Somatic Sensory System

Sensation arising from skin muscle joints

Allow you to survive in your environment and make appropriate

motor responses

Somatic Sensation

• The ability to feel your physical environment, to ache, feel temperature, pain, to know where your body position is

• Pressure, position of joints/muscles, temperature, distension of bladder, stomach

• Over-stimulation of body can be damaging

Sensory Receptors

• Spread throughout body

• Senses 4 types of information aka sensory modality– Touch, pressure, vibration– Body position, propioceptive receptors– Temperature– Pain-Nociception

Encoded Information

• Intensity

• Duration

• Position

• Direction

Propioception

• Sensory Information regarding joint and limb position

• Allow you to know your body position without looking in the mirror

• Muscle spindles- intrafusal fibers and • Golgi Tendon organs are specialized

structures that are innervated by DRGNs and send information to the spinal cord

Touch

• Skin: largest sensory organ– Epidermis (outer layer) & dermis (inner layer)

• Sensitive to raised dot: 0.006mmH x 0.04mmW; braille dot is 167 times bigger

• Skin receptors– Hairy & glabrous (hairless)

12.1

Mechanoreceptors

• Found in skin for sensing contact with physical world

• Found in bladder, blood vessels, heart digestive organs and teeth to sense pressure

• Mechanoreceptors are innervated by myelinated axons

• Axons have mechanosensitive ion channels gated by stretch & tension changes

• Pacinian corpuscle

– Found in dermis

• Meissner corpuscle

– Found in ridges of glabrous skin

• Merkel’s disc in epidermis

• Rufini endings

– Found in hairy & glabrous skin

Types of Mechanoreceptors Found in Skin

Primary Afferent Axon

• Axons of varying diameters with soma in DRG and enter Spinal cord through dorsal roots into dorsal horn

• Different diameter axons carry different types of somatosensory information

• Project locally in spinal cord and have long ascending branches to contact secondary somatosensory axons

Receptive Field

• Area of skin that is monitored by a single mechanoreceptor– Meissner & merkel have small RF (2-3mm)– Pacinian & ruffini have large RF (entire finger/

½ palm)

F 12.2

Dermatomes

• The area of skin innervated by the right and left dorsal roots (1st order neuron) of a single spinal segment

• When mapped, dermatomes form sets of bands representing surface of body innervated by axons in one level of sc.

Add F 12.10-12

2-point discrimination

• Ability to discern 2 closely position points as 2 rather than 1.

• Varies 20 fold throughout body• Fingertips have highest resolution

– Due to high density of mechanoreceptors– Receptor subtypes with small receptive fields– More cortical neurons dedicated to deciphering

sensory information

F 12.6

First and Second Order Neurons

• Primary and secondary somatosensory or afferent neurons– The primary neuron has the sensory receptor– The secondary neurons gets information from

the primary and can project information or modify locally activity of primary neurons.

Second order Neurons aka Interneurons

• Neurons that receive synaptic input form DRG neurons

• Reside in dorsal horn and trigger reflex responses

• Also ascend to brainstem and thalamus

• Also reside in brain stem and are involved in perception

Add 12.13

Dorsal Column-Medial Leminiscus Pathway

• Touch/vibration & position (proprioception) info travels to brain separate from pain/temperature

• Afferent/central axon of large sensory (AB) fibers ascend ipsilaterally in dorsal columns with tactile info and limb position info

• DC also have 2nd order ascending axons from dorsal horn neurons

Dorsal column nuclei

• Axons terminate in DCN in medulla• Then decussate (cross) and ascend as

medial lemniscus tract through pons & midbrain to synapse in ventral posterior nucleus (VPN) of thalamus

• VPN axons then project to primary somatosensory neurons in parietal cortex (S1)

Somotopic Organization

Somotopy

• Mapping of body areas sensation onto the cortex

• Somotopic map called homunculus that shows that the largest number of neurons in S1 receive sensory information from hand and mouth

Somatosensory Cortex

• In Parietal lobe, posterior to central sulcus• Carries on higher order processing of sensory

information. Called S1• Receives synaptic input from VP nucleus of

thalamus• Respond to somatosensory info• Lesions in S1 impair somatic sensation• Electrically stimulate S1 and you “feel” a

sensation on the appropriate body part

S1 Cortex

• Reciprocal (bidirectional) connections between cortical areas

• Association pathways • Restriction of information: some cortex

areas specialize in decoding texture, size & shape

• Thalamic input is to cortical layer IV which send axons to other layers in same area

S2 and Parietal Posterior Cortex

• S2 is lateral to S1 and is association area

• PPC is posterior to S1 and is involved in perception/recognition of sensation

• Neurons in S2 and PPC have complex receptive fields which can include sensory information as well as attention and visual and movement planning.

Posterior Parietal Cortex

• Injury causes neglect syndrome: Do not recognize body part as your own so you do not dress it, wash it.

• Agnosia: Inability to recognize objects including your own body parts.

• Astereoagnosia: inability to recognize something by touch, but recognized by sight

Trigeminal Touch Pathway

• Two trigeminal nerves CN5

• Each divided into 3 PNs that innervate face, mouth and anterior 2/3 tongue & dura mater

•

Sensory CNs

• CN 7 facial, 9 glossopharyngeal, 10 vagus• Large diameter axons carry tactile info from

skin mechanoreceptors• Synapse on ipsilateral trigeminal nucleus in

pons• Decussate and project to medial VP nucleus

of thalamus• Project to somatosensory cortex

Add 12.15-19