Lecture 8 - Somatosensory, Vestibular and Olfactory systems

Question 1

Stimulus

Answer 1

Energy source Internal External

Question 2

Receptors

Answer 2

Sense organs - structures specialized to respond to stimuli Transducers - stimulus energy converted into action potentials

Question 3

Conduction

Answer 3

Afferent pathway Nerve impulses to the CNS

Question 4

Transduction

Answer 4

CNS integration and information processing Sensation and perception

Question 5

Sensory projections

Answer 5

Ascend from the spinal cord into the brain through the brainstem They travel through the thalamus, which acts as a relay processing station of signals to other brain regions

Question 6

Olfactory pathways

Answer 6

From the nose project to the primary olfactory cortex Olfactory system (smell) -> dirrectly to cortex without going through brain stem -> nose is above the brain stem and so it is most efficient for it to go straight to the cortex via nerve

Question 7

Vestibular pathways

Answer 7

Also project to the cerebellum Vestibular system -> Brain stem -> into cerebellum (motor control, balance, movement) as well as cortex

Question 8

Touch

Answer 8

Touch receptors are the most common receptors in the body Giant Washable Stretchable Water proof Boundary between you and the external world Most common receptor in the body

Question 9

Skin

Answer 9

Skin Glabrous Skin Smooth, thick skin on palms and soles of our feet Epidermis = 1.5 mm Dermis = 3 mm Hairy Skin Thin skin populated with hair follicles Epidermis = 0.1 mm Dermis = 1-2 mm Sweat Glands Eccrine – secrete saline (heat regulation) Sebaceous – secrete complex cell cytoplasm (Primary source of human body odor!)

Question 10

Mechanoreceptors

Answer 10

Mechanoreceptors -> mechanically Produce information on tuctile stimulus -> tuctile receptors No tuctile receptors in the Dermis (Meissner and Merkel are in Epidermis) Fast adaptive -> apply stimuls = only fire at onset of stimuli then silent (e.g. cannot feel clothers), fire again when is removed such that this is also noticed Slow adaptive -> can keep sending signal to the brain as long as the stimulus is present (e.g. pain) Large receptive feild = lower resolution (objects must be further apart to be percieved as two individuals as opposed to one large stimulus) Meissner Corpuscle - Touch Small receptive fields Fast adapting Merkel Cells - Touch Small receptive fields Slow adapting Ruffini Ending - Stretch Large receptive fields Slow adapting Parcinian Corpuscle - Vibrations Large receptive fields Fast adapting

Question 11

4-channel model of touch

Answer 11

Question 12

Ascending pathway

Answer 12

Pathway from receptor to brain

Spinal cord -> sliced and look from top = butterfly shape

Dorsal = receives afferent sensory information from sensory receptor 
Ventral = receives efferent motor comands from the brain and transmits to effector organs

Dermatome:
A dermatome is an area of skin that is mainly supplied by a single spinal nerve Although there are 31 pairs of spinal nerves in humans, there are only 30 dermatomes
each area of the skin is named with the nerve that innervates it

Each portion of the skin has a specific nerve of the spinal cord (as shown by colour split) -> high specialisation
First evidence of dermatones -> looking into patients with virus affecting particular area of the body (rash for example) -> this is likely due to it effecting one particular nerve which effects the areas this nerve detecting
Different nerves needed for brain to know where abouts stimulus from

Question 13

Tactile pathway

Answer 13

Tactile signals are sent through the spinal cord via the dorsal column pathway to the primary somatosensory cortex (S1)
in the brain

Dorsal column -> Thalamus -> Primary somatosensory cortex (S1) -> Secondary somatosensory cortex (S2) -> Other brain areas (parietal areas)

Dorsal column switches sides (at megdula) -> left motor sensory cortex (left brain) processing right side stimulus

Dorsal column -> Cross over to opposite side at the medula -> Mid brain -> thalumus (subcortical reagion) -> S1 -> S2

Can be recorded on EEG or MRI -> proccessing of signal

Send current in nerve in hands -> report from brain or other level of pathway (before brain)

Question 14

Samatosensory Pathways

Answer 14

Each part of the skin surface is represented by a specific region of primary somatosensory cortex

“Sensory Homunculus”
Little man
The area devoted to each body part reflects the receptor density in that part

Cortical Magnification
The receptive fields and cortical representations give more acuity to fingers, mouth, nose and tongue

Each area represented in somatosensory cortex in very organised and specific way -> somatotopy (how brain represents body)
Some areas, e.g. finger tips, are much more highly represented at nervous level (many more neurones for that area) than others, e.g. shoulder

Question 15

Pain

Answer 15

“Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage”

SENSORY
Pain and touch are processed by separate neural systems
Pain is detected by special receptors in the skin

Ascending pathway - PAIN
Pain signals travel through the spino-thalamic tract to the brain and run parallel to somatosensory (dorsal column) pathways

PAIN PATHWAY
Afferent to spinal cord laminae I & II -> Cross midline to contralateral anterolateral column -> Thalamus -> Primary somatosensory cortex (S1) -> Secondary somatosensory cortex (S2) -> Anterior Cingulate Cortex, Insula, Amygdala

Question 16

Nociceptors

What are they

Where are they

Key features

Answer 16

Pain receptors = nociceptors (small diameter, afferent)
Nociceptors are free nerve endings that respond to stimuli that can cause tissue damage or when tissue damage has taken place
Small diameter afferent neurons (with A-delta and C-fibers receptors)
Touch, pressure, vibration, skin stretch

skin
muscle
joints
and some visceral tissues

KEY FEATURES:

1. some are specific to one type of stimulus, such as: - mechanical (Mechano-sensitive nociceptors)
   - thermal (Thermo-sensitive nociceptors)
2. but most are polymodal (respond to many stimuli, including chemicals)
3. the number and size of the receptive fields served by each fiber may be small or large, respectively

Question 17

A delta fiber vs. C fiber

Answer 17

Aδ Fiber
Sharp, Prickly Pain
Thin, Myelinated, Fast

C Fiber
Dull, Aching Pain
Thin, Unmyelinated, Slow
Slow (second) pain is delayed, dull, diffuse, and long-lasting

Question 18

Central mechanisms for pain

Answer 18

Somatosensory cortex (S1 & S2)
Mediate perception of location, intensity, and ”quality” of
painful stimuli

EMOTIONAL AVERSIVE
Amygdala, anterior cingulate cortex, insular cortex
Mediate perception of fear, anxiety, and unpleasantness of
painful stimuli

Question 19

Vestibular system

Answer 19

Vestibular stabilisation of the head in space
Head in 3D space

Organ inside inner ear that detect position of head in space -> stabilise head whilst moving
Always providing this information
Onset of movement, direction, velocity etc.
High presision and speed
Only sensory modality not associated with conscious perception

Coding gravity direction -> allows moving without falling

Question 20

Semicircular canals

Answer 20

Sense head rotations (angular acceleration)
Semicircular canals filled with liquid (endolymph)
Rotation of head cause liquid to move opposite to rotation
This bends the jelly-like cupula, causing embedded vestibular hair cells to bend and fire action potentials

Question 21

Otoliths

Answer 21

Sense linear acceleration & gravity
Linear acceleration (e.g. tilting the head) cause the crystals to pull the gelatinous substance downward, bending hair cell stereocilia and causing depolarisation
The otoliths consist of hair cells embedded in a jelly-like substance, covered with heavy calcium carbonate crystals

Question 22

Vestibular pathway

Answer 22

Vestibular signals travel through the Medial Longitudinal fasciculus to the brain

Vestibular Nerve -> Brainsterm -> (Cerebellum) -> Thalamus -> Vestibular Areas

Question 23

3 Vestibular-driven reflexes

Answer 23

Vestibular-Ocular Reflex -> keeps the eyes still in space when the head moves
Vestibular-Collic Reflex -> keeps the head still in space (or on a level plane when you walk)
Vestibular-Spinal Reflex -> adjusts posture for rapid changes in body position

Question 24

Vestibular-Ocular Reflex

Answer 24

Vestibular-Ocular Reflex (VOR)
VOR -> eye movements that stabilize gaze by countering movement of the head
In VOR the semicircular canals measure rotation of the head and provide a signal for the oculomotor nuclei of the brainstem, which innervate the eye muscles VOR allows maintaining gaze while rotating or moving head

1. Detection of rotation
2. Inhibition of extramuscular muscles on one side
3. Excitation of extramuscular muscles on the other side
4. Compensating eye movement

Question 25

Chemoreceptors

Answer 25

Respond to chemicals in aqueous solutions

Chemicals dissolve in mucus of nose & saliva in mouth

Question 26

Smell receptors

Answer 26

• Located in roof of nasal cavity
• Non-motile cilia covered by mucus
• Axons go to the brain
• Lifespan approx. 30-60 days
• Each olfactory receptor has only one type of receptor protein which bind specific odorant molecules

1. Odorants bind with receptors
2. Olfactory receptor cells are activated and send electric signals (Action Potentials)
3. Signals are relayed in glomeruli
4. Signals travel to brain

Question 27

Smell pathway

Answer 27

Signals from the olfactory nerve travel to the pyriform cortex -> Neurons in pyriform cortex respond to odours -> Amygdala (emotional response) -> Thalamus -> Orbitofrontal Cortex

Question 28

Taste receptors

Answer 28

Taste Receptors
• Taste buds – receptors with hairs covered by saliva
• Most found in papillae
• Lifespan = 7-10 days
Five basic tastes
Each triggered by different chemicals:
1. Sweet
2. Sour
3. Salty
4. Bitter
5. Savoury/umami
80% of taste is actually smell Interpretation of the stimuli is KEY

Question 29

Taste pathway

Answer 29

Signals from the taste receptors -> Medulla -> Thalamus -> Gustatory Cortex (in Insula)