Somatosensory Systems

(Transcribed from Dr. Glasser’s lecture, 9 June 2000 by Brian Buschman)

Receptors are specific transducers to convert one type of energy to another. The receptors in the body convert stimuli into nerve signals. The receptor potential is the threshold of stimulation needed to initiate the signal. The greater the stimuli the greater the firing frequency of the nerve.

A receptor field is the area that when stimulated will cause a change in activity of the neuron.

To have perception of the stimuli we must have relays to convey the signal to the cerebral cortex since that is where you actually feel things.

As you ascend in sensory systems a receptor field tends to get larger and larger. That is, at each level when sensory neurons synapse, multiple neurons synapse on a single neuron of the next level.

Sensory Relays

There are two types of sensory relay systems:

1) Labeled line relays take fibers and receptors for a specific sense to the cortex. You will have one for temperature, one for pain and so on.

2) Nonspecific relays take general sensory stuff to the cortex. This is information that plays a role such as when your alarm clock wakes you up in the morning. When the sense is initially carried to your brain it’s taken as a stimulation to wake up regardless of if it’s from your alarm clock, TV, the garbage man or your evil roommate.

Somatosensory Systems

Somatosensory stimulation includes:

1) Proprioception

2) Exteroception, which is sensory information, that comes from outside the body (touch).

3) Interoception, which is sensory information that originates inside the body (gastric distention).

Nociceptors are receptors for pain.

When you are stimulated by the prick of a cold pin you will stimulate three sets of receptors:

1) Touch receptors

2) Cold receptors

3) Pain receptors

This is called feature extraction. It is when you have a number of senses stimulated at the same time the components of the stimulation are broken down and sensed by receptors for the respective stimuli. The different stimuli are transmitted to the brain separately and then assembled into the given feeling in the cerebral cortex. It is the same with words.

Rapid/Slow Adaptation

When you are stimulated the receptors will eventually become accustomed to the stimuli and quit firing. When you put on your clothes you feel them at first but then your receptors adapt and stop sensing the clothes you are wearing.

Rapid adaptation is when the receptors adapt quickly. This is the case for Pacinian and Meissner’s corpuscles.
Slow adaptation in just that. It exists in Ruffaini’s and Merkel’s corpuscles.

Mechanoreceprots

There are four main types of mechanoreceprots:

1) Pacinian for vibratory sense

2) Meissner’s for fine touch

3) Merkel’s for fine touch

4) Raffini’s for genera touch

Physical stimuli also come from free nerve endings for diffuse touch.

One specific type of specialty receptor is the temperature receptor. They have separate receptors for hot and cold. Thermoceprots only work within a specific range. If the temperature is above or below that range then it will be felt as pain by the nociceptors.

Perception of pain is divided into two categories. Fast pain is quickly transmitted to the brain and usually consists of sharp pain. Dull pain is slow pain.

There are two methods for classifying sensory fibers. In both of the systems (below) the top of the chart are the fastest and the bottom the slowest. In both cases the slowest are the unmyelinated neurons (C-fibers and type IV neurons). These are just two different systems of classifying fibers. Any given sensory fiber could be classified into either system.

A alpha	I
A beta	II
A gamma	III
A delta	IV
C fibers

Note that this table cannot be used to convert from one system to another. An A delta fiber is not a type IV fiber. I just used this table to list them.

Somatosensory Tracts

There are two main classes of somatosensory tracts:

Somatosensory Systems

Sensory Relays

Somatosensory Systems

Rapid/Slow Adaptation

Mechanoreceprots

Somatosensory Tracts

Epicritic Relays

Epicritic Pathway (Dorsal Column)