1. Somatosensory system

2. Peripheral Somatosensory neurons Sensory receptors
Peripheral nerve axon
Dorsal root ganglion
Spinal cord
Brain
Peripheral
Somatosensory
neurons

3. Introduction Somatosensation: Skin : Musculoskeletal :
the sensory information from the skin and musculoskeletal systems
Skin :
Superficial sensory or cutaneous
Superficial sensory information :
touch, pain, temperature
Touch
Superficial pressure and vibration
Musculoskeletal :
proprioception and pain

4. Proprioception
stretch of muscles, tension on tendons, position of joints, and deep vibration
both static joint position sense and kinesthetic sense, sensory information about movement
information in the somatosensory system proceeds from the receptor through a series of neuron to the brain
Sensory information:
nerve impulses generated from the original stimuli
Sensation:
awarness of stimuli from the senses
Perception:
the integration of sensation into meaningful forms
(an active process of interaction between the brain and the environment)

5. Pathway convey Somatosensory information share similar anatomic arrangements
Receptors in the periphery encode the mechanical, chemical or thermal stimulation received into receptor potentials
Action potential  peripheral axon soma in dorsal root ganglion  proximal axon in spinal cord via axons white matter brain
The diameter of the axons, the degree of axonal myelination, and the number of synapse in the pathway determine how quickly the information is processed

6. Peripheral somatosensory neurons
Sensory receptors
Somatosensory peripheral neurons
Cutaneous innervation
Musculoskeletal innervation

7. Sensory receptors : located at the distal ends of peripheral nerves
mechanoreceptors, responding to mechanical deformation of the receptor by touch, pressure, stretch, or vibration
chemoreceptors, responding to substances released by cells, including damaged cells following injury or infection
thermoreceptors, responding to heating or cooling
Nociceptors
Tonic receptors
Phasic receptors

8. Somatosensory peripheral neurons
cell body of most peripheral sensory neurons : outside the spinal cord in the dorsal root ganglion or outside the brain in cranial nerve ganglia
Two axons :
distal axons conduct message from receptors to the cell body
proximal axons project from the cell body into the spinal cord or brain stem

9. Classification of afferent axons
Factors that influence nerve conduction velocity
-Diameter of the axons(the larger is the faster)
-The degree of axonal myelination

10. Cutaneous innervation - receptive field : the area of skin innervated by each neuron

11. Receptive field: tend to be smaller distally and larger proximally
Skin sensations:
Touch
Pain
Temperature
Fine touch:
superficial fine touch receptors : Meissner’s corpuscles, Merkel’s disks, hair follicle receptors
subcutaneous fine touch : pacinian corpuscles, Ruffini’s corpuscles

12. Cutaneous receptors Coarse touch: Nociceptors Thermal receptors
-free endings throughout the skin (localized touch or pressure and sensations of tickle and itch)
Nociceptors
-Free nerve endings, responsive to stimuli that damage or threaten tissue.
Thermal receptors
-Also free nerve endings, respond to either warmth or cold within the temperature
Cutaneous receptors
-respond to touch, pressure, vibration, stretch, noxious stimuli, and temperature
Dermatome
-the area of skin innervated by axons from cell bodies in a single dorsal root

17. Musculoskeletal innervation Muscle spindle
Are embedded in skeletal muscle.
the sensory organ in muscle – muscle fibers, sensory endings, and motor endings
Sensory endings
stretch that is changes in muscle length and the velocity of length change
Quick and tonic stretch of the spindle
type Ia afferents
Tonic muscle stretch
type II afferents

19. Intrafusal and extrafusal fibers
Intrafusal fibers contractile only at their ends; the central region cannot contract
nuclear bag fibers
Have a clump of nuclei in the central region
nuclear chain fibers
Have nuclei arranged single fiber
Two different sensory endings
primary endings : phasic and tonic (annulospiral ending)
secondary endings : tonic(flower-spry ending)

21. Muscle length is signaled by type Ia and II afferents reflecting stretch of the central region of both types of intrafusal fibers.
Spindle sensitivity to changes in length is adjusted by gamma static efferents.
Velocity of change in muscle length is signalized only by type Ia afferents,
with information mainly from nuclear bag fibers whose sensitivity is adjusted by gamma dynamic efferents.

22. Golgi tendon organs
- sensitive to very slight changes in the tension on a tendon and respond to tension exerted both by active contraction and by passive stretch of muscle
- Ib afferents
Joint receptors
- respond to mechanical deformation of capsule and ligaments
-ruffini’s endings(II) : extremes of joint range and respond more to passive than to active movement
- paciniform corpuscles(II) : respond to movement
- free nerve endings(Aδ and C) : stimulated by inflammation
- ligament receptors(Ib) : similar to Golgi tendon organs and signal tension

23. Fully normal proprioception
Requires muscle spindles, joint receptors, and cutaneous mechanoreceptors.

25. Golgi tendon organs and muscle spindles: how do they compare?
GTO
Muscle spindle
Location
Within tendon near the muscle-tendon junction in series with muscle fibers
Interspersed among muscle fibers in parallel with the fibers
Stimulus
Increase in muscle tension
Increase in muscle length
responses
1)Inhibits tension development in streteched muscle
2)Initiate tension development in antagonist muscles
1)Initiate rapid contraction of stretched muscle
2)Inhibits tension development in antagonist muscles

26. Summary of the function of different-diameter axons
large-diameter afferents:
muscle, tendons, and joints
medium-sized afferents:
joint capsule, muscle spindles, and cutaneous touch, stretch, and pressure receptors
smallest-diameter afferents:
crude touch, nociceptive, and temperature information

28. Pathway to the brain Conscious relay pathways Divergent pathways
Unconscious relay pathways

30. Conscious relay pathways to cerebral cortex
- touch
- proprioception
- pain
- temperature
Three projection neurons
Two routes
- discriminative touch and conscious proprioception (dorsal columns, medial leminiscus system)
discriminative pain and temperature (anterolateral tracts=neospinothalamic and trigeminospinothalamic) : fast pain
To be aware of sensory information
The information must reach the thalamus

31. Discriminative touch and conscious proprioception pathway
localization of touch and vibration and ability to discriminate between two closely spaced points touching the skin
Conscious proprioception:
awareness of the movements and relative position of body parts
Stereognosis:
the ability to use touch and proprioceptive information to identify an object

32. Three-neuron relay
the primary, or first-order, neuron conveys information from the receptors to the medulla
the secondary, or second-order, neuron conveys information from the medulla to the thalamus
the third-order, neuron conveys information from the thalamus to the cerebral cortex

33. Dorsal Column/Medial Lemniscus System
Fasciculus gracilis
Axons from the lower limb occupy the more medial section of the dorsal column
Nucleus gracilis
Fasciculus cuneatus
Axons from the upper limb occupy the more the lateral section of the dorsal column
Nucleus cuneatus

34. Tactile information from the face
Sensory information essential for identifying objects by palpation, distinguishing between closely spaced stimuli, and controlling fine movement and smoothness of movements travels in the dorsal columns  medial leminiscus VPLS1 cortex
Tactile information from the face
travels in the trigeminal nerve thalamus(VPM)  sensory cortex

37. Discriminative pain and temperature, coarse touch
Anterolateral columns
Pain
Temperature
Coarse touch
Conveys less information than the dorsal column/MLS, and thus coarse touch cannot be tested independently when discriminative touch is intact.
Is involved with pleasant touch and skin-to-skin contacts.
Spinothalamic tract

38. Temperature sensation
warmth and cold
free nerve ending of small myelinated and unmyelinated neurons
Aδ : cooling, C : heat
proximal axon (segment of the spinal cord) the second axon (cross the midline and then ascend contralaterally to VPL nucleus of the thalamus) sensory cortex

39. Neospinothalamic pain: Spinolimbic pain: Both pain:
complex phenomenon; persistent pain affects emotional, autonomic, and social functioning
Pain is composed
Protective and the emotional response
Nociceptive
Describes receptors or neurons that receive or transmit information about stimuli that damage or threaten to damage tissue.
Several different pathways
Neospinothalamic pain:
fast pain (conscious relay pathway)
Spinolimbic pain:
slow pain (divergent pathway)
Both pain:
anterolateral section of the spinal cord

40. Fast, localized pain
the first-order neuron brings information into the dorsal horn of the spinal cord
the axon of the second-order neuron crosses the midline and projects from the spinal cord to the thalamus
the third-order neuron projects from the thalamus to the cerebral cortex

43. Divergent pathways Medial pain system
Projection neurons synapse in medial locations in the central nervous system
Several pathways with variable numbers of projection neurons, not a three neuron pathway like fast pain.
The information from the medial pain systems is not somatotopically organized, so slow pain cannot be precisely localized.

44. Divergent pathways Solw, aching pain: First neuron:
depends on a divergent network, not a three-neuron pathway like fast pain
First neuron:
small, unmyelinated C fibers, sensitive to noxious heat, chemical, or mechanical stimulation

45. Ascending projection neurons:
wide-dynamic-range neuron
The ascending axons reach the midbrain, reticular formation, and thalamus via three tracts
Spinomesencephalic
Spinoreticular
Spinolimbic

47. Spinomesencephalic tract
nociceptive information to the superior colliculus and to an area surrounding the cerebral aqueduct, the periaqueductal gray
involved in turning the eyes and head toward the source of noxious input and in activating descending tracts that control pain
periaqueductal gray is part of the descending pain control system

48. Spinoreticular ascending neurons:
Reticular formation is a neural network in the brainstem that includes the reticular nuclei and their connections.
Arousal, attention, and sleep/waking cycles are modulated by the reticular formation.
reticular formation  midline and intralaminar nuclei of the thalamus
Spinolimbic track
midline and intralaminar nuclei of the thalamuscerebral cotrex involved with emotions, sensory integration, personality, and movement

49. The slow pain pathways:
produces automatic movements and autonomic and emotional responses to noxious stimuli
Slow pain information from the face :
trigeminoreticulothalamic pathway: C fibers in the trigeminal N. reticular formation intralaminar nuclei

50. Tempeature
Reticular formation, nonspecific nuclei of the thalamus, to subcortical nuclei, and to the hypothalamus
This temperature information that does not reach conscious awareness contributes to arousal, provides gross localization, and contributes autonomic regulation

51. Unconscious relay to the cerebellum
Information from proprioceptors and information about activity in spinal interneurons are transmitted to the cerebellum via Spinocerebellar tracts
Adjusting movements

52. High-fidelity pathways
Two pathways
-arranged information to the cerebellar cortex
posterior spinocerebellar pathway
-Transmits information from the legs and the lower half of the body
-1st order; nucleus dorsalis
-2nd order; posterior spinocerebellar tract.
-the tract remains ipsilateral and projects to the cerebellar cortex via the inferior cerebellar peduncle.
cuneocerebellar pathway
-begin with primary afferents from the arm and upper half of the body

53. High fidelity pathway Posterior spinocerebellar pathway :
Transmit information from the leg and the lower half of the body
Dorsal column to the thoracic or upper lumbar spinal cord(T1-L2)  nucleus dorsalis  posterior spinocerebellar tract
Ipsilateral and projects to the cerebellar cortex

54. Cuneocerebellar pathway :
Afferents from the arm and upper half of the body
Lateral cuneate nucleus (first – and second-order neurons) in the medulla  cuneocerebellar tract  ipsilateral inferior cerebellar penduncle cerebellar cortex

57. Internal feedback tracts
the two internal feedback tracts monitor the activity of spinal interneurons and of descending motor signals from the cerebral cortex and brain stem
anterior spinocerebellar tract
Transmits information from the thoracolumbar spinal cord
rostrospinocerebellar tract

58. Internal feedback tracts
Anterior spinocerebellar tract :
Transmit information from the thoracolumbar spinal cord
Lateral and ventral horns -> cross the opposite side and ascend in the contralateral anterior spinocerebellar tract to the midbrain -> cerebellum via superior cerebellar penduncle
Gets information from both sides of the lower body
Bilateral projection : automatic coordination of lower limb activities

59. Rostrospinocerebellar tract :
Transmits information from the cervical spinal cord to the ipsilateral cerebellum and enters the cerebellum via both the inferior and superior cerebellar peduncles
The anterior and rostrospinocerebellar tracts :
- apprise the cerebellum of the descending commands delivered to the neurons that control muscle activity via interneurons located between descending motor tracts and motor neurons that innervate muscles
- the activity of spinal reflex circuits

60. Functions of spinocerebellar tracts - not consciously perceived
- unconscious adjustments to movements and posture
internal feedback tract :
*convey descending motor information to the cerebellum prior to the information reaching the motor neurons
high-fidelity pathway
*convey information spindles, tendon organs, and cutaneous mechanoreceptors; cerebellum obtains information about movements commands and about the movements or postural adjustments
-cerebellum can compare the intended motor output with the actual movement output

61. Information in the spinocerebellar tracts is from proprioceptors, spinal interneurons, and descending motor pathways.
This information, which does not reach conscious awarness, contributes to automatic movements and postural adjustments