Module 6 - PNS Afferent Division Flashcards
Describe the sensation of pain
• Uses nociceptors
• Activated by intense thermal, mechanical, or chemical stimuli
o Accompanied by behavioural and emotional responses unlike other stimuli
• Located everywhere but the brain
• Free nerve endings that do not adapt to sustained or repeated stimuli
• Sensations may be out of proportion to damage or appear for no obvious reason
• Influenced by prostaglandins, histamine, potassium, serotonin, and substance P (peptides)
Describe fast pain
- Uses myelinated A-delta (δ) fibres
- Responds to cold, warmth, and mechanical stimuli
- Occurs immediately after stimulus applied
- Not felt in deeper tissues of the body
- Precisely located to stimulation
Describe slow pain. Why does this pain last even after the stimulus is removed?
• Also called chronic, burning, aching, or throbbing pain
• Uses unmyelinated C fibres
• Responds to pain, heat, cold, and mechanical stimulation
• Activated by bradykinin released to the ECF from damaged tissue
o Contributes to pain and inflammatory response
o These may be the reason the pain sensation lasts after removal of stimuli
• Polymodal nociceptors
o Respond to more than one type of noxious stimuli
• Begins a second or more after stimulus is applied
• Gradually increases in intensity over seconds or minutes
• Can be felt in skin and deeper tissues
• Felt in affected area but more diffuse
Describe afferent neurons
Receive stimuli from the environment and transmit the impulse to the CNS
Single long dendrite that is functionally similar to an axon
Short axon
Smooth rounded cell body
Describe the two types of neurons in the afferent division of the autonomic nervous system
Visceral Afferent Neurons
• Convey information to the CNS from receptors
• Located in the smooth muscle of the visceral organs (mainly thorax, abdomen, and pelvis)
• Usually happens no conscious awareness except pain signals coming from viscera
Somatosensory Neurons • Consists of receptors in the PNS and processing centers in the CNS • Person is consciously aware of this input • 4 primary types of receptors are o Thermoreceptors o Mechanoreceptors o Photoreceptors o Chemoreceptors • Input from the body surface o Somaesthetic sensation from the skin o Proprioception from the muscles, joints, skin, and inner ear • Includes input from special senses
What is perception?
Conscious interpretation of the external world
What is transduction?
o Energy conversion process that converts mechanical or chemical stimulation into an electrical signal
What is adequate stimulus?
o Property of a sensory receoptor that determines the type of energy to which a sensory receptor responds with the intention of sensory transduction
Ex. Receptors in the eye are most sensitive to light etc.
o Some receptors can respond weakly to stimuli other than their adequate stimuli
Receptors are activated to a lesser degree
Same sensation detected
Ex. The eye sees “stars” in response to mechanical pressure
What is modality?
o A particular way of sensing the stimulus
o Determined by the type of sensory neuron that’s activated and its point of termination in the brain
What stimulus do photoreceptors respond to?
Visible wavelengths of light
What stimulus do mechanoreceptors respond to? What are some examples of these receptors and their specific stimulus?
Mechanical energy
Skeletal muscle receptors - stretch
Hair cells in the ear - bend to sound waves
Baroreceptors - blood pressure
Osmoreceptors - change in osmotic pressure
What stimulus do thermoreceptors respond to?
Varying amounts of heat
What receptors are responsible for the sensation of wetness?
A combination of touch, pressure, and thermal input at the central integration area
What stimulus do chemoreceptors respond to?
Specific chemicals
May include receptors for smell and taste
Chemical content of digestive tract
Oxygen and CO2 levels in the blood
What may the structure of the sensory neurons be and what is the depolarization of each type called?
A specialized ending of the afferent neuron - generator potential
A separate receptor cell closely associated with the peripheral ending of the neuron - receptor potential
Where are action potentials created in afferent neurons?
• All receptors have graded potential with action potentials initiated at the peripheral end of the afferent neuron next to the receptor
What is stimulus intensity determined by?
o Frequency of action potentials generated
o Number of receptors activated in the area
What is adaptation?
- Decrease in sensation during prolonged stimulus
* Frequency of action potentials generated in afferent neuron decreases
Describe the 2 types of receptors based on their speed of adaptation?
Phasic Receptors
• Rapidly adapting receptors
• Activated when stimulus is first encountered
• Important to signal a change in stimulus intensity rather than relay status quo information
o Ex. Tactile (touch) receptors so you can wear clothes
Off Response
• Slight depolarization when stimulus is removed
Tonic Receptors
• Slow to adapt or do not adapt at all
• Will maintain sending signals even with a sustained stimulus to allow to tell duration of the sensation
• Located where it is valuable to maintain information about stimulus
o Ex. Proprioceptors
Describe labelled lines of afferent neurons.
• Chains of neurons that create the somatosensory pathways
• Sends specialised information to specific area in the somatosensory cortex
o Allows brain to interpret specific sensations
First Order (Primary) Neurons
• Sensory neurons in PNS carrying information to CNS
• Axons synapse with second order neurons
Second Order (Secondary) Neurons
• Usually located in brain or spinal cord
• Axons ascend to the thalamus and synapse with third order neurons
Third Order (Tertiary) Neurons • Synapse located in the thalamus • Project axons into primary somatosensory cortex
What is the receptive field?
The region of skin surface surrounding a somatosensory neuron in which it will respond to a stimulus
Size is inversely related to number of receptors in the area
What is discriminative ability?
The greater acuity created by a small receptive field the more a neuron is able to sense and discriminate between different sensations (compare sensation of the same object between fingertips and elbows)
What is lateral inhibition?
Occurs within the CNS, the most activated stimulus area inhibits the less exited pathways. It’s what allows us to feel a pin point sensations
Most lateral inhibition occurs with touch and vision, allowing the most accurate localization
Describe Pacinian corpuscles. What do they respond to? What is their appearance? How quickly do they adapt?
Lamellar Corpuscle
• Also called a Pacinian corpuscle
• Respond to
o High frequency vibration
o Touch
o Deep pressure
o Allows for sensation from texture
• Located in the dermis, subcutaneous tissue, and other tissues of the body
• Myelinated neuron ending surrounded by a multilayered connective tissue capsule that resembles a sliced onion
o Onion structure causes constant pressure to slip, resulting in adaptation
• Phasic receptor so is rapidly adapting
o Respond to transient touch rather than sustained pressure