What are sensory receptors?
-Nerve endings, specialised non-neural structures
What do sensory receptors do?
- They are transducers that convert different forms of energy into frequency of AP
- Inform the CNS about the internal and external environment
Sensory modality
A type of stimulus activating a particular receptor e.g. touch, pressure, pain, temperature, light
Adequate stimulus
They type of energy a receptor normally responds to
What are sensory receptors sensitive to?
Highly sensitive to one specific energy form but can be activated by other intense stimuli
What are the types of sensory receptors?
- Mechanoreceptors
- Proprioceptors
- Nociceptors
- Thermoreceptors
- Chemoreceptors
- Photorececeptors
Mechanoreceptors
- Stimulated by mechanical stimuli- pressure, stretch or deformation
- Detect many stimuli- haring, balance, blood pressure and skin sensations of touch and pressure
Proprioceptors
Mechanoreceptors in joints and muscles that signal information related to body or limb position
Nociceptors
Response to painful stimuli- tissue damage and heat
Thermoreceptors
Detect cold and warmth
Chemoreceptors
Detect chemical changes e.g pH, pO2 and pCO2
Photoreceptors
Respond to particular wavelengths of light
What are cutaneous mechanoreceptors and proprioceptor good examples of?
The principles of peripheral sensory processing
What does transduction in all sensory receptors involve?
Opening and closing of ion channels
What does an adequate stimulus cause/
A graded membrane potential change called a receptor potential or a generator potential
Membrane deformation
The adequate stimulus in cutaneous mechanoreceptors and proprioceptors
What does membrane deformation activate?
Stretch sensitive ion channels causing ion flow across the membrane
What does a stimulus cause?
Local current to flow to where the membrane has voltage gated ion channels that generate action potentials
Where does the local current flow to in myelinated sensory neurons?
The start of myelination
Describe the receptor potential.
Graded to stimulus intensity
Frequency coding of stimulus intensity.
The larger the stimulus, the larger the receptor potential and the higher the frequency of AP in a sensory nerve
What also reflects the stimulus intensity?
The number of receptors activated
What is the information a touch receptor in the skin responds to dependent on?
The properties of nerve endings and of accessory, non-neuronal structures
What happens for some mechanoreceptors if the stimulus persists?
APs persist
Adaption
Some mechanoreceptors adapt to a maintained stimulus and only signal change- the onset of stimulation
Why do some mechanoreceptors only signal at the onset of stimulus?
- Stimulus is enough to cause an above threshold generator potential, which triggers APs
- The generator potential declines rapidly and APs cease
- They respond only to change or a novel event
What do rapidly/ moderately adapting receptors include?
- Pacinian corpuscles
- Meissners corpuscles
What are examples of slow-adapting receptors?
- Merkels discs
- Ruffini endings
Why do nociceptors not adapt?
They are free nerve endings that detect painful stimuli and it is important to not ignore painful stimuli
What is a pacinian corpuscle?
A myelinated nerve with a naked nerve ending, enclosed by a connective tissue capsule of layered membrane lamellae separated by fluid
How does the Pacinian corpuscle respond?
- A mechanical stimulus deforms the capsule and nerve ending
- Stretching the nerve ending opens ion channels
- Na influence causes local depolarisation (generator potential)
- APs are generated and fire at the myelinated nerve
Describe adaption in the pacinian corpuscle?
- Stimulus is on
- Fluid redistribution in the capsule rapidly dissipates stimulus, removes mechanical stretch of nerve ending and the AP stops firing
- Withdrawal of the stimulus means the capsule springs back and the AP starts firing again
What happens if the lamellae is removed?
Much of the adaption is lost
What happens when the capsule is removed?
- Bare nerve ending loses much adaption
- Continues to produce a receptor/ generator potential
What does the non-neural accessory structure do?
It is critical to how the sensory receptor works and in general enhances sensory function
Receptive field
A somatic sensory neuron is activated by stimuli in a specific area e.g a touch-sensitive neuron in the skin responds to pressure within a defined receptive field
What does our ability to tell 2 points on the skin apart depend on?
- Receptive field size
- Neuronal convergence
- Determined by 2 point discrimination test
hat do sensory neurons with neighbouring receptive fields exhibit?
Neuronal convergence= multi presynaptic neurons input on a smaller number of post-synaptic neurons
What does convergence of primary sensory neurons allow?
Simultaneous sub-threshold to sum at the secondary neuron, forming a large secondary receptive field and initiating AP
What does convergence and large secondary receptive field indicate/
A relatively insensitive area
2 point discrimination test
Distance between points adjusted until just perceive 2 points rather than 1
Where has a very small 2-point threshold?
Fingers and lips
Where has a large 2 point threshold?
Back and limbs
Acuity
The ability to locate a stimulus on the skin and differentiate it from another closeby
How can we precisely determine the location of a stimulus?
- Lateral inhibition
- Information from afferent neurons with sensory receptors at edge of stimulus is strongly inhibited, compared with information from stimulus centre
What does lateral inhibition do?
Enhances the contrast between relevant and irrelevant information
Where does lateral inhibition occur/
In the spinal cord for cutaneous information
Describe sensory signals in the brain.
- All sensory information goes to the brain
- Relayed through thalamus to somatosensory cortex
- Cortical body map is distorted
- Most sensitive areas occupy biggest cortical space
Merkel receptors
Sense steady pressure and texture
Meissners corpuscle
responds to flutter and stroking movements
Pacinian corpuscle
Senses vibration
Ruffini corpuscle
Responds to skin stretch
Sensory nerves
Carry signals to spinal cord
Free nerve endings of nociceptors
Responds to noxious stimuli