Sensory Systems Flashcards
What is each type of sensory information associated with?
Each type of sensory information is associated with a specific receptor type responding to a specific sensory modality
Give examples of types of receptors.
- Mechanoreceptors
- Chemoreceptors
- Thermoreceptors
- Nociceptors
- Proprioceptors
Give examples of receptors with free nerve endings.
- Nocicpetors
- Cold receptors
Give examples of receptors with complex structures.
- Pacininan corpuscle
- Meissner’s corpsucle
What is the receptive field?
Specific area that response to stimulus acts over
What do tactile (Meissner’s) corpuscle detect?
Light touch
What do tactile (Merkel’s) corpuscle detect?
Touch
What do free nerve endings detect?
Pain
What do lamellated (Pacinian) corpuscles detect?
Deep pressure
What do ruffini corpuscles detect?
Warmth
What do all sensory receptros transduce their adequate stimuli into?
All sensory receptors transduce their adequate stimulus into a depolarisation, the receptor (generator) potential
What does the size of receptor potential encode?
Size of receptor potential encodes intensity of stimulus
What does the receptor potential evoke?
Receptor potential then evokes firing of action potentials for long distance transmission
What does the frequency of the action potentials encode?
Intensity of stimulus
What does the receptive field encode?
Location of stimulus
What is duration of action potentials proportional to?
Stimulus duration
What does neurotransmitter release vary with?
The pattern of action potentials arriving at the axon terminal
What determines acuity?
- Density of innervation
- Size of receptive field
What transmits action potentials to the CNS?
Action potentials are transmitted to CNS by axons
What is cutaneous sensation mediated by?
Cutaneous sensation is mediated by 3 types of primary afferent fibres
- AB
- Adelta
- C
Describe AB fibres.
-Large myelinated
-30-70m/s
Touch
-Pressure
-Vibration
Describe the A delta fibres.
- Myelinated
- 5-30m/s
- Cold,
- “Fast” pain,
- Pressure
Describe the C fibres
- Unmyelinated fibres
- 0.5-2m/s
- Warmth
- “Slow” pain
What is proprioception mediated by?
Proprioception is mediated by 2 types of primary afferent fibres
-Aα & Aβ eg muscle spindles, golgi tendon organs etc
What do the primary afferent fibres enter the spinal cord via?
All enter spinal cord via the dorsal root ganglia (or cranial nerve ganglia for head)
What types of fibres are mechanorecetive?
Aα & Aβ fibres
What types of fibres are thermoreceptive and nociceptive?
Aδ & C fibres
Where do the mechanireceptive fibres project?
Project straight up through ipsilateral dorsal columns
Where doe the mechanireceptive fibres synapse?
Synapse in cuneate & gracile nuclei
Where do the 2nd order mechanoreceptive fibres decussate?
The 2nd order fibres cross over midline (decussate) in the brain stem & project to reticular formation, thalamus and cortex
Where do the thermoreceptive and nociceptive fibres synapse?
Synapse in the dorsal horn
Where do the thermoreceptive and nocicipetive project?
Project up through the contralateral spinothalamic (anterolateral) tract to reticular formation, thalamus and cortex
Where do the 2nd order thermoreceptive and nociceptive fibres decussate?
The 2nd order fibres cross over the midline in the spinal cord
What explain consequences of spinal cord injuries?
Different pathways for transmission of sensory information explains consequences of spinal cord injuries
What does damage to the dorsal columns result in?
Causes loss of touch, vibration, proprioception below lesion on ipsilateral side
What does damage to the anterolateral quadrant result in?
Causes loss of nociceptive & temperature sensation below lesion on contralateral side
Where is the ultimate termination of sensory information?
Ultimate termination is in the somatosensory cortex (S1) of the postcentral gyrus
What are endings grouped according to?
Endings are grouped according to the location of the receptors
What is extent of representation of endings related to?
Extent of representation is related to the density of receptors in each location
What is the order of the sensory homunculus from median to lateral?
- Genitalia
- Toes
- Foot
- Leg
- Hip
- Trunk
- Neck
- Head
- Shoulder
- Arm
- Elbow
- Forearm
- Wrist hand
- Pinky
- Ring
- Middle
- Index
- Thumb
- Eye
- Nose
- Face
- Lips
- Gum and jaw
- Tongue
- Pharynx
- Intraabdominal
What are the 5 of processing in sensory pathways?
- Adaptation
- Convergence
- Lateral inhibition
- not all information reaches the brain
- Perception
What may underlie referred pain?
Convergence
What is the advantages and disadvantage of convergence?
Saves on neurones but reduces acuity
What is lateral inhibition?
- Activation of one sensory input causes synaptic inhibition of its neighbours
- Gives better definition of boundaries
- Cleans up sensory information
How does lateral inhibition occur?
- Primary neuron response is proportional to stimulus strength
- Pathway closest to the stimulus inhibits neighbours
- Inhibition of lateral neurons enhances perception of stimulus
Give examples of different types of pain.
- Sharp, stabbing vs diffuse, throbbing pain
- Fast (initial) pain vs slow (delayed) pain
- Acute vs chronic pain
- Visceral pain
- Referred pain
- Phantom limb pain
What is signal transduction in nociceptors activated by?
- Low pH, heat (via ASIC, TRPV1 etc)
- Local chemical mediators (eg bradykinin, histamine, prostaglandins)
How can the ‘gate’ be closed to prevent pain?
- Segmental controlseg gate control theory of pain: activity in Aα/β fibres activates inhibitory interneurones
- Descending controls: The same inhibitory interneurones are also activate by descending pathways from PAG and NRM, hence also “closing the gate”
- Inhibitory interneurones release opioid peptides (endorphins) that inhibit transmitter release from Aδ/C fibres, hence “closing the gate”
How do NSAIDs work?
- Prostaglandins sensitise nociceptors to bradykinin
- NSAIDs are analgesic (and antipiretic & anti-inflammatory) because they inhibit cyclo-oxygenase which converts arachidonic acid to prostaglandins
- So NSAIDs work well against pain associated with inflammation
How do local anaesthetics work?
Block Na+ action potential and therefore all axonal transmission
How do opiates work?
- Reduce sensitivity of nociceptors
- Block transmitter release in dorsal horn (hence epidural administration)
- Activate descending inhibitory pathways
Give example of analgesia.
- NSAIDs
- Local anaesthetics
- Trans cutaneous electric nerve stimulation (TENS)
- Opiates (e.g. morphine)
