Peripheral Nervous System & Receptor Physiology: Sensory Pathways and Pain

Perception

Conscious interpretation of the external world as created by the brain from a pattern of nerve impulses delivered to it from sensory receptors.

Afferent neuron

Neuron that brings the action potential to the CNS.

Modality

Energy from a stimulus that is determined by which sensory neuron is activated and where the stimulus terminates in the brain.

Transduction

Energy conversion process that converts mechanical or chemical stimulation into an electrical signal through changes in ion permeability at controlled ion channels.

Receptor potential

Local depolarizing change in potential if the receptor is a separate receptor cell that has a junction with an afferent neuron.

Generator potential

Local depolarizing change in potential if the receptor is a specialized ending of an afferent neuron.

Receptor adaptation

The ability of receptors to adapt slowly or rapidly to sustained stimulation.

Tonic receptors

Receptors that do not adapt at all, or adapt slowly, important for maintaining information about a stimulus.

Phasic receptors

Rapidly adapting receptors that stop responding to a maintained stimulus but respond with a slight depolarization called the off response when the stimulus is removed.

General sensory pathway

Chains of neurons, or labeled lines, synaptically interconnected in a particular sequence to process sensory information.

First-order sensory neuron

Afferent neuron with its peripheral receptor that first detects a stimulus.

Second-order sensory neuron

Neuron located in the spinal cord or medulla that synapses with a third-order neuron.

Third-order sensory neuron

Neuron located in the thalamus.

Acuity

Refers to discriminate ability (discrimination) influenced by receptive field size.

Receptive field size

The circumscribed region of the skin surface surrounding a somatosensory neuron that responds to stimulus information.

Baroceptors

Receptors that measure pressure.

Muscle stretch receptors

Receptors that monitor muscle length.

Joint propioceptors

Receptors that measure the degree of joint flexion.

Tactile receptors

Receptors that signal changes in pressure on the skin surface.

Off response

Slight depolarization that occurs when a maintained stimulus is removed.

CNS

Central Nervous System, where afferent neurons transmit information via action potential propagation.

Neurotransmitter

Chemical that excites the receptor by stimulus.

Large receptive field

one point

Small receptive field

two point

Lateral inhibition

process where a neuron's response is suppressed by the activity of neighboring neurons

Stimulus

Y sends off inhibitory neurons, which create IPSP's, X & Z will be shut down

Touch Receptors

are the most common in the body

Free nerve endings

found around hair roots, in the eyes, and many other tissues. abundant, sense hair movement, painful stimuli (noxious stimuli)

Meissner's corpuscle

for sensing light touch. They are concentrated in areas sensitive to light touch such as fingers, lips, and nipples.

Merkel's receptors

are the most sensitive to low frequency vibrations. Found in the superficial layers of the skin and mucosa. They are clustered into 'touch domes'. They are also located in the mammary glands. sensitive to steady pressure and texture

Ruffini corpuscle

in the deep layers of the skin and joints (register mechanical deformation within joints) and respond to skin stretch, skin is being deformed

Pacinian corpuscle

responds to touch and deep pressure and is very sensitive to vibration. sense steady pressure and vibration.

Pain Receptors

Is associated with nociceptor stimulation

Fast pain

Acute, sharp stabbing pain

A-delta fibers

largest and fastest, small myelinated, 6-30m/sec, 1-5 um

Slow pain

Throbbing, aching, burning

C fibers

smallest and slowest, feel pain the next day, less than 1-2m/sec, 0.5-2um

Somatic pain

Body pain from body surfaces

Visceral pain

Pain in internal organs sensed on the surface of the body

Neuropathic pain

chronic pain caused by damage or dysfunction of the nerves, spinal cord, or brain.

Phantom pain

Pain in missing limb or body part after amputation

Substance P

ordered from first-order neuron. Activates ascending pathways that transmit nociceptive(pain) signals to higher levels for further processing.

Glutamate

major excitatory neurotransmitter, can become toxic, has 2 receptors: NMDA and AMPA

NMDA receptor

leads to calcium entry into the dorsal horn cell. Causes excitotoxicity.

AMPA receptor

leads to permeability changes that ultimately result in the generation of action potentials in the dorsal horn cell.

Pain Modulations

The brain has a built-in analgesic system: suppressed transmission of pain pathways as they enter the spinal cord.

Endogenous opiates

Natural opioids that bind to opioid receptors and block pain transmission