Intro to Sensory Systems

• Transduction converts one form of energy into another.
• Sensory receptors are peripheral endings of pseudounipolar afferents, they have cell bodies in the dorsal root ganglia.
• Sensory receptors often have non-neural tissue.
• Encapsulation alters responsive behaviors.

Receptor Classification Systems

• Neuronal characteristics determine the rate of transmission
• Thicker, heavily myelinated axons result in rapid transmission.
• Thinner, less myelinated axons result in slow transmission.
• Unmyelinated axons result in the slowest transmission.
• Axon size correlates with conduction speed.
• Roman numerals are for proprioceptors, which are the fastest conducting primary afferents.
• Letters and Greek letters are for somatic receptors (pain, temperature, touch).
  • A beta
  • A delta
  • C

Cutaneous Receptors

• Pacinian Corpuscles are for vibration and pressure on the skin.
• Meissner's corpuscles detect fine touch on non-hairy skin, like fingertips.
• Merkel's discs are associated with hair follicles and pressure sensation.
• Corpuscles of Ruffini detect low-frequency vibration and pressure.
• Hair follicle receptors detect light touch.

Proprioceptors

• Proprioceptors are located in muscles, tendons, joints, and ligaments.

Muscle Spindle

• Muscle spindles are encapsulated receptors that lie parallel to muscle fibers.
• Detects stretch in muscles.
• Has motor innervation to adjust sensitivity.
• The sensory system includes Ia phasic, tonic, and type II fibers.
• The motor system is the gamma efferent system.
• Intrafusal fibers are muscle fibers inside the spindle, they send action potentials to the ventral horn of the spinal cord.
• Neuron motor pool maintains background muscle tone.
• Nuclear chain fibers are multiple within each spindle.
• Nuclear bag fibers are muscle fibers located inside the capsule.
• Muscle Spindles fire when extrafusal fibers are stretched or when intrafusal fibers are contracted, background activity keeps spindle 'tuned'.
• An intrafusal fiber contraction is alpha-gamma co-activation in voluntary movements.

Joint Receptors

• Joint receptors respond to tension on the joint capsule
• They work with the spindle to execute motor programs
• There are four types of joint receptors:
  • Type 1: Proximal, low threshold, slowly adapting (steady state)
    • Located in hip, shoulder, spine
    • Respond to pressure through joint with co-contraction
  • Type 2: Distal, high threshold, rapidly adapting
    • Supports movement and direction
  • Type 3: Protective, responds to tension on joint capsule (working out)
    • High threshold, slowly adapting
  • Type 4: Nociceptors
    • Mediates flexion
    • Pain receptors for arthritis in the hand

Golgi Tendon Organ

• The Golgi tendon organ lies in a series perpendicular to the muscle fiber
• It uses Ib afferents
• It is an autogenic inhibitor: a brake to contraction.
• It is sensitive to tension
• Used to adjust tension, for example when going down a hill.

Stimulus Modality

• Receptors have a classification and respond to specific stimuli.
• This follows a code of labeled lines or pathways/tracts.

Stimulus Intensity

• Intensity determined by threshold, action potential frequency, and neuron distribution.

Location

• Location of a stimulus is determined by the Homunculus.

Spinal Processing of Sensory Stimuli

• Spinal processing permits participation in spinal-mediated behavior, like reflexes.
• Spinal processing also controls the amount of information sent to higher centers for conscious processing.

Types of Circuits

• Diverging circuits magnify input, sending it along the same pathway.
• Converging circuits in the dorsal horn converge input into a single neuron every second.
• Henshaw circuits turn the circuit off.
• Reverberating circuits keep themselves turned on, continuously firing.

Neuron Potentials

• Action potentials are "all or none," propagated, and cannot be summated.
• Local potentials are non-propagated, can be summated.
• Post-synaptic potentials (EPSP or IPSP)
• Generator or sensory potentials are also local potentials.
• Presynaptic inhibition requires a three-neuron chain.
  • The 1st neuron releases a neurotransmitter on the 2nd, reducing neurotransmitter release.
  • This decreases the likelihood of an EPSP generating an action potential in the 3rd neuron.
  • This is also known as primary afferent depolarization or PAD.

Tactile Sensitivity

• Tactile hypersensitivity is overly sensitive to touch and has a very low threshold.
• It results in sensory avoiding and is often seen in ASD.
• Hyposensitivity is sensory seeking with an overly high threshold.
• Hyposensitivity is activated with a light touch, itch, or tickle.

Management of Hypersensitivity

• Management relies on a graded approach to stimulation.
• Activating large diameter afferents "gate" small diameter afferents through pre-synaptic inhibition.
• Proprioception/ weight bearing activates large diameter afferents.

Higher Sensory Processes

• Sensation involves the registration of sensory stimuli by neural receptors, transmission, and recognition.
• Perception involves processing sensory input, it uses stored memories.
• Perceptual constancy recognizes objects as unchanging, and is irrespective of lighting conditions or the viewing angle.
• Perceptual set causes the bias to perceive some stimuli over others based on experience.
• Perception cannot happen without sensation.

Unimodal Areas

• The parietal lobe processes the somesthetic sensation.
  • Areas 5 and 7 process body image, body in space and organizing body in space.
• The temporal lobe processes auditory sensation.
  • It contains transverse temporal gyri and the hippocampus.
• The frontal lobe processes attention.
• The occipital lobe processes visual information.
• Medial areas cause more registration.

MultiModal association cortex

• Inferior parietal lobe:
  • Supramarginal: somatosensory
  • Angular: visual (reading center)
• Lateral temporal and occipital gyri.

Hemispheric Differences

• Right hemisphere: spatial processing, visualization, intuition, and conceptualization.
  • Unilateral neglect can occur with lesions.
• Left hemisphere: temporal processing, sequencing, and numerical thinking.
• For most, the dominant hand corresponds to the dominant language hemisphere.

Cortical Processing Deficits

• Astereognosis: inability to identify familiar objects by touch
• Dyslexia: reading difficulty, normal intelligence (angular gyrus)
• Agraphesthesia: can't identify numbers/letters traced on the hands with the eyes closed
• Prosopagnosia: inability to recognize faces, including own reflection
• Apraxia: inability to execute a motor command, central processing disorder

Neurobiology of Pain

• Pain is an unpleasant sensation associated with tissue damage and processed through nociceptors.
• Has a cortical pathway and is interpreted individually

Nociceptors

• Nociceptors are free nerve endings and are associated with two axon types:
  • Small and thinly myelinated.
  • Small and unmyelinated.
• Can be silent/sleeping, and only become sensitized after tissue damage.
• Peripheral Sensitization occurs when endings contain glutamate receptors to sensitize pain.
• Pain is perceived in 2 stages:
  • Delta pain (first pain, fast pain), from A-delta fibers.
  • Slow pain (protopathic pain), from unmyelinated fibers, lasts longer.

Pain processing

• Pain may participate in reflexes at the spinal cord level or may be transmitted to higher centers.
• Pain can be processed/gated through spinal circuits before being transmitted
• Glutamate is the main excitatory pain neurotransmitter.
• Substance P is also a pain neurotransmitter and lasts longer.
• Spinal circuits are located in Rexed's Lamina II (substantia gelatinosa)
• Pain may travel a few segments in Lissauer's Tract.

Gate Control Theory

• Melzack and Wall propose a pain gate in the brainstem and cortex

• T-cells fire and cross the commissure and then ascend as part of the ALS

• A-beta fibers relay touch and pressure (fast fibers)

• A-delta fibers relay slow fibers

• 3 Neuron chain

• The brain influences the pain signal it receives from the spinal cord.

  • Cell bodies of Periaqueductal Grey Matter (PAG) surround the cerebral aqueduct
    • Neurons excite serotonin pathways and release opioids.

• The descending pathway mediates conscious awareness of pain.

  • Source: PAG
  • Target: nucleus raphe of the medulla
  • Endogenous opioids (endorphins, enkephalins, dynorphins) are released.

Central Sensitization

• Central Sensitization: repeated exposure to a noxious stimulus increases responsivity in dorsal horn neurons.
  • Form of long-term potentiation and increases neurotransmitter production.
  • Neurons exhibit conversion of nociceptive specific neurons to wide dynamic neurons.
  • Neurons show a progressive increase in responses, expansion of spatial input, and changes that outlast a trigger.

Opioid Receptors

• Mu, Delta, and Kappa opioid receptors differ in distribution.
• All are present in high concentrations in spinal dorsal neurons.
• Activation by agonist closes voltage-gated Ca+ channels.
• Activations allow an influx of K+ (hyperpolarizing neuron).
• Most drugs act on Mu receptors.

Cerebral Cortex

• Cortical areas activated by painful stimuli:
  • Primary sensory cortex
  • Cingulate gyrus
  • Insular cortex.
• Chronic pain changes the cortical map.

Ascending Sensory Pathways

• Describe the ALS system in terms of:
  • Modalities, location, site of crossing and the three-neuron pathway.
• Sensory job: To receive, process and ship information to the cortex.
• The nervous system determines what gets to the brain.
• Action potentials are "all or none."
• Ascending Pathways CROSS.

Pathways

• Route supports code of labeled lines concept for touch, pain, pressure, etc.
• Roads (tracts) are highways that information takes with bundles of axons in the CNS
• Exchanges (change in roads) are synapses between neurons
• Neuron #1 is ALWAYS a primary sensory afferent with cell bodies in dorsal root ganglia.

Ascending (Sensory) Pathway Table

Sensory Pathway Rules

• All ascend (spinal cord to brain), and are three-neuron chains.
• Gray matter is where neuron bodies sit.
• White matter is axons and highways.
• The thalamus is the GATEWAY to the cortex for all sensory info.
• Anterolateral System (ALS) fibers go to the cortex through the thalamus (spinothalamic) and end in the brainstem (spinoreticular).
• Dorsal column medial lemniscus (DCML) enters below T6, forming the fasciculus gracilis, enters above T6, forming the fasciculus cuneatus
• Ipsilateral - Same side of spinal cord.
• Contralateral - Crosses SC.

Ascending System Damage

• Peripheral nerves lose all modalities in the nerve field distribution -may report analgesia.
• Dorsal roots lose all modalities with dermatomal distribution, may report hypoalgesia.
• CNS: side and extent of the deficit depends on where the lesion is.

Sensory Deficits

• Modality, topography, intensity must be described.
• Anesthesia: Absence
• Hypoalgesia: Diminished
• Paresthesia: Pins and needles
• Hyperesthesia: Hypersensitivity

ALS Lesions

• Anterior spinal artery damage causes Bilateral loss of pain and temperature.
  • Maintains some touch and proprioception with the DCML, with paralysis.
• Syringomyelia ALS lesion from cavity
  • Bilateral loss of pain and temperature with specific motor deficits

DCML Lesions

• IPSILATERAL through and to the spinal cord.
• Doesn't become contralateral until they cross in the medulla
• DCML Lesions causes Loss of proprioception and 2-point discrimination.
• Decreased touch and preserved pain and temp.
• Posterior spinal artery damage causes unilateral infarcts ipsilateral to the side of the infarct
  • Tabes dorsalis/tabetic ataxia = Ataxic gait and poor balance.

Brainstem

• Lesions affecting ALS and DCML modalities are located before the sensory decussation of the DCML.

Descending Pathways

• Four auxiliary descending systems from cell groups in the brain stem:
  1. Reticulospinal tracts
  2. Rubrospinal tracts
  3. Tectospinal tracts
  4. Vestibulospinal tracts

Reticulospinal system

• Upper/pontine/medial reticulospinal system:
  • Originates in the upper reticular formation that runs through the pons and lower midbrain.
  • Descends ipsilaterally
  • Targets gamma motor neurons of extensors to contract muscle spindle
  • Facilitate postural extension with more input to medial muscles.
• Lower/medullary/lateral reticulospinal system:
  • Originates in the lower medullary reticular formation.
  • Descends mostly ipsilaterally, but has some cross.
  • Facilitates postural adjustments associated with UE movements and requires a cortical connection.
• Lesions the corticospinal tract + the medial reticular formation leads to loss of axial musculature control.
• CST + loss of lateral reticular formation = loss of independent use of arms.

Vestibulospinal System

• The Lateral vestibulospinal tract facilitates extensors
  • Originates in the lateral vestibular nucleus and descends ipsilaterally
  • Targets alpha motor neurons of extensors.
  • Maintains center of gravity over the feet and enhances posture.
• The Medial vestibulospinal tract is most robust in cervical cords and boosts postural control of head and neck.
  • Originates in the medial vestibular nucleus, descends mostly ipsilaterally (some crossing)
  • Ends in cervical cord, and coordinates head and eye movements, maintaining cervical spine stability.

Tectospinal System

• The tectospinal tract originates in the superior colliculus
  • It crosses and descends as tectospinal pathway.
  • The cortex uses corticospinal fibers to comunicate with rhe superior colliculus
  • Coordinates visual inputs with neck movements and aids in reflexes which turn the head to visual stimuli.

Rubrospinal System

• The rubrospinal tract originates in the red nucleus of the midbrain.
  • It crosses and descends as the rubrospinal pathway
  • The cortex connects with the red nucleus via the corticorubral fibers.
  • Facilitates anti-gravity muscles, like UE flexors (hands, and fingers).

Descending (Motor) Pathway Table

• Corticospinal : Look for cranial nerve involvement when Localizing
• Descending pathways are 2 neuron chain and go from the cortex into the brainstem and spinal cord.
• The postcentral gyrus is for somatosensory input.
• The precentral gyrus controls movement.
• Delta A and C fibers detect V Cranial nerve pain and temperature.

Decerebration

• Decerebration is when arms are extended, the wrists are rotated out and feet rotated internally

Decortication

• Decortication is when arms are flexed and wrists rotated inward Descending

Corticospinal Systems

• Commands for voluntary movements are initiated in the cerebral cortex.
  • The M1 motor strip (area 4 or the primary motor cortex).
  • M1 neurons have the lowest stimulating threshold.
  • Face and hand have large neuronal territories, therefore, they are more challenging to rehabilitate.
• The corticospinal tract does NOT originate entirely from the precentral gyrus.
• Betz cells are located in histological layer of cortex that contain large and pyramidal neurons.
• Can also be described as Pyramidal Tract / Long Tract

CorticalSpinal Tract Types

Blood supply in relation to Cortical Spinal Tract

• Contralateral is 90% of all descending fibers of the tract and anter is 10 percent originating in tract from cortex

• Long term damage shows very Characteristic signs of upper motor neurons

• Blood clots mostly travel in lateral blood flow

• Blood supply chart Structure Blood Supply

• Capsule- Medial Striate artery: ACA and MCA, Internal carotid: anterior choroidal artery

• Midbrain- Primarily PCA

• Pons- Branches of Basilar

• Medulla- Anterior Spinal Arteries

pyramidal -Decussation- Anterior Spinal arteries

• If Lesion here affects fingers descending fibers will drop of first: Meaning the most medial (arms or descending fibers of cst)
• Ascending Fibers drop off later

Descending Fibers rule:

• The higher into the medulla means dropping fibers will have an effect
• The Lower into the Spinal Cholrd, means fibers later drop out, so legs will present

Decorticate posturing:

• Above brain stem And common with cva's

Corticobulbar system:

• The Face area of the cortex- Passes through the genu and thru all fibers passing in pca
• Bilateral Representation: Therefore a unilaterateral lesion will not affect face function
• Some exceptions are where the lower fibers connect:
  • May see drooping of the mouth that raise blink
• Will not have swallow complain and struggle to drink

Reflexes And Motor

• Not cortical proccess
• By using local circuits: When it activates a chain for movement
• Adaptive and Change: Constant Anticipactations and programs for adjustment

Convering Vs. DEverging

Reflexes

• Adapt highly to changes
• Shallow attractor: no strategy
• DEep Attractor: Steroytype method
• Medium: Prefer method

Myototic Relfect

• Receptors are from sensory stimuli, then endings end up and around regions of neculas bag
  • Fiber is in relation to muscle or stretching will cause receptors quickly
  • Stimulatyes afferent as they make its way directly to motor where spinal cor synapes
• Relfects in: Relation of muscles and demands of movement to be adjuted so the
• Gmma must all be loaded and for preperations of streatch
• Reciptical relfex can cause stffness

Alpha motors: And anagosnst muscal are linked- if musle contact, then anagotis will be shut down too

• A lot inhibition for pliant ( flex)
• A lot fascilation for stiff (Cocontractive musles are stiffer

net effect

a lot of bristleness- absent- if no excit

prospinal path

a. Run short and quick along Latern funicus cords in which can adjust small impinging segments b. Runn long and slow: on a large anterior funinculs across sections

colus

( quick streatch that casues hperactive or short quick segments ( beats a limb)

withddwal reflex-

a. the flex to platnus stimulu in which foot is withdrawnl ex

• Dorsi Fexk and hip Fext

• Dont is always so natural now, see is more in NS infants, still in early rapid NS phases can have a reflex, may come BACK to surface ( if NS damaged) even after 2 Years,

• central patterns- Flxiebal notrws fo nural conrl

uppermotpr nuural

A. UMN's only travel the cprtico corcords in ehcih does no go to spinls colods eithr dos is or leavs

• UMN desises: is to damage to cords thar are now" home laone"" to waiting for eithr to be more or less and if the motor has no cnrtol, thnr" They have movement but dis org

b. sign

###Spasticity

• the speed that the stretch is affected , affects that motor can have and be the speed that tne
• Is disordered motor
• is dsoroderrd motor

ab moral relftes

abcent- surface area associated - is released and caused movement by

ex homolatrl- more person more moves- more person is lifted at ( homparatetic gall)

voluntarily :

• hard for movement desiers with a desierss

measure : Ashworth : posative and negtive

type 2 muscle (GTO) Babinskl: dymai track is dangees

Hoffmann s=gn. Fickles the middle

• cord pression- but does not ecluses damage-

umb

therperay