Cutaneous Sensory Lecture (Dr. Karius) TEST 3 Flashcards
Map of Lecture
1) Transduction of Stimulus into Action Potentials
2) Transmission/ Processing of Signal Below Cortex
3) Initial Processing of Signal in Cortex
4) Interpretation of Signal
5) How the Cortex can be changed by Sensation
6) Characteristics of the Sensation
General Information: Sensory Neurons
- Sensory (Afferent) Neurons have SPECIALIZED Channels that are OPENED or CLOSED in Response to a Stimulus
- For Ex, TOUCH Receptors have a SODIUM CHANNEL that is OPENED with Deformation of the CELL MEMBRANE
- Opening of these Channels produces a LOCAL RESPONSE, similar to the SUB-THRESHOLD response of EPSP you saw in the Action Potential and Synapse Lectures
***In Sensory Neurons, this Depolarization is called a “GENERATOR POTENTIAL”!!!!
**** If the Stimulus is STRONG ENOUGH or LASTS LONG ENOUGH, the Generator Potentials will CAUSE the Afferent Neuron to come to THRESHOLD and Generate an ACTION POTENTIAL!!!!!!!
The Generator Potential: Touch/ Pressure
- The PACINIAN CORPUSCLE is probably the best studied of the Receptors that respond to Touch, We will use it as a STARTING POINT
1) Pacinion Corpuscle is comprised of ALTERNATING LAYERS of Membrane with Fluid between them, surrounding the Nerve Ending
2) When we touch something, all the layers of the Membrane are DEFORMED
3) This leads to the OPENING of Mechanosesnitive Na+ Channels on the Membrane and INFLUX of SODIUM
4) And the generation of ACTION POTENTIALS back to the CNS!!!!
5) If the Stimulus is MAINTAINED, the Action Potentials gradually DIE AWAY as ADAPTATION Occurs!!!
6) The Adaptation is due to REDISTRIBUTION of the Fluid in the Corpuscle so that the force is NO Longer transmitted to the Nerve Ending
The Generator Potential: Touch/ Pressure
Lessons
1) Much (But NOT all) of the Adaptation that occurs is the result of CHANGES in the Periphery (Directly altering the Afferent). The Olfactory System showed us something similar
2) In some cases of Receptor adaptation, the REMOVAL of the Stimulus triggers Action Potentials as the ending “REFORMS” … this is an AFTERDISCHARGE!!!!!!
3) Afterdischarge is associated with the PERSISTENCE of the SENSATION after the Stimulus eliciting the Discharge has been Removed. I notice this most as the PHANTOM SUNGLASSES, i Keep reaching for my Sunglasses even though I no longer have them on
Sensory Neurons:
Sensory Unit: The Sensory Nerve and all its Branches
Receptive Field: The AREA from which Stimulation produces ACTIVATION of the Neuron
Coding of Stimulus Intensity
1) The Number of Action Potentials is one way of Coding the INTENSITY
2) With GREATER INTENSITY, we see MORE Action Potentials
3) With further Increases, we may see PATTERNED DISCHARGED (Doublets or Triplets)
4) The Number of Receptors firing also INCREASES with INCREASED INTENSITY
- The Number of Receptors FIRING also Increases with Increased INTENSITY
Coding of Stimulus Intensity: Quantification
NOTICABLE DIFFERENCE:
- The Smallest different can be detected
- A Change of about 10% is usually required for CONSCIOUS RECOGNITION of the Change
RELATIONSHIP BETWEEN PERCEIVED STRENGTH AND ACTUAL (MEASURED) INTENSITY):
- Originally described as the WEBER-FECHNER LAW:
- ** Perceived Intensity = Log (Measured Intensity) ***
- New Experimental data means new formula:
- Perceived Intensity = K (Measured Intensity) ^A
K and A are CONSTANTS!!!
Coding of Stimulus Intensity: Quantification
Important Details***
- K and A vary depending on the TYPE of Sensory Receptor
a) MUSCLE SENSES: Both are close to 1:
- TRANSLATION: Out perceived Intensity matches the actual (measured) intensity very closely
b) CUTANEOUS SENSES: More Variability
- TRANSLATION: What we Perceive may DIVERGE from the ACTUAL rather Substantially
Central Processing of the Senses: To the Brain
There are Multiple Pathways to the Brain:
1) DORSAL COLUMNS:
- Proprioceptive and Discriminative (Fine Touch)
2) SPINOTHALAMIC TRACT:
- Thermal, Nociceptive, and “Coarse” Touch
3) SPINORETICULOTHALAMIC SYSTEM:
- Nociceptive!!!
4) SPINOCEREBELLAR TRACT
Central Processing of the Senses, Modification of Input
- IN the MOST SENSITIVE Parts of Our Bodies, it is very RARE that a Stimulus activates only ONE Receptor due to the OVERLAP of Receptive Fields
Pre-Synaptic Inhibition
Special Case of Inhibition:
- AXO-Axonal Synapse
- The POST-Synaptic Cell is a PRE-Synaptic Terminal
- *****End Result of Pre-Synaptic Transmission:
- REDUCED Neurotransmitter Release form the Inhibited Pre-Synaptic Terminal!!!!!!!!!!!!!!!!!
Pre Synaptic Inhibition Process
1) Start with a “Normal” Chemical Synapse with Neurotransmitter release from A to B
2) Now, add another Neuron the Synapses on the Pre-Synaptic Terminal
3) When activated NEURON C release GABA!!!
4) Which then ACTIVATES CHLORIDE Entrance into Neuron A
5) Causing the Pre-Synaptic Terminal of Neuron A to HYPERPOLARIZE and allowing LESS CALCIUM TO ENTER!!!!
6) Leading to LESS Neurotransmitter RELEASE and REDUCING the PROBABILITY of Action Potentials in Neuron B!!!!!!!
Central Processing of the Senses- Modification of Input
- Pre-Synaptic INHIBITION occurs between the Neighboring Receptors at the FIRST SYNAPSE in their Pathway. This INCREASES THE BRAIN’S ABILITY TO LOCALIZE THE SIGNAL!!!!!!!
Central Processing of the Senses: To the Brain
- Regardless of which pathway is used, every Synapse along the way represents a CHANCE TO MODIFY OR RESPOND TO THE STIMULUS!!!!
Central Processing: Organization of the Cortex
1) The SENSORY CORTEX is arranged SOMATOTOPICALLY!!!!
- Hands, Face, and Mouth get a HUGE CHUNK of Corticotropin Area
2) As we Saw in the Visual Cortex:
- The Somatosensory Cortex is NEOCORTEX so it has Six Layers
- Neurons in the Somatosensory Cortex are arranged in Columns
- Each COLUMN DEALS WITH ONE SENSORY MODULATION IN ONE PART OF THE BODY!!!!!!!!
3) Sensory Information arrives at its Respective Column in LAYER IV (Via the THALAMUS)!!!!!
4) Neighboring Columns receive Information from the SAME PART of the Body, but a different Sensory Modality
