Neuroanatomy Lecture #23: Somatosensory System

Front 1

The Somatosensory System:
• The five modalities and where each originate
• The morphology of neurons in the somatic sensory system
• Where somatic sensory fibers and cell bodies are located

Back 1

Somatosensory system:
• The five modalities and their origin
- touch, vibration, pain and temperature all originate in the skin
- proprioception originates in the skeletal muscle system

• They are all pseudo-unipolar

• Fibers run in peripheral nerves, The cell bodies are located in either DRG (for spinal nerves) or Cranial nerve ganglia (for cranial nerves)

Front 2

The Cutaneous Receptors:

• Touch - one basic type of receptor, two examples of these receptor types, their location in the skin and their fxn

• Vibration - one basic type of receptor, two examples of these receptor types, their location in the skin and their fxn

• Pain - two basic types of receptors, the part of the nerve that transduces them and its location in the skin

• Temperature - two basic types of receptors, the part of the nerve that transduces them and its location in the skin

Back 2

The Cutaneous Receptors:

• Touch - Mechanoreceptor
> Merkel's disks - discriminitive touch, located in the dermis
> Ruffini's endings - skin stretching, located in the dermis

• Vibration - Mechanoreceptors
> Meissner's corpuscles - most sensitive to low frequency vibrations (~50Hz), located in the dermis
> Pacinian - most sensitive to high frequency vibrations (~300 Hz), located in the dermis, but lower spatially than Meissner's

• Pain - Mechano- and Thermo- receptors, transuced by free nerve endings, located in the epidermis

• Temperature - Thermoreceptors, transduced by free nerve endings, located in the epidermis

Front 3

The Muscle Receptors:
• Involves this receptor modality
• The main somatic sensory receptors of the musculo-skeletal system (2)
• Involved in forming two pathways to the cerebellum
- Significant feature of the input carried by these tracts

Back 3

The Muscle Receptors:
• Proprioceptors
• Muscle spindles and Golgi tendon organs
- Dorsal and Spinal cerebellar tracts
- the sensory info does not reach consciousness

Front 4

Muscle Spindles:
• General location
• Its type of primary receptor
> the receptor contains this fiber
> carries this sensory information
> forms this limb of the of this reflex arc

Back 4

Muscle Spindles:
• Embedded in extrafusal fibers of muscle
• Rapidly adapting receptor
> Ia afferent
> carries sensory information about muscle stretch
> forms the afferent limb of the myotactic reflex arc

Front 5

Golgi Tendon Organs:
• General location
• Features about the receptor
> Type of fiber it contains
> Forms this limb of the of this reflex arc

Back 5

Golgi Tendon Organs:
• Located close to the border btwn muscle and tendon
• Features about the receptor
> contains Ib afferent fibers
> Forms the afferent limb of the of reverse myotactic reflex arc

Front 6

Name the four fiber types of somatic sensory receptors
• Which have the highest and lowest conduction velocities
• How are they classified under the Lloyd system of skeletal muscle afferent fibers
• How are they classified under the Erlanger & Gasser system of spinal nerves

Back 6

Large myelinated
Medium myelinated
Small myelinated
Unmyelinated

• Large and Medium myelinated fibers have the highest conduction velocities

• Unmyelinated fibers have the smallest conduction velocities

Lloyd system of skeletal muscle afferent fibers
- Large and medium myelinate muscle fibers are Type I a & b
- Type II in muscle afferents correspond to Type II in Lloyd's
- Small myelinated are Type III
- Small unmyelinated are Type IV

Erlanger & Gasser system of spinal nerves
- Large and Medium myelinated nerves are Type A-alpha
- Small myelinated are Type A-delta
- Small unmyelinated are Type C

Front 7

Large & Medium myelinated fibers:
• are found in these types of cutaneous receptors
• carry these types of information

Back 7

Large & Medium myelinated fibers:
• found in mechano- and propio-receptors
• carry touch, vibration and sensory information

Front 8

Small myelinated and unmyelinated fibers carry this type of input

Back 8

pain and temp

Front 9

The largest drop of in conduction velocity occurs btwn these two sensory fibers

Back 9

Myelinated (4 - 36 m/s) and Unmyelinated (0.4 - 2 m/s) fibers

Front 10

Relative to which vertebrae(s) do the following spinal nerves emerge:
• C1
• C8
• T12
• L1

Back 10

• C1 - Above C1 vertebrae
• C8 - Below the C7 vertebrae
• T12 - Below the T12 vertebrae
• L1 - Btwn the L1 and L2 vertebrae

Front 11

Approximate locations for the following dermatomes:
• C2
• C6
• C7
• C8
• T4
• T10
• L1
• L5
• S1

Back 11

Locations for the following dermatomes:
• C2 - Back of the head
• C6 - Thumb
• C7 - Middle finger
• C8 - Pinky
• T4 - Nipple live
• T10 - Umbilicus
• L1 - Inguinal ligament/groin
• L5 - Big toe
• S1 - Little toe

Front 12

Two pathways in the spinal cord that carry different sensory modalities

Back 12

Dorsal Column/Medial Lemniscus System
Anterolateral System

Front 13

Dorsal Column/Medial Lemniscus System:
• Carry these sensory modalities (3)
• Trajectory
- where the fibers enter
- where they synapse

• Topographical organization of the fibers originating from the leg trunk and arm
- their location in the spinal cord in relation to one another

Back 13

Dorsal Column/Medial Lemniscus System:
• Carry touch, vibration and proprioception
• Trajectory
- fibers enter the dorsal column of the spinal cord
- ascend up the white matter tract tract
- where they synapse in the nuclei of the dorsal columns

• Fibers originating from the leg are medial, the arm is lateral and the trunk is in between.

Front 14

Anterolateral System:
• Carry these sensory modalities (2)
• Trajectory
- where the fibers enter
- the location of 2nd order neurons on to which they synapse
- the remaining pathway of the 2nd order neurons, include the commissure they use
- Final destination of the tract

• Topographical organization of the fibers originating from the leg trunk and arm
- their location in the spinal cord in relation to one another

Back 14

Anterolateral System:
• Carry pain and temperature
• Trajectory
- fibers enter the dorsal column of the spinal cord
- they synapse on 2nd order neurons in the dorsal horn
- The 2nd order neurons cross to the contralateral side through the anterior commissure and ascend in the ALS fiber tract of the spinal cord
- Thalamus

• Fibers originating from the arm are medial, the leg is lateral and the trunk is in between.

Front 15

Lissauer's Tract:
• another name
• Carry these sensory modalities (2)
• Trajectory
- where the fibers enter
- the location of 2nd order neurons on to which they synapse
- the remaining pathway of the 2nd order neurons and the tract they join
- Final destination of the tract

Back 15

Lissauer's tract:
• Dorsolateral fasciculus
• Pain and temperature
• Trajectory
- fibers enter the dorsal column of the spinal cord
- they synapse on 2nd order neurons in the dorsal horn, both above and below the level of entry
- The 2nd order neurons cross to the contralateral side and ascend with the ALS tract, eventually synapsing at the VPLN of the Thalamus

Front 16

Brown Sequard Syndrome:
• Cause and fibers involved
• Tracts involved, Location of and presentation of sx
- Discuss what happens at the level of and below the level of the lesion
• Tract that can help recover some of the lost information and why

Back 16

Brown Sequard Syndrome:
• Hemisection of the spinal cord, ascending & descending fibers are interrupted on one side

• Information
-- At the level of the lesion:
> The DC/ML system is affected and PTV are lost on the ipsilateral side (lesioned/injured side)
> The ALS system is affected and Pain and Temp are lost on the contralateral side
> The Corticospinal Tract is affected with LMNs in the anterior/ventral horn being damaged. Presentation of LMN syndrome at this level will include: flaccid paresis, fasciculations & fibrillations, and hypo- or a-reflexia

-- Below the level of the lesion,
> The DC/ML System is affected and TVP is lost ipsilaterally
> The AL System is affected and Pain and Temp are contralaterally, starting 1-2 segments below the lesion
> The Corticospinal Tract is affected and sx conducive to UMN syndrome are observed ipsilaterally:
Hyperreflexia, clonus, Extensor Plantar reflex and spastic paralysis
> Lissauer's tract, because it sends its input contralaterally to the ALS and has nociceptors that iNN not just at the level of axon entry but 1 - 2 segments above & below their level of entry.

Front 17

Syringomyelia:
• Definition
• Type of sensory information lost, include sidedness

Back 17

Syringomyelia:
• A pathological enlargement of the central canal of the spinal cord, interrupting fibers that cross through the white commissure

• Pain and temperature lost bilaterally