Sensory Systems: An Overview
A sensory system is one in which
information is conveyed to the spinal
cord and brain from peripheral sensory receptors, which in themselves are
either specialized neurones or nerve endings.
•
The
specialized sensory receptor, afferent axon and cell body together
with the synaptic contacts in the spinal cord are known as the primary
afferent. The process by which stimuli from the external environment are
converted into electrical signals for transmission through the nervous system
is known as sensory transduction (see Chapter 23).
•
The signal
produced by the sensory receptor is relayed to the central nervous system (CNS)
via peripheral or cranial nerves and through a series of synapses eventually
projects to a given area of cortex that is then capable of detailed analysis of
that sensory input.
There are five main sensory systems
in the mammalian nervous system:
•
Touch/Pressure,
Proprioception, Temperature And Pain Or The Somatosensory System (See Chapters
31–33);
•
Vision
(See Chapters 24–26);
•
Hearing
And Balance (See Chapters 27–29);
•
Taste (See
Chapter 30);
•
Smell Or
Olfaction (See Chapter 30).
All but the somatosensory pathways
are regarded as ‘special’ senses.
Sensory receptors
Sensory receptors transduce the
sensory stimulus either by a process of direct ion channel activation (e.g.
the auditory system) or indirectly
via a secondary intracellular messenger network (e.g. the visual system). In both cases the sensory
stimulus is converted into an electrical signal that can then be relayed to the
CNS in the form of either graded depolarizations/hyperpolarizations leading on
to action potentials (e.g. visual system) or the direct generation of action
potentials at the level of the receptor.
The specificity or modality
of a sensory system relies on the activation of specialized nerve cells or
fibres which are highly specific for different forms of afferent stimuli.
The receptor will only respond to
stimuli when they are applied within a given region around it (its receptive
field). This area or receptive field from which the receptor can be
activated is recognized by the CNS as corresponding to a specific site or
position in the body or outside world. The receptor will only transmit electrical
information to the CNS when it receives a stimulus of sufficient intensity to
reach the firing threshold.
The incremental response to a
change in stimulus intensity by the receptor gives the receptor its sensitivity.
Many receptors have high sensitivity both to the absolute level of stimulus
detection and to changes in stimulus intensity. This is because they are
capable of both amplifying the original signal by the use of secondary messenger systems and adapting to the
presence of a continuous unchanging
stimulus (see below and Chapter 23).
Ascending sensory pathways in
the spinal cord
With very sensitive receptors the
intrinsic instability of the transduction process is termed the noise and
the challenge for the nervous system is to detect a sensory stimulus response
or signal over this background noise (termed the signal to noise ratio).
The strength of a sensory stimulus
can be coded for at the level of the receptor and its first synapse, either in
the form of action potentials or graded membrane potentials within the
receptor.
The afferent sensory nerve can
code (among other things) for the strength of the stimulus, first by increasing
the number of afferent fibres activated (recruitment or spatial
coding) and, second, by increasing the number of action potentials
generated in each axon per unit time (temporal or frequency coding).
There is a complex relationship between the stimulus intensity and action
potential firing frequency in the afferent nerve – this is defined by the
Weber–Fechner law.
Sensory pathways
Each sensory pathway has its own
unique input to the CNS, although ultimately most sensory pathways provide an
input to the thalamus – the site of that projection being different for each
sensory system. This in turn projects to the cortex, although the olfactory
pathway primarily projects to limbic structures (see Chapter 30) and the muscle
spindle to the cerebellum (see Chapter 40).
Each sensory system has its own
area of cortex that is primarily concerned with analysing the sensory information
and this area of cortex – the primary sensory area – is connected to
adjacent cortical areas that perform more complex sensory processing (secondary
sensory areas). This in turn projects into the association areas (posterior
parietal, prefrontal and temporal cortices; see Chapter 34) which then project to the motor and limbic
systems (see Chapter 35). These latter areas are more involved in the
processing of sensory information as a cue for moving and generating complex
behavioural responses.
The primary sensory cortical areas
project also subcortically to their
thalamic (and/or brainstem) projecting nuclei. This may be important in
augmenting the detection of significant ascending sensory signals. This
augmentation probably involves at least two major processes: lateral
inhibition and feature detection. Lateral inhibition is a process by
which those cells and axons with the greatest activity are highlighted by the
inhibition of adjacent less active ones, which produces greater contrast in the
afferent information. Feature detection, on the other hand, corresponds to the
selective detection of given features of a sensory stimulus, which can occur at any level from the receptor to the
cortex.
