Question

Describe three features of sensory processing that occur at the receptor level where a sensation is first detected. How is information about the stimulus coded before it is passed into the CNS?

Answer 1

There are five special senses smell, taste, balance, vision, and hearing. In addition, we possess somatosensation, (stimuli of temperature, pain, pressure, and vibration). Vestibular sensation, (spatial orientation and balance) proprioception (positioning of bones, joints, muscles), and the limb position. The stimulus is converted to electrical signal called transduction.When a sensory receptor is activated the afferent neuron, carries information about the stimulus to the CNS.

In the second type of transduction, sensory nerve ending responds to a stimulus in the internal or external environment, and this neuron constitutes the sensory receptor. Free nerve endings of pain receptors in gums and teeth can be stimulated by temperature changes, chemical stimulation.

The receptors that respond to stimuli and transmit data about them to the brain are sensory. These receptors detect touch, pressure, vibration, temperature, and pain.In response to stimuli the sensory receptor initiates transduction by creating action potentials in the same cell or an adjacent one.By the release of neurotransmitters an electrical signal, known as an action potential (electrical signal), is converted into a chemical and movement of electrical stimuli occurs.

Receptor -biochemistry

A receptor is a

1. · protein on the cell membrane or within the cytoplasm
2. · cell nucleus that binds to a specific molecule (a ligand) such as a neurotransmitter,
3. · hormone, or
4. · other substance,

and initiates the cellular response to the ligand.

Ligand-induced changes in the behavior of receptor proteins result in physiological changes that constitute the biological actions of the ligands

Binding and activation

Ligand binding to a receptor is an equilibrium process:

Ligands bind to an empty receptor and they dissociate from it .(according to the law of mass action):

[Ligand] ■ [Receptor] ?=* [Ligand — receptor complex] (the brackets stand for concentrations)

Ligand fits into a given receptor and the binding affinity is measured as the dissociation constant Kd (good fit means high affinity and a low dissociation constant). The activation of the second messenger cascade and the final biological response is achieved only when at time when significant number of receptors are activated ligands that are bound.

There are agonists ,antagonists, partial agonists, inverse agonists

Stimulus Encoding

All Stimuli are finally converted into action potentials in the primary afferent neurons

Sensory receptors encode four types of information

1. Stimulus modality

2. Stimulus location

3. Stimulus intensity

4. Stimulus duration

The tip of the central fiber inside the capsule is unmyelinated, but the fiber does become myelinated shortly before. All sensory receptors have one feature in common. Whatever the type of stimulus that excites the receptor, its immediate effect is to change the membrane electrical potential of the receptor. This change in potential is called a receptor potential leaving the corpuscle to enter a peripheral sensory nerve by compression of the corpuscle, ion channels opened in the membrane, allows positively charged sodium ions to diffuse to the interior of the fiber. This positive charge is increases inside the fibre, which is the "receptor potential. The receptor potential in turn induces a current flow, that spreads along the nerve fiber.

The local current flow depolarizes the fiber membrane at the node Ranvier, R which then sets off typical action potentials that are transmitted along the nerve fiber toward the central nervous system. When a sensory stimulus is applied continuously, a high impulse rate and receptor response is seen which decreases progressively at slower rate until finally the rate of actionpotential reaches to very few or none at all