Sensory Systems Flashcards
Sensory receptors may be simple free nerve endings or complex structures, give some examples of both?
Nociceptors & cold receptors are free nerve endings
Pacinian Corpuscles (vibration) and Meissner’s Corpuscles (Touch) are more complex structures
How is tactile acuity determined?
Larger number of smaller receptive fields (i.e. nerve ending density) allows greater acuity as you can isolate a signal to a smaller area.
What is a receptive field?
The area of which a receptor will detect a stimulus
What does an “adequate Stimulus” mean?
The specific mode of stimulus the receptor responds to
Signal transduction occurs differently for mechanoreceptors and nociceptors, how do mechanoreceptors transduce a signal?
The receptor transduces the adequate stimulus into a depolarisation called the receptor or generator potential which then evokes an AP
How is the intensity of a stimulus encoded by mechanoreceptors?
In the size of the receptor or generator potential
Then in the frequency of the APs
What are the various types of sensory primary afferent fibres?
Aalpha & Abeta:
- Large myelinated fibres
- Touch, pressure, vibration (all the innocuous mechanoreceptive modalities)
- Also carry proprioception from muscle spindles & GTOs
Adelta:
- Small myelinated fibres
- Carry cold and “fast” pain
C:
- Small Unmyelinated fibres (slowest)
- Carries Warmth & “slow” pain
Describe the pathway of mechanoreceptive fibres (Aalpha & Abeta)
They all enter through the dorsal root via the DRG
- 1st order neuron projects striaght up through the ipsilateral dorsal column
- 2nd order neuron decussates in the brainstem
- Projects up to reticular formation –> Thalamus –> Cortex
Describe the pathway of thermoreceptive & nociceptive fibres (Adelta & C)
Enters through dorsal root via DRG
- 1st order neuron synapses in the dorsal horn
- 2nd order decussates in spinal cord
- Then projects up the contralateral spinothalamic (anterolateral) tract to the reticular formation –> Thalamus –> Cortex
What happens if you damage the dorsal column?
You lose ipsilateral mechanoreception (Touch, vibration, proprioception)
What happens if you damage the spinothalamic tract?
You lose contralateral thermoreception & nociception
Define adaptation?
Receptors can be divided into slow and fast adapting.
Fast adapting receptors such as pressure receptors adapt to a sustained stimulus, only telling you when the stimulus changes
Slow adapting receptors such as muscle spindles maintain the signal until the stimulus is removed
Define Convergance?
Multiple receptive fields synapsing onto one neuron, this saves on neurons but reduces acuity
What is the difference between a specific and non-specific pathway per convergence?
A specific pathway has receptors of the same modality converging on one neuron
A non-specific pathway has receptors of different modalities converging on one neuron, usually pain and touch. These can’t be distinguished
Explain the concept of lateral inhibition?
The neuron closest to the stimulus inhibits the synapses of the neighbouring neurons.
This cleans up sensory info giving you a better definition of the stimulus boundary
(So the central neuron has a higher than tonic frequency of APs and the neighbouring ones have a lower than tonic level)
