Definitions

Somatosensation: sensory information from the skin and musculoskeletal systems

• info from the skin/cutaneous = touch, pain, temperature

• info from the musculoskeletal system = proprioception and pain

Information from these receptors travels through a series of neurons (1st, 2nd, 3rd order) to the brain

The speed of information being processed is determined by the following

• diameter of axons

• axonal myelination

• number of synapses

Sensory vs Sensation

Sensory: impulses generated from the original stimuli

• Sensory receptors

  • Tonic Receptors: DO NOT adapt to the stimulus

  • ex: light on a photoreceptor, pressure, chemoreception (can me overridden, just turn on/off)

  • Phasic Receptors: ADAPTS to the stimulus, meaning that after a while the stimulus will still be there but not as apparent to the you

  • ex: some types of pain, noise, vibration …happens just on the “outside”, at the location where your body “touches” the “environment”

• Receptors are specialized to respond to only a specific type of stimulus

  • Mechanoreceptors: respond to touch, pressure, stretch or vibration (goes into the Dorsal Column/Medial Lemniscus pathway)

  • Chemoreceptors: responds to substances released by cells, including damaged cells after injury or infection

  • Thermoreceptors: responds to heat or cold

  • Nociceptors: (subset of somatosensory receptors) stimulation results in the sensation of pain…activated when cell damage/death might occur

    • ex: pressure (mechanoreceptors) are stimulated by stubbing a toe..the sensation of pain is experienced

Sensation: awareness of the stimuli based on the senses

Somatosensory Peripheral Neurons: sensory axons have 2 axons

• Distal: conduct messages from the receptor to the cell body

• Proximal: project the message from the cell body into the spinal cord/brainstem

Afferents are classified according to axon diameter: 1a, 1b, 2 or A(beta), A(gamma), C

• 1a: largest and heavily myelinated, prevalent in areas that need quick reactions…maximum speeds = milliseconds (ex: mechanoreceptors)

• 1b: next largest found in retina cells

• C: the smallest and slowest transmitting axon

Cutaneous Receptors

Cutaneous Innervation ( How big is the receptive field?)

• distally = smaller receptive fields

• proximally = larger receptive fields

Distal areas of the body have dense areas of receptors in comparison to proximal areas

… it’s how we are able to distinguish b/w 2 closely applied stimuli on our fingertips versus our forearm

Cutaneous Innervation: Somatic Afferent

Cutaneous receptors respond to touch, pressure, vibration, stretch, noxious stimuli, pain and temperature

• Touch, is categorized as fine or coarse

  • Fine touch = A(beta) afferent

  • Coarse touch = mediated by free endings

These are the receptors are the interface b/w us and our environment

• damage to these sensors → determine our perception of pain/feeling

• 2nd degree burns, the receptors would still be there and sensation/stimulus will be transmitted ( very painful) …unlike if we had 3rd degree burns where it would be deeper then these receptors would not be there and therefore no sensation/stimulus and you would have no feeling (same for deep injury)

• During recovery you hope that there would be some regeneration, you’ll have neighboring axons growing into the region but sensation will never be the same

• Receptors are modified versions of a cell, capable of producing neurotransmitters/sending a AP

  • receptors, wrap around hair follicles so the movement of hair = stimulus

  • Meissner’s & Pacinian’s corpuscle: pressure receptors

  • Free endings: unmyelinated, slow transmission

Cutaneous Innervation-

Pain: free nerve endings

Temperature: free nerve endings

( ^ travel along the spinothalamic/anterolateral pathway: crossing immediately and travels to the brainstem)

Free nerve endings = A(gamma) & C Afferents

Cutaneous receptors…

• innervation of skin by peripheral nerves

• contribute to the sense of joint position and movement

• motor and sensory nerves travel together

  Innervation and Dermatomes…

• Dermatomes = innervation of skin by single spinal nerve

• Myotome: innervation of muscle by single spinal nerve

  ^^^They travel together

Muscle Receptors

Somatic Afferents: starts in the muscle

Muscle spindles

• Sensory Organs in Muscle Belly

  • muscle fibers

  • Sensory fibers

  • Motor ending

• Respond to stretch …change in muscle length and rate of length change = change in input sent out

• essentially the sense the amplitude and velocity of change….this is active only at the muscle system, not a lot of input comes in from the CNS, similar to a “stretch reflex” no CNS input needed to respond

• 1A’s send it quickly (realization that you’re doing something), from Gamma & 2 endings slowly send input

• 1A: just feeling that the muscle is moving

• Gamma motor neurons are what help your muscles stay calibrated, to preform accordingly

Specialized Muscle Fiber

Intrafusal fibers: contract at the ends, the centra; region does NOT contract

2 types of intrafusal fibers:

• Nuclear Bag fibers = clumps of nuclei, helps sense end range

• Nuclear Chain fibers = nuclei arranged in single file…helps sense velocity

2 different sensory endings:

• Primary endings: (of 1a neurons) wrap around the central region of each intrafusal fiber … detect a quick sense of change b/c they are wrapped around the fiber

• Sensory endings: (of 2 afferents) mainly on nuclear chain fibers adjacent to the primary endings

Golgi Tendon Organ

• sensitive to tension in tendons

• Type 1b afferents transmit sensory information from the Golgi tendon organ

Joint Receptors

• ligament receptors

  • Type 1b afferents ….sensing Tension

• Ruffini’s endings

  • Type 2 afferents …extreme join movements, prevent from reaching end range and going beyond it ( in passive movement)

• Paciniform endings

  • Type 2 afferents

• Free Nerve Endings

  • All contained in the joint capsule

  Post knee replacement or surgery, these all get disrupted so there’s a lack of input… this is when individuals express that they don’t trust their knee or “know their knee” feeling

“How are they different?”

• Muscle spindles: respond to quick and prolonged stretches…more about time, aids in stretching and passive movement

  • “How much tension? How much velocity”

• Tendon organs: signal the force generated by the muscle contraction or by passive stretch of the tendon…more about amplitude, adjusts muscle tension (quick jump)

  • input on force & velocity

• Joint Receptors: responds to mechanical deformation of joint capsule…”where you are in space”

  • translation of pressure into 3D formation

  • these can be override by cognitive function and utilizing other input (ex: vision)

  • takes in these small sensory inputs and combines them to put together a big picture that we can understand where we are

• The information they tell us can be cognitively overridden if we know that what it is saying is not accurate

• Golgi tendon organ = protective measures

• muscle spindles = optimization

  Muscle spindles and TO can work together or independently … how we see it is usually that MM spindles will fire and then as the muscle changes the TO’s will kick in

Conscious Relay Pathways

• Information in conscious relay pathways transmit with high fidelity, which means that information is very accurate and detailed orientated

• High input → High/detailed recall

• Information = location of stimulus and type (pressure, cutaneous, pain, temp)

Course Touch , you know its there and can explain the general size and weight

• less localized

• C fiber afferents, this is how we can identify a general area and cannot pinpoint a location

Temperature Sensation, Hot and cold

• Receptors = specialized free nerve endings (typically more C fibers, they do not have special endings just regular dendrites)

• A-delta fibers = carry information in regards COOLING (quick transmission)

• C fibers = carry information in regards to HEAT (slow transmission)

  *speech therapist utilize cooling techniques, due to the quick transmission*

• info does not reach conscious awareness contributes to arousal, provides gross localization (reflexes) and automatic regulation (..still aids in visceral responses)

Muscle Receptors

Pathways to the Brain: Tracts

• Tracts = bundle of axons with the same origin and the common termination

• named for the origin and where they end

Three types that bring sensory info to the brain….(ascending tracts)

• Conscious Relay

• Divergent

• Unconscious Relay

All 4 types of somatosensation reaches conscious awareness

• travel upward (ascending up the spinal cord)

  • Dorsal Column (aka Medial Lemniscus): fine touch, vibration & proprioception (travels ipsilateral)

  • Anterolateral (aka Spinothalamic): pain, temperature, and pressure (travels contralateral)

  • Anterior and Posterior Spinocerebellar: muscle tension & limb position…plans for movement (travels contralateral)

These are 3 neuron pathways…

• 1st order neuron = sensory cell picks up the signal, interacts with with environment …ex: free nerve ending, muscle spindle fiber

• 2nd order neuron = takes the information up to the thalamus

• 3rd order neuron = takes it to the specific region of the sensory cortex

Divergent Pathways: info is transmitted to many location in brainstem and cerebrum, affects a variety of neurons

• ex: aching pain

• diffuse response that can interact with the limbic system

When talking about pathways we reference the major neurons (the main highway…not interneurons)

Clinically Important Tracts:

• Spinothalamic: pain, temp and pressure…poor localization

• Dorsal Column/Medial lemniscal: transmits discriminative touch, proprioception, vibration, 2 pt discrimination to consciousness

• Spinocerebellar: relay information about muscle tension, limb position …planning for movements

ALL are IMPORTANT for output motor function

If there were “break” along any of these pathways the information would not reaction conscious sensation ….system is activated but there is a disconnect along the way

In the face….

In terms of discriminative touch, it works the same way with the trigeminal nerve. There is just a shorter distance for the information to travel to the brainstem (short first order neuron), no spinal cord interaction

1st order….

• trigeminal nerve

2nd order…

• from the brainstem, specifically the pons

3rd order….

• from the pons to the cerebral cortex

Dorsal Column/Medial Lemniscus

Splits into 2 portions, this is where we will see the 1st order neurons run to

• Gracilis region: lower limb, runs centrally

• Cuneatus region: upper limbs, run laterally

This will correlate to understanding where you will see side effects of damage to the spinal cord (tumor, injury, etc…)

2nd order neuron…

• will cross at the midline (MEDULLA) and ascend to the thalamus

3rd order neuron…

• from the thalamus (VPL) to the cortex

Conscious Relay Pathways

Provide input on …

• Discriminative Touch: Localization of touch & vibration, discriminate b/w 2 closely spaced points touching the skin

• Conscious Proprioception: movements and relative position of body parts

• Stereognosis: ability to use touch & proprioception info to identify object…( happens at the cortical level ) dealing w/ sensation and aids in multitasking

Pathways to the Brain

• High fidelity transmission (good accuracy)→ provides accurate details regarding location of stimulation

• allows info to be arranged in a somatotopic map…cortex is split up into regions that correlate to specific body region

• there’s communication b/w these areas

• Places that you need to discern fine movements → large portion of brain contributed to it

• Sensory Homunculus, based on receptive field mapping

  • high fidelity area = larger assigned regions

  • low fidelity areas ( larger limbs w/ low amount of innervation) = smaller assigned region

Somatosensory areas of the Cortex

Primary sensory cortex…1st order neuron: discriminates among size, texture or shape of object

Somatosensory association areas**….referring to the 3rd order neuron** : analyze info from the primary sensory area & thalamus

• provides stereognosis and memory or tactile/spatial environment…gives/connects meaning to what you’re feeling

Anterior Lateral Pathway

3 neuron system:

• 1st: brings info into the dorsal horn, synapses and crosses immediately (synapsing immediately = allows for a quick response)

  • starts in the skin, from limb to spinal cord (dorsal root ganglion)

  • A-delta neuron (slower fiber)

• 2nd: crosses at the midline and goes from the spinal to the thalamus

  • synapses in spinal cord, starts in the dorsal horn and goes up to the thalamus

  • cross of info occurs via anterior white commissure (mini highway for information to cross)

• 3rd: thalamus to cerebral cortex

  • starts in the VPL of the thalamus to the primary/secondary sensory cortices

• disruption along the pathway → misinterpretation of information or no sensation in general (dependent on where it occurs)

• crosses at the spinal level

• transmits the following…

  • pain

  • crude touch

  • temperature

Supplemental Trigeminal Nerve Sensation

Trigeminal Nerve: Sensory & Motor (B1 top - B3 bottom/taste)

• responsible for sensation from face and head…pain, blink reflex etc…

• Communicates with spinal cord for head reflexes (specifically C2-C5)

• information comes in synapses at spinal nucleus and to the thalamus

There is always communication to the spinal cord because there are so many reflexes & muscles involved in keeping our head upright/stable (proprioception)

• “adds on” to the dorsal column/medial lemniscus for Touch and Proprioception

• “adds on” to the anterolateral System for Pain and Temp…more sensation

  • primarily ipsilateral ( all moves together initially, right info goes to the left side)

• still 3 neuron pathway … the nerve is above the medulla so it doesn’t follow the typical “travel rules”

  Other sensory pathways….primarily pain, emotional and postural responses. Slow pathways…

  • spinoreticular

  • spinomesencephalic

  • spinoemotional tract

• These cross at the spinal level and travel to other regions of the brain…travel with other pain pathways

• These pathways can end in…

  • Spinoemotional → cingulate cortex

  • Spinomesencephalic → brainstem (postural responses)

  • spinoreticular → midbrain

• These pathways will transmit slowly to non-discrete regions (subconscious, you know something is wrong versus a specific identification)

Anterior Lateral Pathway (spinothalamic)

Unconscious relay pathway to the cerebellum

• proprioceptive and movement-related information to the cerebellum (accurate but subconscious) → adjustments to movement and posture

• High fidelity, fast, accurate and subconscious

…where else do we see high fidelity …in the primary sensory cortex (conscious pathway)

Medial Pain system

• divergent pathway (visceral pain, like a stomach ache)

• slow pain, NOT localized, low fidelity

• information is not somatopoically organized

Ascending projection neurons:

• Spinomesencephalic → Midbrain, postural response

• carries nociceptive information

• controls turning/control of head

• activates descending tracts that control pain…so you can turn and look at where the pain is coming from

• Spinoreticular → Reticular Formation, postural response

  • modulates arousal, attention and sleep cycles…on top of postural response

  • in a patient who’s comatose, if you can get them upright & applying pressure under their feet then you can increase their arousal/attention level & postural responses ( very night & day response when on/off)

• Spinolimbic → Limbic region, emotional response

  • responsible to automatic movement & emotional response to pain…response we cant really control

  • transmits slow pain information

Tracts are parallel ascending tracts

Unconscious relay tracts to the cerebellum, transmit info from proprioceptors and info about activity in spinal interneurons

• allows for unconscious adjustments to posture and movements

• info is important for complicated movements, unconscious tracts work as “quick fine tuners”

• adjusts movements in size/strength by utilizing sensory info & proprioception

High fidelity pathways transmit info from muscles, tendons & joints → cerebellum (subconscious )

These pathways split into 2…

• Posterior Spinocerebellar pathway: transmits info from the legs and lower half of the body

  • 1st order neuron: thoracic and upper lumbar spinal cord

    • large group comes from your spine and is constantly adjusting your spine

    • stays on the same and crosses (both sides sending info)

  • 2nd order neuron: (are specific) cell body = Nucleus dorsalis and the axon ascends as the spinocerebellar tract

  • damage would stop info traveling to the cerebellum

• Cuneocerebellar (anterior) pathway: transmits info from the arms and upper half of the body

The Cerebellum has Internal Feedback tracts, which means that information doesn’t make it to the cerebral cortex so the cerebellum communicates directly with spinal cord. Making subconscious adjustments/corrections…(little brain has it’s own cortex)

• Anterior spinocerebellar tract: transmits info from the thoracolumbar spinal cord

• Rostro-spinocerebellar tract: transmits info from the cervical spinal cord to the cerebellum (same side)

• Internal feed back from the lateral/ventral horns of the spinal cord communicate directly w/ cerebellum

“Emotional” contribution of pain from regions of the brain in response to nociceptive signals

• So the initial pain/ nociceptive signal from the spinal cord travels to thalamus and from there the somatosensory cortexes this same signal travels (low fidelity) to another region of the thalamus then to the cingulate cortex (also receives indirect stimulus from the somatosensory cortex) & amygdala (limbic system) from here all of this travels to the prefrontal cortex (cognitive center) → this is why we can have a HUGE emotional response to pain