Sensory Reception

Sensory systems

Vision, somatic sensory/touch, gustatory/taste, olfaction/smell, auditory/hearing, balance/movement, and proprioception.

Proprioception

The ability to know where different body parts are in relation to other things.

Why do we need sensory reception?

Maintain homeostasis, detect and react to changes in the environment, and protect the body from noxious stimuli

Stimulus

A change in the environment that triggers a response.

Sensory receptor

Specialized cells that detect and respond to specific stimuli.

Afferent neuron

carries information from the sensory receptor to the CNS

Efferent neuron

Neuron that carries motor commands from the CNS to the effector organ (muscle or gland).

Effector organ

Muscle or gland that carries out the response to a stimulus.

What are sensory receptors required for?

sight, hearing, taste, smell, pain, temperature, pressure and touch

Cutaneous receptors

Receptors that respond to chemical pain, thermal pain, mechanical pain, and touch.

proprioceptors

receptors which detect body position

C fibers

Unmyelinated fibers that transmit slow pain signals.

A-delta fibers

Myelinated fibres which transmit fast, sharp pain signals

Pain receptors

Activated by the release of ATP and prostaglandins from damaged or stressed cells.

Prostaglandins

Chemical messengers that bind to prostaglandin receptors and contribute to pain signaling.

specialised Schwann cells in the skin (2019)

detect mechanical pain and transmit signals to C fibers.

TRPM8

Receptor activated by extreme cold (<15 degrees) and menthol

TRPV1

Receptor activated by extreme heat (>45 degrees) and capsaicin (chillies).

Pacinian corpuscles

specialised receptors in the skin sensitive to pressure and vibration.

Pacinian corpuscle structure

nerve ending surrounded by layers of highly specialised connective tissue. The nerve ending contains specialised stretch mediated sodium ion channels.

Merkel's discs, Messner's corpuscles, and Ruffini's endings

Other sensory receptors in the skin sensitive to various types of touch.

Merkel cells and innocuous mechanical itch

protect against itch via stretch-activated Piezo2 channels.

slowly adapting receptors

allow continual awareness and response eg receptors for pain, temperature and body position

Why are some mechanoreceptors rapidly adapting?

so ‘unimportant’ information can be ignored

Dorsal column system

Sensory pathway which responds to fine touch, pressure and proprioception

Spinothalamic system

Responds to pain, temperature and coarse touch

Decussation

Crossing of sensory pathways to the opposite side of the brain.

Post-central gyrus

Area of the brain where sensory information is processed and consciously experienced

Nociceptors

Receptors for pain, temperature, and body position. A-delta and C fibres

polymodal nociceptors

activated by high intensity mechanical, chemical and thermal stimuli and the fibres are C fibres

Referred pain

Perception of pain in a different part of the body from where the pain signals are generated.

Enkephalins

Neurotransmitters released by interneurons to modulate pain signals

Gate control theory of pain modulation

spinal cord contains a neuronal ‘gate’ which blocks/allows pain signals → brain. Action potentials along A-beta fibres (dorsal columns) may depolarise inhibitor interneurons

Why is sharp pain harder to treat?

A-delta fibres synapse in a different area of the spinal cord to C fibres and there are no interneurons that release enkephalins. Also no opiate receptors

different perceptions of pain

Everybody perceives pain differently due to prior experience, expectation of pain, mood eg anxiety/depression, genetics etc.

Non-steroidal anti-inflammatory drugs (NSAIDs)

block inflammation and can treat pain eg aspirin and ibuprofen block COX-1 and COX-2

Local anaesthetics

block voltage-gated sodium ion channels and block action potentials eg lidocaine

Opioids (eg morphine)

activate descending inhibition and inhibit transmitter release

Acute pain

Beneficial pain that helps keep us safe.

Chronic pain

Non-beneficial pain that persists over a long period of time.

Neuropathic pain

occurs as a result of changes in nociceptor pathways and neuronal death