Somatic senses  There are 4 somatosensory modalities  Touch  Temperature  Nociception (pain and itch)  Proprioception

Pathways for somatic perception – 1 st and 2 nd neurons  Receptors for the somatic sensations are found both un the skin and viscera  Receptor activation triggers AP in the 1 st order neuron  In the spinal cord, sensory neurons synapse with interneurons – 2 nd order neurons  All 2 nd 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 2 nd 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 2 nd neuron and cross over

Pathways for somatic perception – 2 nd and 3 rd neurons  The synapse between the 2 nd and the 3 rd happens in the thalamus  The axons of the 3 rd 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

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  1 st order neurons synapse with the 2 nd in the posterior gray horn at the level of entrance  The 2 nd cross before ascending to the thalamus  3 rd order synapse at the level of the primary somatosensory cortex

Spmatosensation perception  The specific sensation depends on the 2 nd and 3 rd 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 2 nd order in the medulla  2 nd order neurons cross over in the brain stem  3 rd order in the thalamus where the stimuli are sorted by the nature of stimulus and the region of body involved

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  1 st order neurons synapse in the dorsal horn  2 nd order neurons ascend via anterior and posterior spinocerebellar tracts to the cerebellar cortex  Used to coordinate movements  In this pathway there is no 3 rd order neuron

PathwaySensation1 st order2 nd order3 rd orderFinal destination Spinothalamic pathway Lateral spinothalamic Pain and temperature Dorsal root ganglion Posterior hornThalamus Primary sensory cortex (opposite side) Anterior spinothalamic Crude touch and pressure Dorsal root ganglion Posterior hornThalamus Primary sensory cortex (opposite side) Posterior column pathway Fasciculus 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 pathway Anterior and posterior ProprioceptionDorsal root ganglion Posterior hornNot presentCerebellar 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 1 st order neuron usually travel with the autonomic motor fibers innervating the same visceral structures  2 nd 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 (V 1 ), sensory from face  maxillary (V 2 ),  mandibular (V 3 ) supplies motor fibers (V 3 ) 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