Unit 4: Sensory Systems

What is the integration of sensory systems?

• Movement is supported by 3 categories of sensory systems

• Motor Function

  • Vision (see)

  • Vestibular (where body is in space)

  • Somatosensory (touch)

How do we see?

• Neurons in the visual system create perception of world (images) based on electromagnetic radiation (light)

• Eyes have evolved to only detect visible light (400-700nm)

• Colour is not inherent in the world

  • Brain’s interpretation of wavelengths

• Visual Pathway

  • Retina → Thalamus → Primary visual cortex

What is retinofugal projection?

• “Flees the retina”

• Retina to Thalamus to Primary visual cortex

  • Retina – receives sensory information

  • Optic Nerve – before decussation

  • Optic Chiasm – decussation (partial)

  • Optic Tract – after decussation

  • Lateral Geniculate Nucleus (of the thalamus)

  • Primary Visual Cortex (V1 or Brodman’s 17)

**Remember: “N” before “T”

What is the primary visual cortex?

Brodmann’s area 17 in the occipital lobe

• First area of the cortex to receive visual information

• Also known as: V1, striate cortex

• Begins mapping and processing visual information

Two main pathways: Dorsal and ventral stream

What is the Dorsal Stream of V1?

Information passed toward the parietal lobe

Specialized processing of visual motions

• Navigation – perceiving the direction and speed of objects helps us navigate safely

• Directing eye movements – sense motion and quickly react to it

• Motion perception – interpretation of moving objects

What happens if there is damage to Dorsal Stream of V1?

• Case Study 1: Can see objects that aren’t moving, but once objects start moving, they become invisible.

• Case Study 2: Told to pour coffee to fill a cup. Can see coffee on the bottom of the cup, but doesn’t register coffee filling up and coffee overflows.

What is the Ventral Stream in V1?

• Information passed toward the temporal lobe

• Specialized processing of vision other than motion

  • Object perception and facial recognition

    • Not only recognize features, but remembering faces (ex: babies recognizing parents faces)

What is the pathway for processing visual information?

Retina → Thalamus → Primary visual cortex

• “maps” and processes visual information

• Complex integration of parallel sensors and processing (colour, shape, motion, etc.)

What is the Vestibular System?

Balance, equilibrium, posture

Based on the motion of hair cells...

• Lateral line organs (similar to fish)

• Detects movement and vibration in water

What is the Vestibular System in humans?

Humans = Vestibular labyrinth

• Otolith organs – acceleration and tilt

• Semicircular canals – head rotation

• Both use hair cells to detect changes

• ~20,000 vestibular axons

  • Cell bodies in Scarpa’s ganglion

How does the Vestibular System relate to mechanics?

• Acceleration – Change in velocity

  • Airplane at a consistent speed = no change in velocity = no sensation

• Tilt – Orientation of head

  • Due to gravity

• Rotation – more specifically, angular acceleration

  • Due to the semicircular canals

What are the Otolith Organs (Utricle and Saccule)?

Measures acceleration and tilt

• Macula – Epithelium filled pouch with hair cells

• Kinocilium – Tallest, most important cilia

  • When fluid fills the hairs, we feel the small hairs (stereocillia) in relation to the big hair

• Otoconia (Ear stones) – Calcium carbonate crystals

  • Tilt: Gravity pushes on otoconia → pushes on liquid → pushes on little hairs relative to this big hair → hair cells receive information and send down vestibular axons

• Baseline of action potentials; firing rate increase/decreases depending on the amount of tilt

  • Cilia bend toward big hair = depolarization

  • Cilia bend away from big hair = hyperpolarization

What is Macular Orientation?

Array of orientations within organ

• Saccular macula – vertically oriented

• Utricular macula – horizontally oriented

Allows measures of all possible linear movements

What are the Semicircular Canals?

Measures head rotation (angular acceleration)

• Three semicircular canals on each side

• Help sense all possible head rotation angles

What are the Crista ampullaris?

• Crista ampullaris: cupula (bubble) full of cilia found within an ampulla (bulge)

• Similar idea to macula, but principle of inertia

• Endolymph reacts slowly to quick rotations which deflects the cupula (and cilia)

How are the Semicircular Canals organized?

• Each paired on opposite side of head.

• Push–pull activation of vestibular axons (same rotation but opposite response)

• Prolonged rotation will keep fluid in motion (dizziness – opposite direction)

  • A → initiate movement, B → stop movement

What are the Central Vestibular Pathways?

Pathways of vestibular information and reflexes to control head, body, eye movement

1) Otolith organs (Acceleration, tilt) + Semicircular canals (Rotation)

2) Vestibulocochlear nerve (VIII)

• Bipolar neurons

  • Merges with auditory information

3) Vestibular nuclei

• Dorsolateral regions of medulla

• Integrate with other information (visual/motor)

4) Send out information above and below

Where do the Central Vestibular Pathways send information out to?

1) Cerebellum

• Vestibular sensations needed for coordinating movements

2) Thalamus (ventral posterior nucleus)

• Then projects to Postcentral gyrus

• Info received by the cortex maintains a representation of the body in space

3) Extraocular motor neurons

• Reflexive eye movements

• Primary goal: Maintain gaze

4) Limbs

• Reflexive limbs movements

• Primary goal: Keep body upright

5) Neck and trunk

• Reflexive neck/trunk movements

• Primary goal: Keep head upright

What is the Vestibulo-ocular Reflex (VOR)?

• Function: To fixate line of sight on visual target during head movement

• Mechanism: Senses rotations of head, commands compensatory movement of eyes in opposite direction

• Ex: Seen in chickens due to their weak eyesight, this reflex helps them maintain gaze.

How do Vestibular Connections mediate horizontal eye movements?

• Example: Rotating head left but maintaining eyes on a fixed point

• Excite the red side (contract) and inhibit the blue side (relax)

(Vestibular nucleus - medulla/pons)

What are the peripheral vestibular changes with age?

Peripheral changes – likely occur first

• Otolith organs

  • Loss of cilia

  • Alterations in otoconia (size and shape)

• Semicircular Canals

  • Loss of cilia, to a greater extent than otolith organs

  • Greater impact in VOR and fall risk

What are the central vestibular changes with age?

Central changes – likely occur later (after 60 years of age)

• Vestibular nuclei – slow loss of neurons

• Cerebellum – slow loss or change in connectivity

Together, this leads to a reduction in sensory information necessary to control head, eyes, and body and maintain balance

• Add this to a multitude of changes to other sensory structures (vision, touch, proprioception) and loss of muscle strength = Increased fall risk

What is BPPV (most common)?

Benign Paroxysmal Positional Vertigo

• Benign = harmless in the long-term

• Paroxysmal = Sudden onset/recurrence of symptoms (<60sec)

• Vertigo = Sensation of spinning/dizziness

  • Vertigo itself is a symptom (not an illness)

Caused by:

• Ear stones (otoconia) migrating into semi-circular canals

• Disrupting the cupula located in ampulla

Treatment:

• Often resolves on own, but...

• Specific head manoeuvres can reposition debris out

What is Vestibular Neuronitis (second most common)?

Caused by:

• Inflammation of the vestibular nerve

Symptoms:

• Sudden vertigo that can last for several days

• Does not affect hearing

Treatment:

• Anti-nausea medication until inflammation reduces

• Steroids to reduce inflammation

• Physical therapy/activity can help the body compensate (help with balance training)

What are Labyrinthitis?

Caused by:

• Inflammation of the entire inner ear due to infection

Symptoms:

• Sudden vertigo that can last for several days

• Does affect hearing

Treatment:

• Treat infection

• Anti-nausea medication until inflammation reduces

• Physical therapy/activity can help the body compensate

What is Meniere’s Disease?

Caused by:

• Excessive fluid build up in inner ear

• Unknown why this occurs

Symptoms:

• Sudden episodes of: tinnitus, hearing loss, and/or vertigo

• Each episode can last minutes to hours

• May occur in clusters, then subside for years

Treatment:

• No cure; managing symptoms

• Can lead to permanent hearing loss, but rare

What does Somatosensory system respond to?

• Distributed all over body

• Responds to many kinds of stimuli

• Subgrouped into 4 senses:

  • Touch

  • Pain

  • Temperature

  • Proprioception

What are mechanoreceptors in skin?

Most somatosensory receptors are mechanoreceptors

• Receptive to physical distortion

4 primary receptors in skin

• Pacinian corpuscles

• Meissner's corpuscles

• Ruffini’s endings

• Merkel's disks

Vary in terms of:

• Receptive field (large vs. small)

• Adaptation (rapid vs. slow)

Which two mechanoreceptors in skin are likely measuring a small receptive field?

Merkel’s disk and Meissner’s corpuscle (closer to surface)

What are Pacinian Corpuscles?

Largest and deepest mechanoreceptor in skin

Get compressed and detect pressure and vibration

• Large receptive field

• Rapid adapting

  • React quickly to initial contact, but not sustained contact

  • Best at detecting finer textures and high frequency vibrations

What are Meissner’s Corpuscles?

Small receptors in upper dermis; common in fingers

Detect fine touch and pressure

• Small receptive field

• Rapid adapting

  • React quickly to initial contact, but not sustained contact

  • Best at detecting heavier textures and lower frequency vibrations

Explain rapid adapting mechanoreceptors and vibrations.

Lower frequency vibration signal (Contours/waves are further apart)

• Meissner's corpuscles better at low frequency (Heavier texture)

• E.g. Moving fingers across keys on a keyboard

Higher frequency vibration signal (Contours/waves are closer together)

• Pacinian corpuscles better at high frequency (Finer texture)

• E.g. Moving hand across the surface of smooth table

What are Ruffini Endings?

Large receptors in the dermis layer

Detect stretch and deformation

• Large receptive field

• Slow adapting

  • React to sustained deformations

  • Best at detecting grip/position

What are Merkel’s Disks?

Small receptors in epidermis, common in fingers

Detect fine touch and pressure

• Small receptive field

• Slow adapting

  • React to sustained deformations

  • Best at static discrimination of shapes/textures

What is the receptive field size and adaptation rate?

• We can combine all 4 into a 2x2 chart

• Most things we feel involve some input from all

Examples:

1. Catching a football? Pacinian corpuscle

2. Reaching into your backpack to find your favourite pen? Merkel’s disk

3. Holding someone’s hand? Ruffini’s ending

4. Petting your dog? Pacinian corpuscle

5. Holding guitar cord? Merkel’s disk

What is Two-Point Discrimination?

Sensitivity to discriminate small points varies greatly across the body

More sensitive in important places

Accomplished by:

• Greater density of mechanoreceptors

• Smaller field size (2 M’s - Meissner/Merkel)

• Greater brain tissue devoted to those areas

Could barefoot running or walking be better than “shod”?

• Improve sensory information coming from the feet

• Could this improve/speed up the rapid mechanical receptor time period?

• Can we train our fast adaptors?

• Highly debated!

What is the path to the brain, beginning with an axon?

• Primary Afferent Axon

  • AKA: First order neuron; Sensor to spinal cord

• Enters spinal cord at dorsal root

• Cell bodies lie in dorsal root ganglion

  • Pseudo-unipolar neurons

Four types of primary afferent axons:

• Aα (proprioception), Aβ (touch), Aδ (pain), C axons (slow pain/itchy)

• Aβ mediates touch

What are the various sizes of primary afferent axons?

All A’s are myelinated

• Larger + myelination = faster

• Aα proprioceptors are faster than Aβ mechanoreceptors for quick reflexes

C’s are NOT myelinated

• Smaller and slower

What is the path to the brain, into the spinal cord?

Two Aβ branches:

• Directly ascending the spinal cord to the brain

• Synapses with second-order sensory neuron (for reflexes)

Most second-order sensory neurons lie in the dorsal horn

What is the Dorsal Column (Touch Information)?

Dorsal Column-Medial Lemniscal Pathway (DCML)

1. Ascending branch goes up the dorsal column

2. Synapse on the dorsal column nuclei in medulla

3. Dorsal column nuclei axons decussate and ascend the medial lemniscus

4. Synapse in the VP nucleus of the thalamus

5. Neurons in the VP nucleus project to somatosensory cortex

Terms: Ipsilateral (Same side) vs. Contralateral (Opposite side)

What is the DCML?

DCML = 3 neuron pathway with 3 synapse points required to reach S1

• These exist for a reason other than simply passing along information

• We can assume information is altered at each synapse

• Adjacent inputs can be inhibited to enhance tactile stimuli

What are the 3 neurons, and 3 synapses of DCML?

3 neurons (Red)

• First-order, second-order, third-order

3 synapses (Green)

• First to second, second to third, third to cortex

• No synapse until it reaches medulla

What is the segmental organization of spinal cord?

Dermatomes diagram – The distribution/mapping of spinal nerves

• Comes from an evolutionary standpoint (all 4s → bipedalism)

What is a Herniated Disc?

Most common in 30-50s (~2% of adults)

Most common in the lower back (L4/5 & L5/S1 -95% of cases)

• Pain – Back and leg (glutes, thigh, calf, even foot)

• Numbness or tingling

• Weakness

**Reported as pairs, because we talk about the disc in between the 2 vertebra

How to test for Herniated Disc?

Physical exam, Imaging, and/or even nerve tests for diagnosis

• Herniated discs cause localized and radiating pain by compressing the spinal cord or nerve roots, with pain location often indicating the affected level

What are treatment options for Herniated Disc?

1. Rest, physical therapy, pain medications

• 85% resolve in 8-12 weeks

2. Surgical – Discectomy/Microdiscectomy

• Conservative failed to resolve

• Progressive/debilitating pain, numbness, and weakness

Why is lateral inhibition (overlapping neurons) important for touch information?

• Inhibit adjacent inputs to enhance tactile sensitivity

• Increases contrast to allow for more precise/finer location of sensation

Graph: Purple shows frequency of APs

What is Sensory gating?

• Corticothalamic feedback influences sensory processing

• Cortex helps to filter irrelevant or repetitive information

• “Feel what you want to feel”

• However, these complex pathways remain unclear

• May be related to many cognitive disorders...

  • Ex: Schizophrenia (extreme), ADHD (less extreme)

How do we know 3b is the primary input site?

Primary somatosensory cortex = 3b

1. Receives inputs from VP nucleus

2. Highly responsive to somatosensory input

3. Damage impairs sensation

4. Electrical stimulus creates sensations

What are the areas adjacent to 3b?

Somatosensory 3a

• Dense thalamus input, but more body position

Somatosensory 1 & 2

• Receives information from 3b

• Generally related to texture, size, and shape

What is the contribution of Wilder Penfield?

• American-Canadian neurosurgeon

• Discovered stimulating parts of the cortex could evoke vivid and specific memories, including sounds and smells

What is Cortical Somatotopy, Homunculus?

• Mapping of the somatosensory cortex

• Homunculus represents the density of sensory input

What is the Posterior Parietal Cortex?

Allows for the processing of basic sensory information and integration with other senses

Posterior parietal cortex 5 (orange)

• Sensory integration for the planning and organization of movement

Posterior parietal cortex 7 (pink)

• Sensory integration for object recognition and spatial relationships

What is pain and nociception?

Nociceptors: receptors of painful stimuli

Activated by stimulus that may damage tissue

• Strong mechanical stimulation, temperature extremes, oxygen deprivation, chemicals; even substances released by damaged cells (lactic acid, histamine, etc.)

Nociception ≠ pain

• Nociception = sensory process that provides the signals that MAY trigger pain

• Pain = sore, aching, throbbing sensations we “feel”; can be influenced by past experiences

  • Nociception can exist without pain

  • Pain can exist without nociception

What are types of Nociceptors?

Free nerve endings, which bring the sensation of pain to CNS

Types of Nociceptors

• Mechanical nociceptors - Respond to damage such as cutting, crushing, or pinching

• Thermal nociceptors - Respond to temperature extremes

• Chemical nociceptors - Respond to histamine and other chemicals

• Polymodal nociceptors - Respond equally to all kinds of damaging stimuli

How do different types of pain involve different types of fibers?

What is the Spinothalamic Tract?

To carry nociceptive information to brain

• Cell bodies in dorsal root ganglion

• Axons enter dorsal horn of spinal cord

What is the pathway of carrying nociceptive information through Spinothalamic Tract?

1. Enter zone of Lissauer (ascend or descend slightly)

2. Synapse in the substantia gelatinosa (in the dorsal horn)

3. Second order neurons in the spinal cord immediately decussate

4. Ascend to the brain in the ventrolateral surface of the spinal cord

5. Synapse with VP nucleus (and other areas) in the thalamus

6. Information then projected the somatosensory cortex

NOTE: Pain is complex and can be difficult to localize in the brain. It is also highly integrated with medial structures related to emotion/memory

How can you compare the 2 major ascending pathways?

• # of neurons?

  • DCML: 3

  • STT: 3

• Ipsilateral vs. contralateral and where?

  • DCML: Ipsilateral to start, contralateral at medulla

  • STT: Contralateral immediately and stays contralateral

NEXT TEST

• Area of spinal cord?

  • DCML: Dorsal column

  • STT: Lateral spinothalamic tract

• Type of axons in the pathways?

  • DCML: Aα (proprioception), Aβ (touch), Aδ (pain)

  • STT: Aδ (pain), C axons (slow pain/itchy)

What is the general organization of these pathways?

DCML

• Upper body tracts more lateral

• Lower body tracts more medial

Spinothalamic

• Upper body tracts more deep

• Lower body tracts more superficial

Pain Regulation - Afferent Regulation

Pain can be reduced by the activity of mechanoreceptors

Gate control theory of pain

• Neurons in the spinothalamic tract may be inhibited by Aα or Aβ sensory nerves (touch) in the dorsal horn of the spinal cord.

Pain Regulation - Descending Regulation

Brain can do powerful things when it comes to controlling pain!

• Strong emotion, stress, etc. can suppress pain

Periaqueductal gray matter (PAG) - in medulla

1. Receives input from many areas in cortex (often emotional)

2. Neurons descend to medulla (Raphe nuclei)

3. Neurons descend to spinal cord to depress activity

Hyperalgesia

Reduction in the pain threshold, increased sensitivity, or spontaneous pain

• Primary Hyperalgesia: super-sensitivity within the damaged area

• Secondary Hyperalgesia: super-sensitivity in the surrounding area

Primary changes occur peripherally:

• Inflammation = bodies attempt to eliminate injury and stimulate healing

• A variety of neurotransmitters, peptides, lipids, etc. are released which can attach to receptors in/around injury to lower their threshold for activation

Hyperalgesia vs. Allodynia:

• Allodynia is a similar concept, but pain response from stimuli that would normally not cause pain

Central Sensitization

• Amplification of neural signaling (e.g., nociceptive information) within the CNS that elicits pain hypersensitivity or even normal stimuli (allodynia)

  • Changes in the synapses and potentially the organization of interconnecting neurons may increase excitability/reducing inhibition of pain pathways

  • Contributions are difficult to identify and treatments difficult to target

What is referred pain?

Cross-talk between sensory neurons

• Convergence of visceral and somatic afferent neurons (in dorsal root ganglion)

• Ex: Chest and left arm pain before heart-attack

Issue here?

• In STT, the signal should synapse, then cross over and go up to the brain

• More accurate diagram:

What is temperature?

Thermoreceptors

• Varying sensitivities to hot and cold temperatures

• Cold (Aδ & C fibers) and hot (C fibers)

• Adapt to long durations of stimuli

Follow the same pathway as pain

What is proprioception?

Our perception of the location and movement of our body

Allows us to control limb and joint position for optimal movement

Group I neurons

Two primary receptors:

• Muscle spindle – amount of stretch in a muscle

• Golgi tendon organ – amount of force

Conscious vs. Unconscious proprioception

Conscious proprioceptive information

• Dorsal column medial lemniscus pathway

Unconscious proprioceptive information

• Spinocerebellar tracts – To cerebellum

• Spinal interneurons – Spinal reflexes

While holding a dime for a period of time between your fingers, you are able to feel the contours along the edge. The most important receptor for this type of sensory information is the...

A. Messner’s corpuscle

B. Merkel’s disk

C. Pacinian corpuscle

D. Ruffini’s endings

B. Merkel’s disk

You stub your right toe on your bedpost and the free nerve endings immediately send nociceptive information to the brain through the spinothalamic tract. What side of the body (spinal cord/brain) will this information be when it: i) is ascending the spinal cord at the level of thecervical spine, ii) reaches the brain.

A. i) left, ii) left

B. i) left, ii) right

C. i) right, ii) right

D. i) right, ii) left

A. i) left, ii) left

Which of the following statements is NOT true about the dorsal column medial lemniscal pathway?

A. The body of the second order neuron is found within medulla

B. It contains 3 neurons (first, second, and third order)

C. Decussation occurs immediately after the axon enters the spinal cord

D. The body of the first order neuron is within the dorsal root ganglion

C. Decussation occurs immediately after the axon enters the spinal cord