From stimulation to nerve signals

You lift the cup from the table. Light reflects from the porcelain, a faint scrape is heard, and the handle presses against your fingers. These are different kinds of physical influence. Yet the nervous system needs to use them together: which cup is it, where is the handle, and how firmly should you hold it?

Sensory receptors connect these events to electrical activity in the nervous system. Light can alter a light-sensitive pigment, a vibration can bend a hair cell's projections, and pressure can deform a sensitive nerve ending. When this influence changes the cell's electrical state, transduction occurs. The change is the beginning of sensory processing; it is not yet the experience of a cup.

A change across the cell membrane

Cells maintain differences in ion concentration and electrical voltage between their inside and outside. When ion channels open or close, ion flow across the membrane changes, and so does the membrane potential. A receptor potential is graded: its size and time course can vary with stimulation.

Distinguish this local change from an action potential. Action potentials are brief impulses that propagate along nerve fibres. In a given fibre, variations in stimulation are encoded mainly through when impulses occur and how often they occur, together with which neurons are active. A stronger influence therefore does not mean that every impulse has a larger amplitude.

Terms: receptor potential · action potential.

Two routes to an electrical change

When light is absorbed in a rod or cone, it starts a chemical cascade that closes certain ion channels. The photoreceptor hyperpolarises: its inside becomes more negative, and glutamate release decreases. In darkness these channels are more open and glutamate release is greater. Rod and cone signals are graded; they do not send action potentials along the optic nerve.

Retinal networks then process the change. ON and OFF pathways respond differently to increases and decreases in light. Ganglion cells carry the retina's output as action potentials. More light therefore does not make every cell more active. Some signals increase, others decrease, and the pattern of differences makes the information useful.

In the cochlea, the mechanical movements caused by sound instead bend the hair cells' stereocilia. When an inner hair cell's bundle bends in the excitatory direction, more mechanically sensitive channels open. The cell depolarises and releases more glutamate onto auditory nerve fibres, where action potentials arise. Bending in the opposite direction has the opposite effect. The same underlying principle is at work as in vision, with a different receptor and a different conversion mechanism.

Explore the seven retinal cell types → · Start with the eye →

The visual lab includes a return link to the vision section.

Several kinds of processing at once

As you look at the cup, your visual system works simultaneously with contrast, colour, shape, motion and position. Hearing simultaneously processes a sound's frequency content, timing and cues to its location. Parallel processing means that different parts of a problem are handled at the same time in interacting neural circuits. Some computations depend on others, but the whole system does not wait for a single stage to finish.

This is a recurring organising principle across human sensory systems: multiple processes cooperate, and information influences activity in several directions. The detailed architecture differs between senses. Smell's early route to cortex, for example, does not pass through an initial thalamic relay in the same way as the main visual pathway.

Parallel processing · Sources on sensory-system organisation

What you are trying to do changes processing

Imagine first looking for the red cup, then judging whether a cup is about to tip over. The scene can stay the same while the task makes different features relevant. Attention to colour or motion can change activity in visual areas that process those features. Intention can thus influence how information is prioritised along parallel streams, while sensory evidence constrains what we can perceive.

Descending influence from higher to earlier processing levels is often called top-down. It can help adjust sensitivity and selection to the task. Feedback operates alongside ascending processing; it need not wait for a finished picture of the world. Nor is every feedback signal a conscious decision or a prediction.

Feedback reaches different levels

In vision, extensive feedback runs from visual cortex to the lateral geniculate nucleus, the LGN, in the thalamus. Even there, the onward transmission of retinal signals can be influenced. Studies in cats show that cortical feedback synaptic contacts can outnumber retinal contacts onto LGN relay cells. This compares synapses, not the number of impulses or the amount of information. Retinal contacts are fewer but provide a strong driving input; many other contacts alter how signals are processed. Not all nonretinal contacts are cortical feedback: brainstem inputs and inhibitory circuits contribute too.

The LGN is therefore an important level in the cortical feedback loop followed here. Saying that all influence from the brain stops there would be too categorical: a small number of fibres from the brain to the retina have also been demonstrated in macaques, among other animals. These findings do not show that conscious intention directly controls human photoreceptors.

In hearing, descending pathways pass through the brainstem and reach the cochlea itself. Medial olivocochlear fibres contact outer hair cells and can alter the cochlea's mechanical amplification. Lateral olivocochlear fibres influence auditory nerve endings near inner hair cells. These connections make it possible to influence processing within the sensory organ; there is no direct nerve fibre from auditory cortex to a hair cell.

This anatomy and experiments in animals show that cortex can influence peripheral auditory responses. How much voluntary attention changes cochlear responses in humans is a more specific research question. When we listen for one voice among several, selection therefore takes place at several brain levels as well as potentially involving the ear.

Perceiving something involves registering changes, processing several kinds of information at once, and continuously adapting processing to what we are doing. Receptors anchor the process in the physical world. Interacting networks make that information useful for recognition, orientation and action.