Some Basic Sensory Principles

Whether of internal or external stimuli, sensory perception begins at the level of the cell. This raises two general questions: first, what kinds of signals do cells sense and what mechanisms are used? Second, what do they do with the signal? (Here, inevitably trapped by language, we use words like "information" or "signal" without implying an intentional sender or coder nor that incoming data are neatly packaged).

Some sensory mechanisms cause cells to change gene expression. Other sensory cells respond to stimuli by transmitting an impulse to another cell, as in neural perception. Much of our own sensory perception is based on central nervous system (CNS) processing of such signals, and there are several ways they are received. There are in mammals a few senses not transmitted to the CNS by neurons (e.g., light receptors in the skin), but as discussed in the introduction to this chapter, we should not be too parochial because many organisms sense their environment in wholly nonneuronal ways.

Receptor Mechanisms

Recognition of mechanical and chemical stimuli begins with the binding of a ligand to a cell surface receptor. The ligands are in the form of chemical or energy signals, and the receptors are transmembrane proteins that bind specific signals and initiate the transformation of the signal into a neural event (in organisms with an organized nervous system, at any rate). This transduction of signal into perception can take either a direct or an indirect path from the receptor cell to the CNS.

In vertebrates, the organization of the sensory cranial nerves is essentially the same for all senses. The receptor cells are either true neurons or neuronlike cells. Receptor cells that are true sensory neurons have either free sensory terminal endings or specialized sensory terminals. The axons from these cells terminate in the CNS. The senses of touch, pain, and stretch—which are perceived by the skin, muscle, and joints—generally involve this type of receptor, as does the sense of smell. Receptor cells that are not neurons are specialized cells with no axons. They relay receipt of their specific signal to the CNS by contact with neurons that transmit along their own axon. Hair cells of the vertebrate ear and lateral line of aquatic vertebrates as well as photoreceptors for light transduction are examples of this type of receptor. The basic cellular structures of different types of neurons are shown in Figure 12-1.

As with all neurons, when a sensory receptor is at rest—not activated—a resting potential is maintained across the cell membrane. That is, there is a difference in electrical potential across the membrane of the cell, which is generally negatively charged, and it is maintained by the sodium-potassium pump, which discharges more positive charge from the cell than it allows in. When the cell is activated by the specific stimulus to which it responds, this resting potential is disturbed and receptor potential results—a nerve impulse is generated. The electrochemical process that converts signals to receptor potentials is called sensory transduction, and generally begins with the activation of ion channels in the membrane of the receptor cell, controlling the passage of cations (positively charged ions such as sodium or potassium) into the cell.

When the flow, or flux, of ions into or out of the cell reaches sufficient magnitude in neuronal receptors, an action potential results in the axon. That is, the electric potential on the surface of the cell membrane changes, creating an electrochemical impulse that sweeps along the axon of the neuron to transmit the signal to terminals located in the brain or spinal cord. The terminals release their neuroactive chemical, which triggers sensation.

In a receptor cell that is not a true neuron the flow of receptor current is indirect, that is, it must be passed from the receptor to a neuron through a synapse. This often requires the intervention of a cascade of events, relayed by the second messengers, that leads to the opening of ion channels. The signal is then transmitted to a neuron to be carried along the axon to the nervous system. Photoreceptors and chemoreceptors are examples of this type of receptor. The use of secondary messengers produces a slower response than in mechanosensory transduction. Perception of sound is almost instantaneous, for example, whereas there is a delay between the stimulus and response in photo- or chemoreception.

Mechanoreceptors

Mechanosensory transduction is the process by which mechanical forces are converted into electrical signals. In this case, the signal is not molecular in nature, but must be translated by genetically based response mechanisms. Mechanoreceptor cells are the basis of a diversity of senses in multicelled organisms, including pro-prioception (the body's ability to orient itself in space and sense the movement of its own parts), balance, hearing, touch, which involves the detection of pressure on the surface of the body, the detection of stretching or twisting such as in joints or muscles or the digestive system, or the displacement of feathers or hairs or whiskers. Although they differ widely in the range of sensations they govern and the receptor that initiates the detection, the basis of each of these senses is a response to mechanical deformation—stretching, bending, displacement by air, and the like.

Monopolar

Bipolar

Multipolar

Monopolar

Bipolar

Multipolar

Figure 12-1. Basic cellular structure of different types of neurons. The signal travels to the cell body from the dendrites and away from the cell body, or soma, along the axon. The nucleus plays no neurological role, but functions in growth and metabolism of the cell. Multipolar neurons are the most common type in vertebrates; bipolar neurons are much less common and are found as olfactory receptors, receptors in the retina of the eye, and several cranial nerves, e.g. unipolar neurons are found in the spinal ganglia of the spinal nerves and some of the cranial nerve nuclei, where they transmit signal to the CNS.

(*) ) Second bipolar neuron

Figure 12-1. Basic cellular structure of different types of neurons. The signal travels to the cell body from the dendrites and away from the cell body, or soma, along the axon. The nucleus plays no neurological role, but functions in growth and metabolism of the cell. Multipolar neurons are the most common type in vertebrates; bipolar neurons are much less common and are found as olfactory receptors, receptors in the retina of the eye, and several cranial nerves, e.g. unipolar neurons are found in the spinal ganglia of the spinal nerves and some of the cranial nerve nuclei, where they transmit signal to the CNS.

They all depend on a mechanically gated ion channel for transduction of the mechanical force into the sensation that is ultimately perceived.

The molecular basis of the mechanical gating of ion channels is not yet well understood (Walker et al. 2000). Homologies found in this class of transduction mechanism, such as the fact that they send their message nearly instantaneously, never requiring the action of a second messenger such as a G protein cascade, suggest to some a common evolutionary origin and/or a single molecular mechanism underlying these diverse responses.

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