Properties of Receptors in Sensory Physiology

Properties of Receptors

• Muller's law states that each receptor responds to a specific stimulus, known as the adequate stimulus for that receptor.
• Receptors exhibit excitability, which is the ability to respond to stimuli, producing a receptor or generator potential that is proportionate to the intensity of the stimulus.
• Adaptation is a decrease in the frequency of discharge of action potentials in the afferent nerve under constant continuous stimulation of its receptors.

Somatic Sensations

• Somatic sensations are feelings produced by changes in the environment or by the application of a stimulus to the receptors.
• Somatic sensations are classified into two categories: special senses and general sensations, which include somatic sensations, organic sensations, and emotional sensations.

Somatic Sensory Pathways

• Each sensory pathway is composed of three order neurons.
• The first-order neuron is located in the posterior root ganglia outside the CNS.
• The second-order neuron differs for each group of sensations, and its axons cross to the opposite side.
• The third-order neuron is located in the ventrobasal nuclei of the thalamus.

The Sensory System

• The sensory system consists of sensory receptors, neural pathways, and parts of the brain that process information.
• Sensory receptors are specialized structures that receive stimuli from the external or internal environment and convert them into action potentials.
• Receptors detect, transduce, and generate action potentials in afferent fibers.

Types of Receptors

• Mechanoreceptors detect mechanical stimuli such as touch, pressure, stretching, and sound waves.
• Thermoreceptors detect changes in temperature.
• Pain receptors (nociceptors) detect pain.
• Electromagnetic receptors (photoreceptors) detect light.
• Chemoreceptors detect chemical changes in the body.

Somatic Sensory Pathways

• The dorsal column pathway carries fine sensations such as touch, pressure, vibration, and stereognosis.
• The anterolateral system pathway carries crude sensations such as pain, temperature, crude touch, and pressure.

The Dorsal Column and Spinothalamic Pathways

• The dorsal column pathway involves first-order neurons in dorsal root ganglia, second-order neurons in gracile and cuneate nuclei, and third-order neurons in ventrobasal nuclei of the thalamus.
• The spinothalamic pathway involves first-order neurons in dorsal root ganglia, second-order neurons in ipsilateral posterior horn cells of the spinal cord, and third-order neurons in ventrobasal nuclei of the thalamus.

Somatic Sensations from the Head

• Somatic sensations from the face are transmitted by the trigeminal nerve, which has three branches: ophthalmic, maxillary, and mandibular.
• The ophthalmic branch transmits sensations from the upper part of the face and forehead.
• The maxillary branch transmits sensations from the middle part of the face, the hard palate, and upper teeth and gums.