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 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.
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.