Lab 9: Nervous system II – Special Senses
Special senses
Learning Objectives
When you are prepared for the Test on Week 9 Learning Objectives in Week 10, you will be able to:
- Identify structures of the eye and their functions.
- Identify layers of the retina.
- Identify the extrinsic eye muscles, their actions, and innervations.
- Identify structures of the ear and their functions.
- Identify structures of the cochlea.
Senses
A major role of sensory receptors is to help us learn about the environment around us, or about the state of our internal environment. Stimuli from varying sources, and of different types, are received and changed into the electrochemical signals of the nervous system. This occurs when a stimulus changes the cell membrane potential of a sensory neuron. The stimulus causes the sensory cell to produce an action potential that is relayed into the central nervous system (CNS), where it is integrated with other sensory information—or sometimes higher cognitive functions—to become a conscious perception of that stimulus. The central integration may then lead to a motor response.
Describing sensory function with the term sensation or perception is a deliberate distinction. Sensation is the activation of sensory receptor cells at the level of the stimulus. Perception is the central processing of sensory stimuli into a meaningful pattern. Perception is dependent on sensation, but not all sensations are perceived. Receptors are the cells or structures that detect sensations.
Sensory Receptors
Stimuli in the environment activate specialized receptor cells in the peripheral nervous system. Different types of stimuli are sensed by different types of receptor cells. One way to categorize sensory receptors is through a functional classification: which type of stimulus the receptor can detect and how it transduces the stimulus into a nervous signal.
Chemoreceptors can detect ions and macromolecules (chemicals), which leads to an interpretation of taste or smell. Nociceptors detect chemicals from tissue damage or other intense stimuli, which leads to an interpretation of pain. Physical stimuli, such as pressure and vibration, as well as the sensation of sound and body position (balance), are interpreted through a mechanoreceptor. Another physical stimulus that has its own type of receptor is temperature, which is sensed through a thermoreceptor that is either sensitive to temperatures above (heat) or below (cold) normal body temperature. Other stimuli include the electromagnetic radiation from visible light and are detected by photoreceptors. For humans, the only electromagnetic energy that is perceived by our eyes is visible light. Some other organisms have receptors that humans lack, such as the heat sensors of snakes, the ultraviolet light sensors of bees, or magnetic receptors in migratory birds.
Gustation (taste)
Gustation is the special sense associated with the tongue. The surface of the tongue, along with the rest of the oral cavit