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.
Sensory pathways
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.
