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
Pathway Sensation 1st order 2nd order 3rd order Final destinationSpinothalamic pathwayLateral spinothalamic
Pain and temperature
Dorsal root ganglion
Posterior horn Thalamus Primary sensory cortex (opposite side)
Anterior spinothalamic
Crude touch and pressure
Dorsal root ganglion
Posterior horn Thalamus Primary sensory cortex (opposite side)
Posterior column pathwayFasciculus gracilis
Proprioception, fine touch and pressure from inferior half of the body
Dorsal root ganglion
Medulla oblongata
Thalamus Primary sensory cortex (opposite side)
Fasciculus cuneatus
Proprioception, fine touch and pressure from superior half of the body
Dorsal root ganglion
Medulla oblongata
Thalamus Primary sensory cortex (opposite side)
Spinocerebellar pathwayAnterior and posterior
Proprioception Dorsal root ganglion
Posterior horn Not present Cerebellar cortex
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
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