculus gracilis

• synapse in gracilis or cuneatus nuclei (caudal medulla)

pathway of secondary neurons of DCML pathway

• synapse in nucleus gracilis or cuneatus

  • become internal arcuate fibers

• decussate in central medulla

  • become medial lemniscus in medial region of rostral medulla

• synapse in ventral posterolateral nucleus of thalamus

travel of the medial lemniscus

• located along midline and dorsal to pyramids, ventral-dorsal orientation, in rostral medulla

• oriented medial-laterally in pons, dorsal to the big pons belly area

• located more laterally between substantia nigra and red nucleus, ventral-dorsal orientation in midbrain

pathway of tertiary neurons of DCML pathway

• synapse in ventral posterolateral nucleus of thalamus

• go through posterior limb of internal capsule

• go through corona radiata

• terminate in primary somatosensory cortex in relative somatotopic arrangement

location of primary somatosensory cortex

postcentral gyrus

primary neurons of lower limbs of DCML pathway enter where

lumbar plexus, into fasciculus gracilis

primary neurons of upper limbs of DCML pathway enter where

brachial plexus, into fasciculus cuneatus

division between fasciculus cuneatus and gracilis

dorsal/posterior intermediate septum

somatotopic arrangement of posterior/dorsal column

lumbar = medial, thoracic, cervical = lateral

relative location of the decussation of DCML pathway axons

• in central medulla

  • rostral of decussation of pyramids (corticospinal pathway)

  • caudal of open medulla

landmarks of rostral medulla

• open medulla, so fourth ventricle

• medullary pyramid

• inferior olivary colliculi (spiral looking things)

• inferior cerebellar peduncle

landmarks of pons

• smaller fourth ventricle “area”

• corticospinal fibers (islands)

• pontocerebellar fibers (lines between islands)

landmarks for midbrain

• cerebral aqueduct (hole in the middle)

• cerebral peduncle (ventral)

• substantia nigra (just dorsal of cerebral peduncle)

• superior colliculi (ventral bump)

secondary sensory neurons synapse with tertiary sensory neurons where if going to face

• ventral posteromedial nucleus of thalamus

  • via trigeminal nerve

what information goes through thalamus in DCML pathway

• relay center for all conscious sensation en route to primary cortices

  • except olfaction

what contributes to the genu of the internal capsule

• somatosensory information from the face

  • (and corticonuclear fibers of the face from motor pathways)

somatotopic arrangement of somatosensory axons in spinal cord compared to brainstem and internal capsule

• spinal cord = lumbar is medial

• brainstem and internal capsule = cervical is medial

fastest sensory fibers construction

relatively thick axon, but thicccc myelin

slowest sensory fibers construction

relatively thin axons, but thinnnn myelin, if any at all

dorsolateral tract location

dorsal tip of dorsal horn (white matter)

dorsolateral tract function

axons ascend/descend

what are substantia gelatinosa and nucleus proprius

dorsal horn subnuclei

where are spinal border cells

dorsolateral edge of ventral horn (lumbar and cervical)

nucleus dorsales

columns of stacked nuclei between ~C8-L2

pathway of primary neurons of spinothalamic pathways

• enter dorsal horn

  • and ascend or descend 1-2 spinal segments via Lissauer’s tract

• synapse in dorsal horn

  • especially in substantia gelatinosa and nucleus proprius

pathway of secondary neurons of spinothalamic pathways

• synapse in dorsal horn

  • especially in substantia gelatinosa and nucleus proprius

• decussate immediately via anterior commisure

  • into spinothalamic tract (ventral of ventral horn)

• become spinal lemniscus when they enter brainstem

• synapse in ventral posterolateral thalamic nucleus

travel of the spinal lemniscus

• located laterally, dorsal of olivary nuclei in rostral medulla

• located dorsolaterally of medial lemniscus, along lateral edge, 90 degree angle, in pons

• located dorsally of medial lemniscus (which is between substantia nigra and red nucleus), oriented ventral-dorsally, in midbrain

pathway of tertiary neurons of spinothalamic pathways

• same as DCML

• synapse in ventral posterolateral nucleus of thalamus

• go through internal capsule

• go through corona radiata

• terminate in primary somatosensory cortex

  • in relative somatotopic area

what to consider for somatosensory pathway lesions

• where could a lesion be to only affect one dermatome?

  • or to affect every dermatome below the lesion?

• would the lesion involve primary, secondary or tertiary neurons?

which neurons decussate in conscious somatosensory pathways

secondary neurons

what lesion location would result in contralateral sensory deficits (or ipsilateral or bilateral)?

• for spinothalamic, effects would be contralateral unless lesion is immediately where primary neuron enters/synapses

• for DCML, effects would be ipsilateral unless lesion is in or above rostral medulla

spinocerebellar pathways

• nonconscious sensory pathways

• carries information from proprioceptors and exteroceptors to ipsilateral cerebellum

lesions of spinocerebellar pathways

• typically involve varying degrees of ataxia

• rarely damaged in isolation of other pathways

ataxia

loss of muscle coordination

proprioceptors (and cutaneous receptors) go to…

dorsal columns and spinal cord grey matter

proprioception info from spinal cord grey matter goes to…

• nuclei dorsales or accessory nucleus cuneatus

• spinal border cells

• interneurons of motor neurons

proprioception info from nuclei dorsales or accessory nucleus cuneatus goes to…

• dorsal spinocerebellar

• cuneocerebellar

proprioception info from spinal border cells goes to…

• ventral spinocerebellar

• rostral spinocerebellar

proprioception info that goes to cerebellum came from…

dorsal, ventral, rostral spinocerebellar and cuneocerebellar

proprioception info that goes to somatosensory cortices comes from…

DCML

proprioception info that goes to deep tendon/stretch reflexes comes from…

interneurons or motor neurons

receives and processes nonconscious proprioception

cerebellum and somatosensory cortices (info comes from different pathways)