General Sensation Flashcards
What are sensory receptors?
- Each type of sensory information is associated with a specific sensory receptor (mechan/chemo/thermo/nociceptors
- Can either be free nerve endings (nociceptor) or complex structures (Pacininan corpuscle)
- These receptors respond to stimulus over a specific area = RECEPTIVE FIELD
What are the different types of sensory receptors found in the skin?
Tactile: Light touch: Meissner’s Corpuscle
Tactile: Touch: Merkle’s Corpuscles
Pain: Free nerve ending
Deep pressure: Pacinian corpuscle
Warmth: Ruffini Corpuscle
What’s the process of Sensory transduction?
- Sensory receptor s transduce (physical energy into a nervous signal) their adequate stimulus into a depolarisation, the receptor (generator) potential
- The size of the receptor potential encodes the intensity of the stimulus - greater stimulus great chance of breaching threshold
- Receptor potential then evokes firing of action potentials for long distance transmission
- The frequency of action potentials encodes the intensity of stimulus with the receptive field encoding the location of stimulus
- Ends in neurotransmitter release
What determines Acuity (ability to discriminate between 2 points)?
The density of innervation and size of receptive field
- Some parts of the body have neurons with small receptive fields - able to discriminate between 2 points
- Some parts of the body have neurons with large receptive fields -unable to distinguish between 2 points of contact in that area
What are Primary afferent fibres?
Nerve fibres/axons which transmit action potentials to the CNS
Mediated by 3 types of Primary afferent fibres
- Aβ = large myelinated (30-70m/s) touch, pressure, vibration (mechanoreceptors)
- Aδ = small myelinated (5-30m/s) cold, “fast” pain, pressure
- C = unmyelinated fibres (0.5-2m/s) warmth, “slow” pain
Proprioception is mediated by 2 types of primary afferent fibres:
- Aα & Aβ eg muscle spindles, golgi tendon organs etc
ALL THESE FIBRES ENTER THE SPINAL CORD VIA THE DORSAL ROOT GANGLIA (OR CRANIAL NERVE GANGLIA FOR HEAD)
How do mechanoreceptive fibres (Aα & Aβ) transmit sensory information?
- Project straight up through ipsilateral dorsal columns
- Synapse in Cuneate & Gracile Nuclei
- 2nd order fibres cross over midline (decussate) in the brain stem & project to reticular formation, thalamus and cortex
How do thermoreceptive & nociceptive (Aδ & C) fibres transmit sensory information?
- Syanpse in the dorsal horn
- 2nd order fibres cross over the midline in the spinal cord
- Project up through the contralateral spinothalamic (anterolateral) tract to reticular formation, thalamus and cortex
How does damage to the dorsal column and anterolateral quadrant differ?
Dorsal column = mechano = cause loss of touch, vibration, propriception below lesion on ipsilateral side
Anterolateral quadrant = = thermoreceptive & nociceptive (pain) = loss of nociceptive and temperature sensation below lesion on contralateral side
Where does sensory information terminate?
- Somatosensory cortex (S1) of the postcentral gyrus
- Endings are grouped according to the location of the receptors
- Extend of representation of location is related to density of receptors in each location = produces the sensory HOMUNCULUS on parietal lobe
What’s adaption in terms of processing sensory information; in terms of rapidly and slowly adapting?
Rapidly adapting: even with constantly high stimulus intensity, action potentials stop surpassing threshold - don’t respond to stimuli
= putting a hat on - you quickly forget and lose sensation
Slowly adapting: action potentials constantly breach threshold in response to the stimuli
What’s convergence and what are its drawbacks and benefits?
Convergence is where 1 postsynaptic cell receives convergent input from a number of different presynaptic cells - allows 1 neuron to communicate with numerous neurons in a network
Benefits: Reduces the number of neurons needed
Drawbacks: Reduces neural acuity (ability to distinguish between 2 individual signals
- Underlies referred sensations
What’s the Specific ascending pathway in terms of convergence?
- 2 receptors for touch OR 2 synapses for temperature synapse at the same post-synaptic cell – inability to distinguish between them = unable to tell where the high temp is actually coming from initially
Non-specific ascending pathway:
- Temperature and touch synapse at the same post-synaptic cell – unable to distinguish between the signals - just aware of something
What’s Lateral Inhibition?
- Activation of 1 sensory input causes synaptic inhibition of its neighbours
- Gives better definition of boundaries and cleans up sensory information
How does Lateral inhibition work?
- Stimulus on the skin (pin) detects pressure change etc
- Primary neuron response is proportional to stimulus strength (closest to pin contact = greater stimulus strength)
- Pathway closest to the stimulus inhibits neighbours
- Inhibition of lateral neurons enhances perception of stimulus
Why doesn’t all the sensory information reach the brain?
- Higher brain centres possess descending controls; either excitatory or inhibitory neurons - can be pre OR postsynaptic
