3. Sensory Overview Flashcards
Afferent Nerve Fibers
Carry sensory info to CNS
Afferent Neuron (components)
Peripheral process extending to sensory receptor
Centra process entering CNS through dorsal root/cranial nerve
Sensory Transmission (steps)
Sensory receptors First order neuron Second order neuron Third order neuron Fourth order neuron
First Order Neuron
Primary sensory afferent neuron
Cell body in dorsal root ganglion
Second Order Neuron
In spinal cord or brainstem
Interneurons can modify here (excitatory or inhibitory)
Third Order Neuron
Typically in thalamus
Fourth Order Neuron
Sensory cerebral Cortex
Types of Sensory Receptors
- Specialized peripheral ending of primary afferent neuron (free naked nerve ending)
- Accessory sensory cell (transmits info to primary afferent neuron)
Adequate Stimulus
Form of stimulus energy that a receptor is most sensitive
Not absolute (can respond to other stimuli) (think eyes closed and pressure)
Classification of Sensory Receptors
Classified by:
- the kind of stimulus they are sensitive to
- location in body
Mechanoreceptors
Respond to mechanical energy
Touch, pressure, vibration
Thermoreceptors
Respond to temperature changes
Warm and cold receptors
Photoreceptors
Respond to light energy
Chemoreceptors
Respond to circulating or applied chemicals
Exteroreceptors
Detect stimuli affecting external surface of the body
mechanoR, thermoR
Interoreceptors
Lie inside the body
taste, olfaction, things that effect gut
Proprioceptors
Located in internal mass of body
muscles, joints, tendons
Special Receptors
Vision, audition, taste, olfaction, balance
Superficial Receptors
Touch, pressure, flutter, vibration, tickle, warmth, cold, pain, itch
Deep Receptors
Position, kinesthesia, deep pressure, deep pain
Visceral Receptors
Hunger, nausea, distention, visceral pain
Nociceptors
Receptors associated with pain sensations
Sensory Receptor (function)
Transduce/translate energy from one form to another
End result: train of impulses (APs) which carries info about the stimulus applied
Sensory Trandsuction
- stimulus energy
- receptor
- receptor/generator potential
- action potentials
Impulse Initiation
- Stimulus activates R
- Local, graded potentials occur across membrane (generator or receptor)
- In afferent nerve fiber, separate local potentials summed at first node of Ranvier
- AP generated when membrane reaches threshold
Generator Potential
Graded potential
Receptor is modified afferent nerve ending
Receptor Potential
Graded potential
Receptor is a separate cell
Neural Encoding
Process by which information is transmitted from periphery to brain
Sensory Information
Transmitted by sensory R
Coded as AP
- Modality (quality/nature of sensation)
- Location
- Intensity
- Duration
Modality
Sensations that are referred to a single type of receptor
Doctrine of Specific Nerve Energies
Sense organ is sensitive to many forms of energy
Sensation is always that of adequate stimulus - despite what form of energy was applied
Labeled Lines
Modality of a sensation depends upon which particular cell, pathway, nucleus or lobe is activated by the stimulus
Pathways of sensory neurons dedicated to that modality
Location
Ability to locate the side of stimulation and discriminate between two closely spaced stimuli
Encoded by receptive field of sensory neurons
Receptive Fields
Area of the body that when stimulated results in a change in firing rate of sensory neuron
Areas supplied by different sensory units will overlap
More complex moving from primary to secondary etc neurons
Receptive Fields: Increase in stimulus intensity …
Stimulus will spread
The smaller the receptive field …
The greater the density of receptors - more precisely the sensation can be localized
Two Point Discrimination Test
Minimal distance between two detectable stimuli
Better discrimination where receptive fields are very small and densely innervated
Receptive Field: Secondary Neurons
Determined by:
- summed inputs from primary neurons
- input from interneurons
Intensity
Methods of transmission:
- Frequency code
- Population code
- Intense stimuli activate different types of receptors
Frequency Code
As stimulus intensity rises, firing frequencies of sensory neurons rise
More APs (number of AP per unit time)
Population Code
Intense stimuli recruit greater numbers of nerve fibers to fire
Bigger stimulus - more neurons activated
Duration
Encoded in the way sensory nerve fibers change their firing frequencies over time
Phasic vs. Tonic
Phasic Receptors
Rapidly adapting
Signals only onset and cessation of stimulation
Tonic Receptors
Slowly adapting
Continuously signal a persistent signal (fires as long as the stimulus is present)
Sensory Dysfunction: Negative
Taking something away
Due to disruption of nerve activity
Numbness
Loss of cold or warmth
Blindness
Deafness
Sensory Dysfunction: Positive
Gain sensation (in a way we don’t want)
Due to excitation or disinhibition
Pain
Paresthesias (tingling, pins and needles)
Visual sparkles
Tinnitus
