Always in motion is the future.

Week 7: Neural

Sensory Receptors and Neuronal Circuits

Sensory Receptors

Five basic types of sensory receptors:

Mechanoreceptors: detect mechanical deformation of receptor or adjacent tissues.
Thermoreceptors: detect temperature changes. Some detect cold, some warmth.
Nociceptors (pain receptors): detect physical or chemical tissue damage.
Electromagnetic Receptors: detect light on the retina of the eye.
Chemoreceptors: detect taste in the mouth, smell in the nose, gas levels in blood, body fluid osmolality, etc.

Each receptor type is highly sensitive to its target stimulus, and almost nonresponsive to other types of stimuli. The stimulus changes the membrane potential of its receptor, and when the receptor potential rises above its threshold for eliciting action potentials in the nerve fiber attached to the receptor, action potentials begin to appear.

All sensory receptors adapt partially or completely to their stimuli in time. So...when you rest your chin in your hand, receptors in the hand initially respond to the pressure, but will discontinue in a short time even though compression continues. Rapidly adapting receptors don't transmit a continuous signal, but rather react strongly during change. Hence, these are known as rate or movement receptors. Rate receptors help the individual predict body status.

Intensity of a signal can be transmitted by:

Spatial Summation: increasing signal strength is transmitted by using progressively greater numbers of parallel fibers whose nerve endings overlap to cover a field as large as 5 cm in diameter.
Temporal Summation: increasing signal strength is transmitted by increasing the frequency of nerve impulses in each nerve fiber.

Neuronal Pools

The CNS is composed of thousands of separate neuronal pools, groups of interconnected neurons with their own organizational characteristics signal processes, allowing the total consortium of pools to achieve the multitude of functions of the nervous system. Different neurons in the pool can be either excitatory or inhibitory. Input fibers in a pool branch hundreds to thousands of times into terminal fibrils which synapse with dendrites and cell bodies of neurons in the pool, the majority of terminals lying on the nearest neurons. When an input fiber provides enough terminals to cause an action potential in the postsynaptic neuron, it provides an excitatory stimulus. If the fiber doesn't contribute enough to cause excitation by itself, it contributes to inputs from other fibers, and is said to facilitate the postsynaptic neuron.

Signals passing through neuronal pools can diverge by spreading an input signal to increasing numbers of neurons as it passes through successive orders of neurons in the path. Signals from multiple inputs can also converge to excite a single neuron, allowing summation of weak signals to achieve a positive output response.

Reciprocal Inhibition Circuit: an incoming signal may cause an output excitatory signal in one direction and an inhibitory signal in another at the same time. Ex: in the process of picking up your book, one set of spinal cord neurons transmit an excitatory signal to the front of your arm to pick up the book, while a separate set of neurons inhibits the muscles on the back of the arms to they wont oppose the movement.