Chapter 10
1. Sensory stimuli are divided into the special senses of vision, hearing, taste, smell, and equilibrium, and the SOMATIC SENSES of touch, temperature, pain, itch, and proprioception. 2. Sensory pathways begin with a stimulus that is converted by a receptor into an electrical potential 3. If the stimulus is above threshold, action potentials pass along a sensory neuron to the central nervous system. We become aware of some stimuli but are never conscious of others 4. Sensory receptors vary from free nerve endings to encapsulated nerve endings to specialized receptor cells. -TABLE 10.1

SOMATIC STIMULI: muscle length and tension, proprioception
VISCERAL STIMULI: Blood pressure, distension of gastrointestinal tract, blood glucose concentration, internal body temperature, osmolarity of body fluids, lung inflation, pH of cerebrospinal fluid, pH and oxygen content of blood FIGURE 10.1

a. Simple receptors are neurons with free nerve endings. They may have myelinated of unmyelinated or unmyelinated axons b. Complex neural receptors have nerve endings enclosed in connective tissue capsules. This illustration shows a Pacinian corpuscle, which senses touch c. Most special senses receptors are cells that release neurotransmitter onto sensory neurons. Initiating an action potential. The cell illustrated is a hair cell, found in the ear. 5. There are four types of sensory receptor, based on the stimulus to which they are most sensitive: CHEMORECEPTORS, MECHANORECEPTORS, THERMORECEPTORS, and PHOTORECEPTORS. TABLE 10.2 examples of stimuli

a. Chemoreceptors: oxygen, pH, various organic molecules such as glucose b. Mechanoreceptors: Pressure (baroreceptors), cell stretch (osmoreceptors), vibration acceleration, sound c. Photoreceptors: photons of light

d. Thermo receptors: varying degrees of heat
6. Each receptor type has an ADEQUATE SSTIMULUS, a particular form of energy to which it is most responsive 7. A stimulus that is above THRESHOLD creates a graded potential in the receptor 8. Multiple sensory neurons may converge on one secondary neuron and create a single large RECEPTIVE FIELD. Figure 10.2

a. Convergence creates large receptive fields
b. Small receptive fields are found in more sensitive areas
9. Sensory information from the spinal cord projects to the thalamus, then on to the sensory areas of the cerebral cortex. Olfactory information does not pass through the thalamus. FIGURE 10.3 Sensory pathways in the brain (most pathways pass through the thalamus on their way to the cerebral cortex) 1. Olfactory pathways from the nose project through the olfactory bulb to the olfactory cortex 2. Most sensory pathways project to the thalamus. The thalamus modifies and relays information to cortical centers. 3. Equilibrium pathways project primary to the cerebellum

10. The central nervous system is able to modify our level of awareness of sensory input. The PERCEPTUAL THRESHOLD is the level of stimulus intensity necessary for us to be aware of a particular sensation. 11. The modality of a signal and its location are indicated by which sensory neurons are activated. The association of a receptor with a specific sensation is called LABELED LINE CODING 12. Location of auditory information depends on the timing of receptor activation in each ear. 13. LATERAL INHIBITION enhances the contrast between the central of the receptive field and the edges of the field. In POPULATION CODING, the brain uses input from multiple receptors to calculate location and timing of a stimulus. 14. Stimulus intensity is coded by the number of receptors activated and by the frequency of their action potentials FIGURE 10.6

1. Receptor potential strength and duration vary with the stimulus > receptor potential is integrated at the trigger zone >Frequency of action potentials is proportional to stimulus intensity. Duration of a series of action potentials is proportional to stimulus duration > Neurotransmitter release varies with the pattern of action potentials arriving at the axon terminal. 15. For TONIC RECEPTORS, the sensory neuron fires action potential as long as the RECEPTOR POTENTIAL is above threshold. PHASIC RECEPTORS respond to a change in stimulus intensity but adapt if the strength of the stimulus remains constant FIGURE 10.7

a. Tonic receptors are slowly adapting receptors that respond for the duration of a stimulus b. Phasic receptors rapidly adapt to a constant stimulus and turn off 16. There are four somatosensory modalities: touch, proprioception, temperature, and nociception 17. SECONDARY SENSORY NEURONS cross the midline so that one side of the brain processes information from the opposite side of the body. Ascending sensory tracts terminate in the SOMATOSENSORY CORTEX. FIGURE 10.8

a. Fine touch proprioception vibration: primary sensory neuron synapses in the medulla, secondary sensory neuron crosses midline of body in medulla b. Primary sensory neuron synapses in dorsal horn of spinal cord ,secondary neuron crosses midline of body in spinal cord BOTH : Synapse with tertiary sensory neuron in the thalamus, tertiary sensory neuron terminates in somatosensory cortex. 18. Touch receptors come in many varieties. Temperature receptors sense heat and cold. 19. NOCICEPTORS are free nerve end...

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