Somatosensory system



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SOMATOSENSORY SYSTEM

The senses collect information about the modality, intensity, duration, location of events in the world. The initial contact with the external world occurs through specialized neural structures: sensory receptors. Each receptor is sensitive to a form of physical energy, e.g. mechanical, chemical, electromagnetic energy. The receptors transform energy into electrical energy (common language for all sensory systems; a series of action potentials; neural encoding) which is called stimulus transduction.



Sensory Receptors Convert Different Forms of Energy into Electrical Signals
1- We have sensors in our bodies that can detect heat, pressure, stretch, acceleration, sound, light, smells, tastes and other forms of energy

2- Sensory receptors act as transducers, converting many forms of energy into action potentials that the brain can interpret

3- Many receptors, such as those in the skin, are simply constructed; others, such as the eye and ear, are very elaborate

The Type of Sensation Perceived Depends Upon Connection in Brain
1- Almost all sensory input passes through the thalamus but sensory action potentials in the thalamus look alike-it is impossible to tell the type of sensation by looking at the action potential

2- The type of sensation felt depends upon which part of the cortex the sensory nerve goes to

3- Presumably, if you were able to cut the auditory and optic nerves and reattach them to the wrong stumps, you should be able to hear the lightning and see the thunder

A Stronger Stimulus Gives More Action Potentials per Minute (Frequency Coding)
1- Stimuli cause sensory receptors to depolarize and produce a voltage called a generator potential- this is not an action potential, it is not transmitted

2- The generator potential causes the sensory nerve to produce a series of action potentials that are sent to the brain

3- When a stimulus gets stronger you do not get bigger action potentials- that would violate the all-or-none principle

4- Instead you get more action potentials per minute

5- The brain knows that a higher frequency of action potentials (more per minute) means a stronger stimulus




If a Stimulus is Continued Sensory Receptors Adapt & Become Less Sensitive
1- If a stimulus is maintained at a constant intensity for a long time the nerve seems to lose interest in it- the nerve has adapted and become less sensitive

2- This allows us to tune out background noise, to ignore the touch sensation from our clothing , to lose awareness of the temperature of the room, etc..

3- Some nerves, such as those for pressure and touch, are fast-adapting; others, such as those for muscle stretch and some types of pain, are slow-adapting- the sensation lasts a long time

4- Example: temperature receptors

1- Two types: warm & cold receptors

2- If one hand is placed in warm water and the other is placed in cold water, the temperature receptors will adapt and become less sensitive



3- After adaptation, if both hands are placed in lukewarm water, the hand originally in warm water will feel cold, and the hand originally in cold water will feel warm




Skin Sensations are Usually Perceived at the Location of the Receptor
1- Somatic (body) senses are perceived to be coming from the location of the sensory receptor

2- Sometimes the body is fooled: phantom limb pain- person feels a limb which is no longer present

3- Skin sensations are quite complicated:

1- Merkel cells and Ruffini endings respond to steady pressure

2- Pacinian corpuscles and Meissner's corpuscles give the sense of vibration

3- There are separate warm and cold receptors

4- Receptors associated with skin hairs allow you to feel the displacement of hairs

5- Several types of pain receptors respond to mechanical trauma or very high or low temperatures



4- Uneven distribution of receptors: close together on finger tips & face; far apart on back, legs, arms, belly

RECEPTORS
RECEPTORS cover the surface of the body, and different dimensions of the world leave traces on them. Receptors convert energy into electrical activity.



For different kinds of sensations, different kinds of receptor cells. Rod and cone cells of the eye's retina are specialized to respond to the electromagnetic radiation of light. The ear's receptor neurons are topped by hair bundles that move in response to the vibrations of sound. Olfactory neurons at the back of the nose respond to odorant chemicals that bind to them. Taste receptor cells on the tongue and back of the mouth respond to chemical substances that bind to them. Meissner's corpuscles are specialized for rapid response to touch, while free nerve endings bring sensations of pain.




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