What is the somatic sensory system?
Sensation arising from skin, muscle, organs & joints
Allow you to Sense your environment and make appropriate motor responses for survival
Two Major Sensory Pathways
Spinothalamic - Pain and temp
Dorsal Column - Everything else
Comes from skin and muscles
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
Sensory receptors?
Senses 4 types of information aka sensory modality
Body position, propioceptive receptors
Touch, pressure, vibration
Temperature
Pain-Nociception
Spread throughout body
Encoded Information
(DDIP)
Duration
Direction
Intensity
Position
Propioception
Sensory information regarding joint and limb position
Allows you to know your body position without looking in the mirror
Muscle Spindles-Intrafusal fibers
encode muscle length
Golgi Tendon Organs
encode muscle tension
Both Muscle spindles-interfusal fibers and golgi tendon organs:
Are specialized structures innervated by DRGNs and send into to spinal cord
Touch
Involves skin, skin receptors and skin on fingers
Skin
Largest sensory organ
Made up of-epidermis (outer) and dermis (inner)
What are mechanoreceptors?
Found in skin for sensing contact with physical world
Also found in: bladder, blood vessels, heart, digestive organs and teeth (to sense pressure)
Mechanoreceptor axons
Innervated by myelinated axons
Have mechanosensitive ion channels gated by stretch and tension changes
Types of Skin Mechanoreceptors?
(PMR)
Pacinian corpusle
Meissner corpuscle
Rufini endings
Pacinian corpusle
Found in dermis and hairy and glabrous skin
Most sensitive to vibrations 200-300 Hz, high frequencies
Not responsive to continual pressure
Football Capsule with 20-70 concentric layers of connective tissue (onion-like)
Nerve terminal at center
Compression
Meissner corpuscle
Found in ridges of glabrous skin
Best at 50Hz of vibrations
Rufini endings
Found in dermis and hairy and glabrous skin
Lower than Meissner and feels like 'fluttering'
Merkel's disc is found in?
Epidermis
Properties of Mechanoreceptors
Receptive Fields
Stimulus Tuning
Adaptation
Stimulus Tuning
Preferred frequency of vibration
Preferred pressure
Receptive field
Area of skin that is monitored by a single mechanoreceptor (Portion of skin that can elicit
a response in a single neuron with sensitivity to
that particular sensory modality)
Meissner & merkel have small RF (2-3mm)
Pacinian & ruffini have large RF (entir
2-point discrimination
Ability to discern 2 closely positioned points as 2 rather than 1
Varies 20 fold throughout body
Fingertips have highest resolution
Why do finger tips have the highest resolution?
Due to high density of mechanoreceptors
Receptor subtypes with small receptive fields
More cortical neurons dedicated to deciphering sensory information on fingertips
Maybe special neural mechanisms devoted to high resolution discrimination
Differences in 2-Point Discrimination due to?
Density of mechanoreceptors
Receptor subtypes with small receptive fields
**MORE CORTICAL NEURONS DEDICATED TO DECIPHERING SENSORY INFORMATION ON FINGERS
Stimulus Preference
Tuned for sensing different types of mechanical energy
-Pressure
-Vibration
Adaptation, Activation/Inactivation
Response to long-lasting stimuli
Rapidly adapting:Meissner & pacinian**
Respond quickly at first, then stop
Slowly adapting: generate sustained response to stimulus
-Merkel Disc and Ruffini Corpuscle**
Inactivation
Caused by fluid between layers absorbing the energy so that axon is no longer deformed
Lowenstein 1960s stripped pacinian corpuscles to bare axons and found that rapid adaptation no longer occurred
Who stripped Pacinian corpuscles?
1960s Lowenstein, found that layered capsule is responsible for the pacinian being sensitive to vibrations. After he stripped it away the nerve was not as good at sensing vibrations
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
Size of primary afferent axons
Proprioceptors are the largest (Skeletal muscles)
Then Mechanoreceptors of skin
The pain, temperature
And the slowest are Temperature, pain, itch
Propioceptive receptors
Touch, pressure, vibration and body position
Nociception receptors
Temperature and pain
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 spinal cord.
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. (Lies in dorsal horns)
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
2 major ascending sensory pathways?
Dorsal column-medial lemniscal pathway
Spinothalamic pathway
Dorsal column medial lemniscal pathway
Touch/pressure/vibration & body position (proprioception) info travels to brain separate from pain/temperature
Afferent/central axon of large sensory fibers (A) ascend ipsilaterally in dorsal columns with tactile info and limb position info
DC also have 2
Spinothalamic pathway
Senses pain, temp and some touch
1.Dorsal root axon
2.Lateral spinothalamic tract
3.Thalamus
4.Cerebral cortex
Dorsal Column-where is S1?
S1 is in the Post central gyrus of the parietal lobe
Dorsal Column-Neuron that projects to cortex?
Thalamo-cortical neuron
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)
S1 Cortex
Thalamic input is to cortical layer IV** which send axons to other layers in same area
Some cortical areas specialize in decoding texture, size & shape
Reciprocal (bidirectional) connections between cortical areas & association pathways
Somotopic Organization
Somotopy: Mapping of body area sensation onto the cortex
Somotopic map called homunculus are?
Somotopic map that shows that the largest number of neurons in S1 receive sensory information from hand and mouth
Trigeminal touch pathway
Sensation in face
Trigeminal nerve (cranial nerve V)
Innervates face, mouth area, 2/3 tongue and dura mater
Pathway: Large mechanoreceptors in face => Trigeminal nerve => Thalamus (VP nucleus)
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 stimul
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 a
S2 and Parietal Posterior Cortex
S2 is lateral to S1 and is an association area
PPC is posterior to S1
-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.
Columnar Organization of S1
Alternating columns with neurons that have neurons with rapid or slow adapting properties receiving information from a given body area
Same concept as in auditory and visual cortex
Hubel and Weisel
ID columnar organization of visual cortex in monkeys
Mountcastle
discovered it for somatosensory cortex (columnar organization)
Pennfield
neurosurgeon who mapped somatosensory cortex in surgery patients by electrically stimulating areas of cortex
Posterior Parietal Cortex
Injury causes neglect syndrome: Do not recognize body part as your own so you do not dress it, wash it.
Agonosia
Inability to recognize objects including your own body parts.
Astereoagnosia
inability to recognize something by touch, but recognized by sight
Neglect Syndrome
Damage to **Posterior parietal cortex can cause
Neglect syndrome, Agnosia and Asterioagnosia
Needed for perception and interpretation of Spatial relationships, body image and body coordination
Somatosensory Cortex-Cortical Map Plasticity
Remove digits or overstimulate - examine somatotopy before and after
Maps are dynamic
How are pain and temperature sensed?
Anterolateral System
Ascending Pathway in the Spinal Cord
Spinothalamic Pathway
Spinothalamic Tract
DRGN enters dorsal horn at each spinal cord level
Ascend/descend in Lissaur's tract
Synapse in Substantia Gelatinosa
Decussate in ventral spinal cord
Ascend ventrally in spinal cord as the Spinothalamic Tract
Synapse in thalamus (Ventral Posterior and int
Two types of fibers for temp?
Cold and warm
Two types of fibers for pain?
C fiber (unmyelinated) and A delta (thinly myelinated)
C fiber
Unmyelinated, conveys long term pain (dull and achy)
A delta
Myelinated, conveys fast sharp, sudden pain (initial)
Nociception & Pain
Nociception is the sensory process that signals potential damage to body called nociceptors (signals for pain and not pain itself)
Located in skin, bones, muscles, most internal organs, blood vessels and heart (NOT IN BRAIN ITSELF, but in meninges)
Pain (
Types of Stimuli
Mechanical stimulation
Temperature extremes
Oxygen deprivation
Chemical exposure
Nociceptive Ion Channels
Ion channels that open when exposed to painful stimuli (extreme heat or cold, compression)
Present in membrane of axons of nociceptor neurons
Mechanically gated channels
Nociceptive Ion Channels-Thermoreceptors
Temperature sensitive neurons called thermoreceptor neurons have **temperature gated channels
-Sense cold or warm: burning is sensed by different neurons called nociceptors which signal damaging temperature extremes
Polymodal Nociceptors
Respond best to one but a little to all
-Thermal
-Mechanical
-Chemical
NTs: glutamate and Substance P
Capsaicin causes release of Sub P from nociceptor axons
Large amounts of capsaicin cause analgesia due to depletion of sub P from synapses
Pain
Primary Afferents and Spinal mechanisms
-First pain and second pain
-Referred pain: Angina
Shingles
Infection of single dorsal root ganglion
Medial lemniscus
Conspicuous white matter tract
Rises through pons, medulla and midbrain
Axon synapse on neurons of ventral posterior nucleus (VP)