Download Somatic senses

Somatic senses
 There are 4 somatosensory modalities
 Touch
 Temperature
 Nociception (pain and itch)
 Proprioception
Pathways for somatic perception – 1st and 2nd neurons

Receptors for the somatic sensations are found both un the skin
and viscera

Receptor activation triggers AP in the 1st order neuron

In the spinal cord, sensory neurons synapse with interneurons – 2nd
order neurons

All 2nd order neurons cross over at some point (sensations are
being integrated in the opposite side)

Neurons associated with nociception, temperature and coarse
touch synapse with 2nd neurons shortly after entering the spinal
cord and cross over in the spinal cord

Most fine touch, vibration and proprioceptive neurons have very
long axons that project all the way to the medulla where they
synapse with the 2nd neuron and cross over
Pathways for somatic perception – 2nd and 3rd neurons
 The synapse between the 2nd and the 3rd happens in the
thalamus
 The axons of the 3rd order neurons project to the
appropriate somatosensory area in the cerebral cortex
Thalamic Function


The thalamus is the “gateway to the cerebral cortex”
Major relay station for most sensory impulses that arrive to the
primary sensory areas in the cerebral cortex:
 taste, smell, hearing, equilibrium, vision, touch, pain, pressure,
temperature
 Contributes to motor functions by transmitting information from
the cerebellum and basal ganglia to the cerebral primary motor
area
 Connects areas of the cerebrum
 Impulses of similar function are sorted out, edited, and
relayed as a group
http://medinfo.ufl.edu/year2/neuro/review/images/thalamus.jpg
3 major somatosensory pathways –1) spinothalamic
pathway
 Conscious sensation of poorly localized sensations
 Anterior spinothalamic tracts – crude touch and
pressure
 Lateral spinothalamic tracts – pain and temperature
 1st order neurons synapse with the 2nd in the posterior
gray horn at the level of entrance
 The 2nd cross before ascending to the thalamus
 3rd order synapse at the level of the primary
somatosensory cortex
http://webanatomy.net/anatomy/spinothalamic.jpg
Spmatosensation perception
 The specific sensation depends on the 2nd and 3rd
neurons
 The ability to localize the specific location of a
stimulus depends on the stimulation of a specific area
in the primary somatosensory cortex
 A sensory “homunculus” (little human) is a functional
map of the primary somatosensory cortex
Somatosensory Association Cortex
 Located posterior to the primary somatosensory cortex
and has connection with it
 Integrates sensory information like temperature and
pressure coming from the primary somatosensory
cortex.
 Forms understanding of the stimulus like size, texture,
and relationship of parts
 Ex.: putting the hand in the pocket and feeling
something. The center integrate previous information to
identify objects without seeing them
Pain pathways

Pain is a subjective perception

It is individual and can vary depending on emotional state

Types of pain sensations:
 Fast pain – sharp and localized
 Rapidly transferred to CNS by small myelinated fibeers
 Slow pain – more diffused pain
 Carried by small unmyelinated fibers

Often slow pain will follow a fast one

Pain from the body – via spinal cord

Pain from face – via trigeminal (V) that enters the pons, descend
to the medulla where they cross over and ascend to the thalamus
Pain pathways
 The ascending pathway sends branches not only to
thalamus and the cerebral cortex but also to the limbic
system (emotions) and hypothalamus (autonomic
reaction)
 The result is that pain may be accompanied by
emotional distress and autonomic reactions such as
nausea, vomiting or sweating
Pain perception
 Pain can be felt in skeletal muscle when anaerobic
metabolism
 In cardiac muscle, pain is a result of ischemia (lack of
oxygen due to reduced blood flow) during myocardial
infraction (heart attack)
 Visceral pain is poorly localized and called referred
pain
3 major somatosensory pathways - 2) Posterior column
pathway
 Sensation of precise touch, vibration and
proprioception
 Includes
 Left and right fasciculus gracilis (inferior part of
the body)
 Left and right fasciculus cuneatus (superior part of
the body)
 First order neurons enter the CNS at the dorsal roots
and the sensory roots of cranial nerves.
 Synapse with 2nd order in the medulla
 2nd order neurons cross over in the brain stem
 3rd order in the thalamus where the stimuli are sorted by
the nature of stimulus and the region of body involved
http://webanatomy.net/anatomy/gracilis_cuneatus.jpg
3 major somatosensory pathways – 3) The spinocerebellar
pathway
 Information about muscle, tendon and joint position
from the spine to the cerebellum
 This information is subconscious
 1st order neurons synapse in the dorsal horn
 2nd order neurons ascend via anterior and posterior
spinocerebellar tracts to the cerebellar cortex
 Used to coordinate movements
 In this pathway there is no 3rd order neuron
http://webanatomy.net/anatomy/spinocerebellar.jpg
1st order
2nd order
3rd order
Pathway
Sensation
Spinothalamic pathway
Lateral
Pain and
spinothalamic temperature
Dorsal root
ganglion
Posterior horn
Thalamus
Anterior
Crude touch and
spinothalamic pressure
Dorsal root
ganglion
Posterior horn
Thalamus
Medulla
oblongata
Thalamus
Primary sensory
cortex (opposite
side)
Medulla
oblongata
Thalamus
Primary sensory
cortex (opposite
side)
Posterior horn
Not present Cerebellar cortex
Posterior column pathway
Fasciculus
Proprioception, fine Dorsal root
gracilis
touch and pressure ganglion
from inferior half of
the body
Fasciculus
cuneatus
Proprioception, fine Dorsal root
touch and pressure ganglion
from superior half of
the body
Spinocerebellar pathway
Anterior and Proprioception
posterior
Dorsal root
ganglion
Final destination
Primary sensory
cortex (opposite
side)
Primary sensory
cortex (opposite
side)
Visceral sensory pathways
 Collected by interoceptors within the closed ventral body
cavities
 The interoceptors include nociceptors, thermoreceptors,
tactile receptors, baroreceptors and chemoreceptors
 The axons of the 1st order neuron usually travel with the
autonomic motor fibers innervating the same visceral
structures
 2nd order neurons within the spinal cord use the spinothalamic
pathway and arrive to the medulla oblongata
 Cranial nerves V, VII, IX and X carry visceral sensory
information also to the medulla
 (all parasympathetic and will be discussed with the ANS)
Cranial Nerve V: Trigeminal
 Fibers run from the face to the pons, to the thalamus
and to the primary somatosensory cortex
 Three divisions:
 ophthalmic (V1),
sensory from face
 maxillary (V2),
 mandibular (V3) supplies motor fibers (V3) for
mastication
Cranial Nerve VII: Facial
 Fibers leave the pons to the lateral
aspect of the face
 Mixed nerve with five major
branches
 Motor functions include facial
expression, and the transmittal of
autonomic impulses to lacrimal
and salivary glands
(subconscious)
 Sensory function is taste from the
anterior two-thirds of the tongue
(taste buds to pons, to the
thalamus, to the insula and parietal
cortex for taste perception)
Cranial Nerve IX: Glossopharyngeal
 Fibers emerge from the
medulla and run to the
throat
 Nerve IX is a mixed nerve
 Motor – innervates part of
the tongue and pharynx,
and provides motor fibers
to the parotid salivary
gland (autonomic)
 Sensory – fibers conduct
taste and general sensory
impulses from the tongue
and pharynx
Cranial Nerve X: Vagus
 The only cranial nerve that
extends beyond the head and
neck
 Fibers emerge from the
medulla The vagus is a
mixed nerve
 Most motor fibers are
parasympathetic fibers to the
heart, lungs, and visceral
organs
 Its sensory function is in
taste