SECTION A: GENERAL PRINCIPLES

1. SENSORY PROCESSING
   1. Sensory processing begins with the transformation of stimulus energy into graded potentials and then into action potentials in nerve fibers.
   2. Information carried in a sensory system may or may not lead to a conscious awareness of the stimulus.

2. RECEPTORS
   1. Receptors translate information from the external world and internal environment into graded potentials, which then generate action potentials.
      1. Receptors may be either specialized endings of afferent neurons or separate cells at the end of the neurons.
      2. Receptors respond best to one form of stimulus energy, but they may respond to other energy forms if the stimulus intensity is abnormally high.
      3. Regardless of how a specific receptor is stimulated, activation of that receptor always leads to perception of one sensation (the doctrine of specific nerve energies). Not all receptor activations lead, however, to conscious sensations.

   2. The transduction process in all sensory receptors involves the opening or closing of ion channels in the receptor. Ions then flow across the membrane, causing a receptor potential.
      1. Receptor potential magnitude and action potential frequency increase as stimulus strength increases.
      2. Receptor-potential magnitude varies with stimulus strength, rate of change of stimulus application, temporal summation of successive receptor potentials, and adaptation.

3. NEURAL PATHWAYS IN SENSORY SYSTEMS
   1. A single afferent neuron with all its receptor endings is a sensory unit.
      1. Afferent neurons, which usually have more than one receptor of the same type, are the first neurons in sensory pathways.
      2. The area of the body that, when stimulated, causes activity in a sensory unit or other neuron in the afferent pathway is called the receptive field for that neuron.

   2. Neurons in the specific ascending pathways convey information to specific primary receiving areas of the cerebral cortex about only a single type of stimulus.
   3. Nonspecific ascending pathways convey information from more than one type of sensory unit to the brainstem reticular formation and regions of the thalamus that are not part of the specific ascending pathways.

4. ASSOCIATION CORTEX AND PERCEPTUAL PROCESSING
   1. Information from the primary sensory cortical areas is elaborated after it is relayed to a cortical association area.
      1. The region of association cortex closest to the primary sensory cortical area processes the information in fairly simple ways and serves basic sensory-related functions.
      2. Regions of association cortex farther from the primary sensory areas process the sensory information in more complicated ways.
      3. Processing in the association cortex includes input from areas of the brain serving arousal, attention, memory, language, and emotions.

5. PRIMARY SENSORY CODING
   1. The type of stimulus perceived is determined primarily by the type of receptor activated. All receptors of a given sensory unit respond to the same stimulus modality.
   2. Stimulus intensity is coded by the rate of firing of individual sensory units and by the number of sensory units activated.
   3. Perception of the stimulus location depends on the size of the receptive field covered by a single sensory unit and on the overlap of nearby receptive fields.
   4. Lateral inhibition is a means by which ascending pathways emphasize wanted information and increase sensory acuity.
   5. Stimulus duration is coded by slowly adapting receptors.
   6. Information coming into the nervous system is subject to control by both ascending and descending pathways.

SECTION B: SPECIFIC SENSORY SYSTEMS

1. SOMATIC SENSATION
   1. Sensory function of the skin and underlying tissues is served by a variety of receptors sensitive to one (or a few) stimulus types.
   2. Information about somatic sensation enters both specific and nonspecific ascending pathways. The specific pathways cross to the opposite side of the brain.
   3. The somatic sensations include touch-pressure, the senses of posture and movement, temperature, and pain.
      1. Rapidly adapting mechanoreceptors of the skin give rise to sensations such as vibration, touch, and movement, whereas slowly adapting ones give rise to the sensation of pressure.
      2. Skin receptors having small receptive fields are involved in fine spatial discrimination, whereas receptors having larger receptive fields signal less spatially precise touch-pressure sensations.
      3. The major receptor type responsible for the senses of posture and kinesthesia is the muscle-spindle stretch receptor.
      4. Cold receptors are sensitive to decreasing temperature; warm receptors signal information about increasing temperature.
      5. Tissue damage stimulates specific receptors that give rise to the sensation of pain, which may also induce emotional and reflex responses.
      6. Stimulation-produced analgesia, transcutaneous nerve stimulation (TENS), and acupuncture control pain by blocking transmission in the pain pathways.

2. VISION
   1. Light is defined by its wavelength or frequency.
   2. The light that falls on the retina must be focused by the cornea and lens.
      1. Lens shape is changed in response to viewing near or distant objects (accommodation) so that both are focused on the retina.
      2. Presbyopia interferes with accommodation. Cataract decreases the amount of light reaching the retina.
      3. An eyeball too long or too short relative to the focusing power of the lens causes nearsighted or farsighted vision, respectively.

   3. The photopigments of the rods and cones are made up of a protein component (opsin) and a chromophore (retinal).
      1. The rods and each of the three cone types have different opsins, which make each of the four receptor types sensitive to a different wavelength of light.
      2. When light falls upon the chromophore, the photic energy causes the chromophore to change shape, which triggers a cascade of events leading to hyperpolarization of the photoreceptors and decreased neurotransmitter release from them. When exposed to darkness, the rods and cones are depolarized and release more neurotransmitter.

   4. The rods and cones synapse on bipolar cells, which synapse on ganglion cells.
      1. Ganglion cell axons form the optic nerves, which lead into the brain.
      2. The optic nerve fibers from the nasal half of each retina cross to the opposite side of the brain in the optic chiasm. The fibers from the optic nerves terminate in the lateral geniculate nuclei of the thalamus, which send fibers to visual cortex.
      3. Visual information is also relayed to areas of the brain dealing with biological rhythms.

   5. Coding in the visual system occurs along parallel pathways, in which different aspects of visual information, such as color, form, movement, and depth, are kept separate from each other.
   6. The colors we perceive are related to the wavelength of light. Different wavelengths excite one of the three cone photopigments most strongly.
      1. Certain ganglion cells are excited by input from one type of cone cell and inhibited by input from a different cone type.
      2. Our sensation of color depends on the output of the various opponent-color cells and the processing of this output by brain areas involved in color vision.

   7. Six skeletal muscles control eye movement to scan the visual field for objects of interest, keep the fixation point focused on the fovea centralis despite movements of the object or the head, prevent adaptation of the photoreceptors, and move the eyes during accommodation.

3. HEARING
   1. Sound energy is transmitted by movements of pressure waves.
      1. The sound wave frequency determines pitch.
      2. The sound wave amplitude determines loudness.

   2. The sound transmission sequence is as follows:
      1. Sound waves enter the external auditory canal and press against the tympanic membrane, causing it to vibrate.
      2. The vibrating membrane causes movement of the three small middle-ear bones, and the stapes vibrates against the oval-window membrane.
      3. Movements of this membrane set up pressure waves in the fluid-filled scala vestibuli, which cause vibrations in the cochlear duct wall, setting up pressure waves in the fluid there.
      4. These pressure waves cause vibrations in the basilar membrane, located on one side of the cochlear duct.
      5. As this membrane vibrates, the hair cells of the organ of Corti move in relation to the tectorial membrane.
      6. Movement of the hair cells' stereocilia stimulates them to release neurotransmitter, which activates receptors on the peripheral ends of the afferent nerve fibers.

   3. Each part of the basilar membrane vibrates maximally in response to one particular sound frequency.

4. VESTIBULAR SYSTEM
   1. A vestibular apparatus lies in the temporal bone on each side of the head and consists of three semicircular ducts, a utricle, and a saccule.
   2. The semicircular ducts detect angular acceleration during rotation of the head, which causes bending of the stereocilia on their hair cells.
   3. Otoliths in the gelatinous substance of the utricle and saccule move in response to changes in linear acceleration and the position of the head relative to gravity and stimulate the stereocilia on the hair cells.

5. CHEMICAL SENSES
   1. The receptors for taste lie in taste buds throughout the mouth, principally on the tongue. Different types of taste receptors operate by different mechanisms.
   2. Olfactory receptors, which are part of the afferent olfactory neurons, lie in the upper nasal cavity.
      1. Odorant molecules, once dissolved in the mucus that bathes the olfactory receptors, bind to specific receptors (protein binding sites). Each receptor cell has one of the 1,000 different receptor types.
      2. Olfactory pathways go to the limbic system.