Outline-3, BIO 3360, Sensory Systems I

I. Sensations –part of nervous system, animal’s perception of external/internal world

II. Sensory receptors, neural pathways, integrating system

III. Receptors

A. Dendrites of afferent neurons or cells that affect afferent neurons

B. Respond to stimuli in the internal/external environment

1. Stimuli include different energy forms

2. Pressure

3. Temperature

4. Light

5. Sound waves

6. Receptors change these energy forms into graded potentials and these can trigger action potentials that travel to the CNS via afferent neurons

C. Receptor (membrane) can be in an afferent neuron or in a separate cell. In the first case stimulus will change the membrane potential of the neuron directly. This results in a GENERATOR POTENTIAL, and if it reaches threshold becomes an ACTION POTENTIAL. In the second case stimulus will result in the release of a NT (with or without that separate cell having an action potential) that will then affect the membrane potential of an adjacent neuron. This results in a RECEPTOR POTENTIAL which causes a graded potential in the afferent neuron. If this graded potential reaches threshold it becomes an ACTION POTENTIAL.

D. Receptor & Generator Potentials

1. Receptor/Generator potentials are graded potentials that depolarize, or lead to the depolarization of, the afferent neuron. If threshold is reached, the afferent neuron will fire an AP.

2. Factors determining the magnitude of the receptor/generator potential

-Stimulus Strength As strength of stimulus increases, the graded potential increases in magnitude. Once the graded potential reaches threshold, an AP is produced. At this point, a stronger stimulus leads to an increased frequency of AP’s.

-Temporal summation of receptor potentials A second stimulus arrives before the effects of the first have disappeared leading to a larger receptor potential and may even lead to an AP.

-Rate of change of stimulus strength A larger response to a rapidly changing stimulus.  For example, a dimming of a light gradually has a smaller response but suddenly turning on/off a light has a greater response.

Adaptation – the receptor stops responding to the stimulus. Pain does not adapt.

E. Stimulus Transduction –how stimuli are transformed into electrical responses (graded and action potentials)

1. Due to changes in membrane permeability –either opening or closing of membrane ion channels

2. Receptors are specific in that they respond better to certain stimuli, but many respond to high intensity stimuli, especially in regards to pain.

F. Sensory Reception is an afferent pathway to the CNS, but CNS modulates receptor sensitivity & may be excitatory or inhibitory thus providing efferent control – as in “mind over matter.”

G. Primary Sensory Coding (interpretation of sensation)

1. Type or Modality –receptors are specific to a certain type of stimulus

2. Intensity –determined by frequency of APs and recruitment of neighboring receptors

3. Duration –different receptors adapt at different rates, some rapidly & some slowly.  While adaptation does not occur with pain, an example of slow adaptation is stretching of the skin or feeling something rough from your skin.  In this case, the feeling is persistent.  An example of rapid adaptation is the sudden odor of a predator coming but if the stimulus is gone, the response is transient and the odor is gone.

4. Location

a. Projection The brain projects the sensation back to the receptor. An example of the effectiveness of this response is seen with phantom pain.

b. Size of receptive field – part of body served by one particular afferent neuron; measured by two point discrimination test on your skin.

c. Amount of convergence –more convergence – less precision

d. Overlap of nearby receptive fields –The denser the receptors, the easier it is to pinpoint locationReceptors respond more vigorously to stimuli applied to the center of their receptive field. Lateral Inhibition occurs when receptors that are the most strongly stimulated inhibit their neighboring receptors that are weakly stimulated. This leads to a more localized and relevant pathway