Introduction To Sensory Systems
The brain obtains its information about the external and internal environment and about the body’s relation to the external environment by sensory experience emanating from sensory receptors (sense organs). There are a number of common steps in sensory reception: (i) a physical stimulus (i.e. touch, pressure, heat, cold, light, etc.); (ii) a transduction process (i.e. the translation of the stimulus into a code of action potentials); and (iii) a response (i.e. taking a mental note or triggering a motor reaction).
The specialized nerve ending (sensory receptor), afferent axon and its cell body, together with the central synaptic connections in the spinal cord or brain stem, are known as primary afferents.
Information is transmitted to the brain in the form of action potentials. These action potentials carry this information in the form of frequency-coded signals and can signal the following information:
1. The modality (specificity) of the system. Such modalities include the ‘five special senses’: sight, hearing, balance, taste and smell. However, it is easy to list others. The skin itself not only senses pressure and touch, but also cold and warmth, vibration and pain (somatosensation). In addition, the body senses both the external environment and the internal environment (its own state). Examples are the sense of equilibrium (balance) and a knowledge of the relative positions of the limbs (proprioception). Other modalities that are related to information about the state of the body, and that are not directly apparent, are the senses that assess Pco2 and Po2, blood pressure, and lung and stomach stretch receptors, the so-called interoceptors. Each modality can often be subdivided into further divisions of quality, i.e. in the case of taste (sweet, sour, salt, bitter and umami), light (red, green and blue) and hearing (tonal pitches).
2. The intensity (quantity) of the stimulus (Fig. 54a). The quantity of a sensory impression corresponds to the strength of the stimulus. As the stimulus strength increases, so does the amplitude of the receptor potential (amplitude-coded signal) and, when this eventually reaches a threshold, it causes action potentials that increase in their frequency of firing as the receptor potential rises (temporal or frequency coding). Another way in which the strength of the signal is coded is by increasing the number of afferent fibres that are activated (spatial or recruitment coding).
3. The duration of the stimulus. Many receptors will continue to fire impulses as long as the stimulus is applied; others will signal when a stimulus is applied and when a stimulus is removed. However, in most cases, even if a stimulus persists (e.g. constant touch to the skin), the sensation/perception of it wanes. This involves a process called adaptation. Adaptation occurs at all stages of the transformation of the stimulus: in the transduction process, in the conductance mechanism of the receptor potential, in the synaptic transmission from a secondary sensory cell and in the generation of the action potential. It can also be a function of the central nervous system (CNS) itself once the action potentials reach that far.
4. The localization and resolution (acuity) of the stimulus. The sensory system detects the location of a stimulus, and its fine detail. Both depend on the spacing of receptors (better localization and acuity occur with greater receptor density). The receptive field of a sensory neurone itself (sometimes called the receptor field) is the area of sensory surface from which that neurone receives an input. Receptor neurones converge onto second-order neurones (usually in the CNS), and then to third-and higher-order neurones. These transitions are made in relay nuclei. The receptor field of the primary receptor is usually a small excitatory area. The receptive field of the second or higher-order neurone is larger and more complex (because of both convergence and divergence, and excitatory and inhibitory pathways).
The net result is sensation and, when interpreted at a conscious level in the light of experience, this becomes perception.
The coded signals from each of the sensory receptors are relayed to the CNS by peripheral and cranial nerves. Each modality is associated with specific nerves and pathways, e.g. gustatory information is trans- mitted via facial and glossopharyngeal nerves, and the somatosensory system is transmitted via the dorsal column–medial lemniscal system for the larger afferent fibres (Aα and Aβ) and the anterolateral system (anterior and lateral spinothalamic tracts) for the smaller afferent fibres (Aδ and C). Each sensory system has its unique pathway into and through the CNS to eventually provide an input into the thalamus. The thalamus, in turn, provides an input to the cortex. Each sensory system projects to a specific area of the primary sensory cortex which is primarily concerned with the analysis of the sensory information, and these neurones, in turn, project to the secondary sensory cortex in which more complex processing occurs. There are further projections to associated areas, such as the posterior parietal, prefrontal and temporal cortices, which can again project to the limbic and motor systems. The latter systems are involved in the processing of the sensory information, leading to responses such as complex behavioural and motor responses.
Lateral inhibition. Figure 54b shows a neural network comprising two mechanoreceptors in the skin and their associated neurones at the next two synaptic levels. The two receptors are each excited equally by a stimulus applied between them. The divergent and convergent connections seem to impose an avalanche-like spread of excitation at progressively higher levels of the CNS. Pinpoint stimulation appears to lead to an enlarged, less precise and more diffuse representation at each successive synaptic level ( A ). However, this situation is encountered only under pathological conditions (e.g. strychnine poisoning, which blocks inhibiting synapses in the CNS). Inhibition normally prevents the spread of excitation by a phenomenon called lateral inhibition ( B ). At each synaptic relay, each excitatory neurone exerts an inhibitory effect by exciting inhibitory interneurones. The neurone with the greatest input (the one in the middle) imposes the strongest inhibition on those on either side of it. Lateral inhibition has been shown to exist at all levels of sensory systems: in the dorsal horn of the spinal cord, in the dorsal column nuclei, in the thalamus and in the cortex, as well as in the visual system. The result is an increased spatial sharpening in the CNS of the representation of the distant peripheral stimulus on moving through the synaptic levels.
Descending inhibition. In practically all sensory systems, higher centres can also exert inhibitory effects on all those at lower levels. Such central inhibition can act at a point as far peripheral as the receptor or at the afferent ending in the spinal cord. Like lateral inhibition, descending inhibition can be considered to function as a means of regulating the sensitivity of the afferent transmission channels.
The types of synaptic mechanism described above indicate that there is great flexibility in the sensory pathways, and that they are not as hard-wired as many pathway diagrams suggest.