Lecture 8 - Somatosensory, Vestibular and Olfactory systems Flashcards
Stimulus
Energy source Internal External
Receptors
Sense organs - structures specialized to respond to stimuli Transducers - stimulus energy converted into action potentials
Conduction
Afferent pathway Nerve impulses to the CNS
Transduction
CNS integration and information processing Sensation and perception
Sensory projections
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
Olfactory pathways
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
Vestibular pathways
Also project to the cerebellum Vestibular system -> Brain stem -> into cerebellum (motor control, balance, movement) as well as cortex
Touch
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
Skin
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!)
Mechanoreceptors
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
4-channel model of touch
Ascending pathway
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
Tactile pathway
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)
Samatosensory Pathways
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
Pain
“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
Nociceptors
What are they
Where are they
Key features
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:
- some are specific to one type of stimulus, such as: - mechanical (Mechano-sensitive nociceptors)
- thermal (Thermo-sensitive nociceptors) - but most are polymodal (respond to many stimuli, including chemicals)
- the number and size of the receptive fields served by each fiber may be small or large, respectively
A delta fiber vs. C fiber
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
Central mechanisms for pain
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
Vestibular system
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
Semicircular canals
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
Otoliths
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
Vestibular pathway
Vestibular signals travel through the Medial Longitudinal fasciculus to the brain
Vestibular Nerve -> Brainsterm -> (Cerebellum) -> Thalamus -> Vestibular Areas
3 Vestibular-driven reflexes
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
Vestibular-Ocular Reflex
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
- Detection of rotation
- Inhibition of extramuscular muscles on one side
- Excitation of extramuscular muscles on the other side
- Compensating eye movement
Chemoreceptors
Respond to chemicals in aqueous solutions
Chemicals dissolve in mucus of nose & saliva in mouth
Smell receptors
- 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
- Odorants bind with receptors
- Olfactory receptor cells are activated and send electric signals (Action Potentials)
- Signals are relayed in glomeruli
- Signals travel to brain
Smell pathway
Signals from the olfactory nerve travel to the pyriform cortex -> Neurons in pyriform cortex respond to odours -> Amygdala (emotional response) -> Thalamus -> Orbitofrontal Cortex
Taste receptors
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
Taste pathway
Signals from the taste receptors -> Medulla -> Thalamus -> Gustatory Cortex (in Insula)
