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