2. SOMATIC SENSATION
Enables body to feel, ache, chill. Responsible for feeling of touch and pain
Different from other systems because receptors are widely distributed
throughout all the body and responds to different kinds of stimuli
Types and layers of skin
Hairy and glabrous (hairless)
Epidermis (outer) and dermis (inner)
Functions of skin
Protective function
Prevents evaporation of body fluids
Provides direct contact with world
Mechanoreceptors
Most somatosensory receptors are
mechanoreceptors.
Pacinian corpuscles
Ruffini's endings
Meissner's corpuscles
Merkel's disks
Krause end bulbs
4. TOUCH RECEPTORS
Two-point discrimination varies across the body surface (Importance of fingertips over
elbow). Difference in density of receptors, size of receptive fields, brain tissue devolved in
processing the information
Big toe
sole
calf
back
lip
forearm
thumb
Index finger
5. PRIMARY AFFERENT AXONS
white matter
Gray matter
Dorsal root
Big toe
Dorsal root ganglion
Dorsal root
ganglion cell
receptor
Dorsal Spinal
root nerve
lip Primary Afferent Axons
Aα, Aβ, Aδ, C
C fibers mediate pain and temperature
A β mediates touch sensations
7. THE SPINAL CORD
Divided in spinal segments (30)- spinal nerves within 4 divisions
Dermatomes (area of the skin innervate by the R and L dorsal roots of a single
spinal segment) have 1-to-1 correspondence with segments
8. THE SPINAL CORD
Division of spinal gray matter: Dorsal horn; Intermediate zone; Ventral
horn
Myelinated Aβ axons (touch-sensitive) mainly synapses in the dorsal
horn with the second order sensory neurons
9. ASCENDING PATHWAYS
Dorsal Column–Medial Lemniscal Pathway The Trigeminal Touch Pathway
Touch information ascends through dorsal Trigeminal nerves
column, dorsal nuclei, medial lemniscus, Cranial nerves
and ventral posterior nucleus to primary
somatosensory cortex
S1
S1
dorsal column
nuclei
VPN trigeminal
nucleus VPN
dorsal column Medial lemniscus
From
face
10. SOMATOSENSORY CORTEX
Primary is area 3b
Receives dense input from VP nucleus
of the thalamus
Lesions impair somatic sensations
Electrical stimulation evokes sensory
experiences
Area 3a receive information from
vestibular system
Area 1 receive information from 3b and
code for texture
Area 2 receive information from 3b and
code for size and shape
Other areas
Posterior Parietal Cortex (5,7)
12. SOMATOSENSORY CORTEX
Cortical Map Plasticity
Remove digits or overstimulate – examine
somatotopy before and after
Showed reorganization of cortical maps
13. SOMATOSENSORY CORTEX
The Posterior Parietal Cortex
Involved in somatic sensation, visual stimuli, and movement planning
Lesion has been associated to: Agnosia, Astereoagnosia and Neglect
syndrome
14. PAIN
Pain - feeling associated to nociception
Nociception - sensory process, provides signals that trigger pain
Nociceptors: Transduction of Pain
Bradykinin , Mast cell activation: Release of histamine
Types of Nociceptors: Polymodal, Mechanical, Thermal and Chemical
Hyperalgesia: higher
sensitivity to pain in tissue
already damaged
Primary occurs in the
damaged tissues and
secondary hyperalgesia in
the surroundings
Bradykinin, prostaglandins,
and substance P
(secondary hyperalgesia)
15. PAIN
Primary Afferents First pain mediated by fast axons and second pain by slower C fibers
Spinal mechanisms
brain
Dorsal root
Ventral root
16. PAIN ASCENDING PATHWAYS
Main differences between touch and pain pathway
Nerve endings in the skin Spinothalamic Pain Pathway
Diameter of axons
Connections in spinal cord
Touch – Ascends Ipsilaterally
Pain – Ascends Contralaterally
Two pathways:
1) Spinothalamic Pain Pathway
2) The Trigeminal Pain Pathway
18. REGULATION OF PAIN
Afferent Regulation: gate theory of pain
Dorsal
horn
To dorsal column
To spinothalamic tract
19. REGULATION OF PAIN
Descending pain control pathway. Use of serotonin
Stimulation of the PAG cause deep analgesia
The endogenuos opiates
Opioids and endomorphins Primary auditory cortex
Secondary auditory cortex
20. TEMPERATURE
Thermoreceptors
“Hot” and “cold” receptors.
Varying sensitivities
The Temperature Pathway
Identical to pain pathway
Cold receptors coupled to Aδ
and C
Hot receptors coupled to C