1. Ch 9 Sensory System
In order to maintain homeostasis (ie stable internal environment), it is necessary to detect changes in the external environment and react appropriately. Several sensory systems exist that detect external changes rapidly. These systems include: the somatosensory (touch, pressure, pain etc) system, visual system, auditory and vestibular system, olfactory (smell) system, and gustatory (taste) system. A major objective of this section is to look at how events in the outside environment are detected, converted to action potentials, travel to the brain, and become consciously perceived.
As we discussed in module number 1, it is essential for the body to maintain homeostasis—the maintenance of relatively stable conditions in the internal environment. In order to do this it is necessary for the body to detect changes in the external environment so it can react appropriately in order to maintain its internal environment. The human body has several sensory systems that allow it to detect these external changes rapidly. These systems include: the somatosensory (touch) system, the visual system, the auditory and vestibular system, the olfactory (smell) system, and the gustatory (taste) system. In this section, we will look at how events in the outside world are detected, converted to action potentials, travel to the brain, and become consciously perceived. In doing so we will examine most of the sensory system of the body (with the exception of smell and taste), their structure, function, and pathways to the brain.

2. Sensations or senses are categorized into 2 types
1. General senses = receptors widely scattered throughout body and detect
touch
pressure
temperature
pain
vibration
itch
proprioception (position of limbs/muscle sense

3. Sensations or senses
2. Special senses = receptors are housed in specialized organs
ear = hearing & equilibrium
eye = vision
tongue = taste
nose = olfaction (smell)

4. Sensory Receptors detect internal & external changes (stimuli)
they are specialized cells or sensory neurons (dendrites or dendritic processes)

5. Sensory information from receptors are relayed to the CNS
CNS interprets information & conveys a sensation or sense back to original area
neurons

6. Types of Sensory Receptors
Receptor types can be associated with both the general and special senses
Pain receptors (nociceptors) = detect tissue damage
Mechanoreceptors = detect changes in or distortions of cell membranes, respond to mechanical stimuli such as stretching or bending of receptors, or movement
Touch
Pressure
Hearing
Equilibrium

7. General Senses: Touch, pressure receptors
Examples
proprioceptors = tendons & muscles (muscle spindle)
baroreceptors = stretch receptors

8. Touch, pressure quickly adapts Examples
mechanoreceptors
quickly adapts
Examples
fine touch & pressure = Merkels & Meissner’s corpuscles
deep pressure = pacinian
corpuscles

9. Other Types of Sensory Receptors
Thermoreceptors = detect changes in temperature (hot & cold)
Photoreceptors = changes in light (vision)
Chemoreceptors = changes in chemical composition (taste & smell)

10. Chemoreceptors respond to chemicals  odor molecules
in blood vessels detect blood pH, O2 and CO2

11. Receptor Characteristics
Sensory adaptation = ability of a receptor to adapt (reduce its sensitivity) to a constant stimulus
What type of receptors adapt?

12. Olfaction and touch
adapt quickly to a
constant stimuli

13. Pain common in skin, joints, bones, blood vessels, eye(s)
few in deep tissue, viscera (in fact very dull)
none in the brain
doesn’t adapt or does so slowly!

14. Pain
referred pain = the perception of pain coming from parts of body not actually stimulated
common with viscera pain receptors = often dull
Ex. Heart, Gallbladder, or Bladder

15. Temp. located in: cold receptors 3-4x more numerous quickly adapts
skin, eye(s), muscles, liver, hypothalamus
cold receptors 3-4x more numerous
quickly adapts

16. Transduction Transduction (2 viewpoints)
Takes external stimuli and converts it into an electrical signals (done in the lab)
Takes environmental information (environmental stimuli) from the periphery and turns it into the language the brain understands  Action Potentials
Transduction of Environmental Information
Transduction of environmental information is how information from the external environment is turned into language the brain understands—action potentials. In order for the brain to know what is happening outside the body, environmental stimuli (energy) like light, heat, touch, or sound must first be detected by sensory receptors which then convert the information into action potentials. Let's begin by examining how stimuli in the outside world are converted to action potentials—the language of the nervous system

17. Environmental Stimuli
In order for the brain to consciously perceive an environmental stimulus, that stimulus must be detected by a sensory receptor. Environmental stimuli come in different forms and, therefore, will require different receptors to detect the stimulus and then convert it to action potentials. For example, a mechanical stimulus, like touching or vibrating the skin, will stretch sensory receptors in the skin and open ion channels, causing a depolarization of the sensory neuron producing an action potential. A chemical stimulus, like a sour taste on the tongue or an odor in the nose, binds with a receptor, causing a depolarization and then an action potential. Light energy is absorbed by photoreceptors of the eye (rods and cones in the retina) and eventually produces action potentials. Gravity and motion can also be detected by hair cells in the vestibular system, which convert this form of external stimulus to action potentials.

18. Primary Somatosensory Cortex
Once sensory information reaches the brain through specific nerve tracts, it travels to the primary somatosensory cortex, which is located in the parietal lobe on the postcentral gyrus behind the central sulcus.
Primary Somatosensory Cortex
Once the sensory information has reached the brain, it travels to the primary somatosensory cortex, which is located in the parietal lobe on the postcentral gyrus behind the central sulcus.

19. Homunculus Somatosensory
cortex is arranged in a topographical manner on the postcentral gyrus. This area of the brain is called the somatosensory homunculus. It receives sensory information from each part of the body
Primary Somatosensory Cortex—The Somatosensory Homunculus (cont.) The somatosensory area in the brain is arranged in a topographical manner on the postcentral gyrus. This area of the brain, called the somatosensory homunculus, receives sensory information from each part of the body
You should notice that the "picture" of the human body represented in the homunculus is somewhat out of scale. Some of the representative areas are out of proportion (much larger than they should be). This is because some areas on the cortex, like the areas dealing with the hand, tongue, and lips, receive more sensory information and require more of the brain to process that information. The hands, tongue, and lips are the most sensitive parts of the body; they contain many more sensory receptors than any other part. After all, when you really want to experience how something feels, you use your hands (although babies prefer to use their mouths