sensory pathways

Question 1

draw and label the anatomy of the peripheral sensory nervous system

and segmental spinal nerves

Answer 1

30 pairs of spinal nerves

Question 2

draw and label spinal cord anatomy (sensory)

Answer 2

Question 3

what are dermatomes?

what are their characteristics?

Answer 3

the regions of skin supplied by individual spinal nerves

they do not have sharp boundaries

Question 4

what is the face/scalp innervated by?

Answer 4

NOT by DRG neurons

by trigeminal ganglion neurons/trigeminal nerves

Question 5

why do dermatomes arising from DRG neurons not have sharp boundaries?

Answer 5

sensory branches may innervate the edges of different dermatomes

Question 6

how can dermatomes be useful in diagnosis of visceral problems?

Answer 6

knowledge of which spinal nerves innervate which dermatome

• damage to internal organs can be perceived as pain in skin
  
  • aiding to identify damaged organ

Question 7

how is skin sensation achieved?

Answer 7

most peripheral sensory neurons have endings in skin

naked endings are from small non- or thinly myelinated DRG fibres (nociceptive neurons)

Question 8

name the sensory endings and what type of sensation they receive

draw and label where in skin

Answer 8

sensory ending type of sensation

naked ending pain, temperature, itch

hair follicle ending hair movement

Merkel’s endings sustained pressure - texture & shape

Meissner’s ending touch - brief response, transient contact

Pacinian corpuscle vibration

Ruffini ending shear stress

Question 9

describe the spinothalamic tract

Answer 9

pain, temperature, itch, simple touch

1. Drg neuron central axon into dorsal horn
2. Projection interneuron crosses midline – until reach thalamus
3. Synaptic connection with thalamic nuclei :

• Either
  
  • Ventral posterior lateral thalamic nuclei
  • Ventral posterior inferior thalamic nuclei
    
    • Can be precisely localised

4. Thalamicortical neurons project from thalamus to primary sensory cortex
5. Perceive pain

Question 10

describe the dorsal column pathway

Answer 10

discriminative touch & conscious proprioception

1. DRG neuron projects to dorsal horn
2. Main branch of DRG neuron travels towards brain in dorsal columns
3. Axons project the length of the spinal cord and make synaptic connections in the medulla with dorsal column nuclei.

• Either
  
  • Nucleus cuneatus
  • Nucleus gracilis
    
    • Axons have not crossed to opposite side yet

4. Neurons in dorsal column nuclei project axon which crosses midline travels through fibre tract medial lemniscus
   * synaptic connection with ventral posterior thalamic nuclei
5. Projection to primary sensory cortex

Question 11

what is two point discrimination?

Answer 11

The ability to distinguish between two points of sensory contact on the skin varies for different regions of the body and depends on the density of sensory innervation

Question 12

describe the spinocerebellar pathway for non-conscious proprioception

Answer 12

reflex circuit

Proprioception – limbs spatial awareness

Reflex circuit

• Proprioceprtive neuron innervates skeletal muscle
• Passes through dorsal horn and branches
• Other branch synapses with nucleus of Clarke
  
  • Project axon to synapse with lateral cuneate nucleus
    
    • To cerebellum
      
      • Coordinates balance
      • Outcome of motor movements
• Therefore is non-conscious

Question 13

what are the ascending sensory tracts within the spinal cord?

draw and label where they are located

Answer 13

Dorsal columns

Spinothalamic tracts

Spinocerebellar tracts

Question 14

describe the trigeminal sensory pathway

Answer 14

central axons from the nociceptors make synaptic connections with the trigeminal nuclei :

• mesencephalic nucleus
• pontine nucleus
• spinal nucleus

decussate

form the trigemino-thalamic tract

reach thalamus

pass thalamocortical neuron

to sensory cortex

Question 15

what are the trigeminal nuclei?

what are their roles?

Answer 15

mesencephalic nucleus

• cell bodies of trigeminal proprioceptors
• innervate muscles of mastication & facial expression

main / pontine nucleus

• receives input from trigeminal ganglion neurons that detect discriminative touch sensation

spinal nucleus

• receives input from trigeminal ganglion neurons detecting pain, temperature, itch, simple touch
• somatic sensory input also from cranial nerves VII (geniculate ganglion), IX (petrosal ganglion) and X (nodose ganglion) and spinal nerves C2-C4

Question 16

how is the face innervated and mapped?

Answer 16

spinal trigeminal tract

• Input to the spinal trigeminal nucleus is arranged in a posterior to anterior fashion.
• As a result, the face is mapped in a series of five concentric dermatomes with sharp borders for pain, temperature, itch and simple touch.

Question 17

describe sensory innervation of the head and neck?

draw and label

Answer 17

• each circular dermatome is innervated by all three branches of the trigeminal nerve
• cranial nerves VII IX and X provide somatic sensory innervation to the external ear and auditory canal
  
  • cranial nerve VII associated somatic sensory neurons are in geniculate ganglion
  • cranial nerve IX associated somatic sensory neurons are in petrosal ganglion
  • cranial nerve X associated somatic sensory neurons are in nodose ganglion
• Spinal nerves C2-C4 provide somatic sensory innervation to the lower rear of scalp, lower edges of the jaw and neck
• The central axons of somatic sensory neurons from C2-C4 DRG and geniculate, petrosal and nodose ganglia innervate second order neurons in the spinal trigeminal nucleus

Question 18

draw and label a diagram showing the sensory innervation of the internal structures of the head

Answer 18