Physiology III
Neuroscience
Overview

Reading: Handout

Keywords:

transduction:  The conversion of one form of energy into another (Haines 40).  For example...turning mechanical pressure (touch) into electrical (sensory nerve propagation).

adequate stimulus:  I'm guessing here.  It is the stimulus that a sensory neuron is specifically tuned into.  It filters out everything but what it is tuned to and only allow that stimulus to activate the neuron.

absolute threshold:  I'm guessing here too.  A receptor potential must rise to at least the absolute threshold before eliciting action potentials in the nerve fiber that is attached to the receptor (assumed from reading Guyton 585).

modality:  Vision, Audition, Taste, Touch, Smell.  

The class of sensation to which stimulation of a receptor gives rise.  Thus, light receptors in the retina are specific for light and sub-serve the visual modality.  This modality specificity is maintained in the central connections of sensory axons, so stimulus modality is represented by the set of receptors, afferent axons, and central pathways that it activates.  The sensory system activated by a stimulus determines the nature of the sensation.  

There is also specialization for different types of stimuli within a modality.  For example, different cutaneous receptors respond best to different stimuli, evoking the different sensations of touch, itch, tickle, flutter, vibrations, fast and slow, pain, cold, and heat  (Haines 40).

receptor/generator potential:  All sensory receptors have one feature in common.  Whatever the type of stimulus that excites the receptor, its immediate effect is to change the membrane potential of the receptor.  This change in potential is called a receptor potential (Guyton 584).

labeled line codes:  Each nerve tract terminates at a specific point in the central nervous system, and the type of sensation felt when a nerve fiber is stimulated is determined by the point in the nervous system to which the fiber leads (Guyton 584).  Groups of cells and axons that specialize in representing a specific type of stimulus (example...mechanical vs. thermal) (Haines 41)

tonic receptors (slowly adapting):  The frequency of discharge may decrease during sustained presentation of a fixed-amplitude stimulus.  Afferent axons that show little or no drop in discharge rate during a maintained stimulus are called tonic or slowly adapting (Haines 40).

phasic receptors (rapidly adapting):  The frequency of discharge may decrease during sustained presentation of a fixed-amplitude stimulus.  Afferent axons that show a substantial drop in discharge rate during a maintained stimulus are called phasic or rapidly adapting (Haines 40).

receptive field:  The area of skin innervated by branches of a general somatic afferent fiber, the stimulation of which activates its receptors.  Small receptive fields are found in areas, such as the finger tips, where receptor density is high and each receptor serves an extremely small area of skin.  In such regions the individual is able to discriminate small variations in a variety of sensory inputs.  In other regions, receptor density is low and each receptor serves an expansive area of skin, creating large receptive fields with resultant reduction in discriminative ability (Haines 222).

lateral inhibition:  The reason for a high degree of resolution in the Dorsal column-medial lemniscal system is the result of inhibitory mechanisms such as lateral (surr9und) inhibition.  This mechanism, which sharpens and enhances the discrimination between separate points on the skin (Haines 220).

spatial summation:  Currents from multiple inputs add algebraically (Haines 46).  Increasing signal strength is transmitted by using progressively greater numbers of fibers (Guyton 588).

temporal summation:  Increasing strength by increasing the frequency of nerve impulses in each fiber (Guyton 589).  Presynaptic action potentials arrive at intervals shorter than the durations of the postsynaptic potentials they produce.  There there is a cumulative effect as postsynaptic potentials overlap in time, and their charges "pile up," or summate, on the membrane capacitance (Hines 46).

Objectives:

compare the relationship between a sensory modality, labeled line codes and the adequate stimulus:  Modality is the "type" of sensation that we can experience...such as pain, touch, sight, sound etc.  Yet despite the fact that we experience these different modalities of sensation, nerve fibers transmit only impulses...SO...each nerve tract terminates at a specific point in the central nervous system (Labeled Line Codes), and the type of sensation felt when a nerve fiber is stimulated is determined by the point in the nervous system to which the fiber leads.  I'm Guessing about the adequate stimulus.  Adequate stimulus is the stimulus that a sensory neuron is specifically tuned into.  It filters out everything but what it is tuned to and only allow that stimulus to activate the neuron (Guyton 583-584).

discuss the significance of receptor/generator potentials:  All sensory receptors have one feature in common.  Whatever the type of stimulus that excites the receptor, its immediate effect is to change the membrane potential of the receptor.  This change in potential is called a receptor potential (Guyton 584).  When the receptor potential rises above the threshold for eliciting action potentials in the nerve fiber attached to the receptor, then action potentials begin to appear.  The more the receptor potential rises above the threshold level, the greater becomes the action potential frequency.  Thus, the receptor potential stimulates the sensory nerve fiber in the same way that the excitatory postsynaptic potential in the central nervous system neuron stimulates the neuron's axon (Guyton 585).

discuss the significance of adaptation of sensory receptors:  A special characteristic of all sensory receptors is that they adapt either partially or completely to their stimuli after a period of time.  That is, when a continuous sensory stimulus is applied, the receptors respond at a high impulse rate at first and then at a progressively slower rate until finally many of them no longer respond.  Some sensory receptors adapt to a far greater extent than others.  The slowly adapting receptors continue to transmit impulses to the brain as long as the stimulus is present (or at least for many minutes or hours).  Therefore, they keep the brain constantly apprised of the status of the body and its relation to its surroundings.  They are called Tonic receptors.  Receptors that adapt rapidly cannot be used to transmit a continuous signal because these receptors are stimulated only when the stimulus strength changes.  Yet they react strongly while a change is actually taking place.  Furthermore, the number of impulses transmitted is directly related to the rate at which the change takes place.  Therefore, these receptors are called Rate receptors, Movement receptors, or Phasic receptors (Guyton 586-587).

be able to identify a fiber type in either of two commonly used systems (I-V; A-C) and state the significance of the classification:  

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