I. Introduction
A. Sensory receptors
B. General processing patterns
II. SR classification
A. Location
B. Type of stimulus detected
C. Structural complexity
1. simple receptors - modified dendritic endings of sensory neurons
a. free (naked) dendritic endings -- cutaneous senses
b. encapsulated endings -- nerve endings associated with connective tissue coverings
- cutaneous senses
- proprioception
2. complex receptors
- sensory cells
III. Electrical and chemical events occurring in receptors
A. Receptor potentials (RP)
1. mechanism
- stimulus produces a change in membrane potential directly proportional to its intensity; if receptor threshold reached SAP produced.
2. receptor potentials -- in simple receptors
3. receptor potentials -- in complex receptors
4. ionic basis of generator potential
5. adaptation
a. tonic receptors -- slow adapting
b. phasic receptors -- fast adapting
c. basis of adaptation phenomena
- opening of channels that cause repolarization -- decay GP rapidly
- accessory structures decrease amount of stimulus reaching receptor
IV. Coding of sensory information
A. Modality and location
1. Sensation invoked by impulses generated in a receptor depends of specific part of brain they ultimately activate.
- specific sensory pathways are discrete from sense organ to cortex: labeled line coding.
a. modality: brain associates signals from specific receptors with a specific modality.
b. location
- no matter where a particular sensory pathway is stimulated along its course to cortex, conscious sensation produced is referred to the location of the receptor.
- phantom limb example in amputees
- lateral inhibition - helps isolate the location of stimulus; involves pre-synaptic inhibition.
B. Intensity and duration
1. Intensity
a. coded by frequency of APs generated - porportional to intensity of stimulation.
b. coded by number of sensory units firing
- a sensory unit is a sensory axon and all its peripheral branches
- receptors of a primary neuron pick up information from a specific area -- receptive field; thus each sensory unit has a receptive field
- primary and secondary sensory neurons do not always exist 1:1
- often multiple primary sensory neurons converge onto a single secondary sensory neuron -- individual receptive fields merge onto a single, large receptive field.
- intensity of stimulus also determined by number of sensory units being stimulated
- recruitment of sensory units by increasing stimulus intensities can result in perception of a stronger stimulus.
c. examples of two point discrimination.