Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Nervous system
Sensory systems
General structural principles of sensory systems

Each sensory system (or analyzer, as termed by Pavlov) consists of several divisions. In the peripheral division, a signal from the external or internal environment is transduced into an electrical process—a Nerve Impulse. This Transduction is mediated by specialized structures called receptor formations. Impulses from the periphery travel along nerve fibers to the Brain AND SPINAL cord, and ultimately reach the Cerebral Cortex. The cortex acts as the central, or cortical, division of any sensory system, where final signal Processing occurs. The pathways connecting the receptor and cortical divisions make up the conducting division of the sensory system (analyzer).

Stimuli acting on receptors can span various modalities: light, sound, mechanical, chemical, etc. Each modality is perceived by its own specific type of receptor and transmitted along highly dedicated Neural Pathways. Consequently, we speak of distinct Sensory systems: visual, auditory, vestibular, somatosensory, gustatory, and olfactory.

Receptors are Cells or neuronal endings specialized for detecting a particular type of stimulus. If the stimulus is perceived by a specialized dendritic ending of an afferent neuron, the receptor is termed primary. Skin Receptors responding to mechanical stimulation are prime Examples. Conversely, if the receptor is a specialized Cell upon which an afferent nerve fiber forms a synaptic contact, it is called a secondary receptor. Examples include the receptor Cells of the gustatory, auditory, and vestibular sensory systems.

Receptors that perceive stimuli from the external environment are called exteroceptors. These are divided into distant (visual, auditory) and contact (gustatory, tactile) receptors. Interoceptors signal the state of Internal Organs, as well as Changes in the Chemical composition of Blood, interstitial fluid, and gastrointestinal contents. Proprioceptors convey information about the state of The Musculoskeletal System. Thus, receptors possess Specificity, meaning they are most effectively excited by a stimulus of a particular modality.

Each afferent fiber makes contact with multiple receptors. The area from which a given fiber collects information is called its receptive field. Receptive fields of neighboring fibers overlap, thereby ensuring greater reliability of receptor function.

The Cell bodies of afferent Neurons typically reside in sensory spinal or cranial ganglia. Exceptions include the visual and olfactory systems, where sensory neurons are located directly in the retina (ganglion cells) or the olfactory bulb, respectively. The processes of these neurons enter the Spinal Cord or brain, where they synapse onto a next-order neuron. The signal then propagates along a neural network in an ascending direction. For most sensory systems, except the olfactory one, the penultimate neuron lies in specific nuclei of the thalamus. From there, information is transmitted to the corresponding projection and association areas of the cortex, where memory processes are integrated into signal processing. It has been noted that impulses from receptors reach the primary projection areas of the cortex via the shortest route, whereas the activation of association areas occurs slightly later due to the involvement of polysynaptic neural networks (see Fig. 3.45). Sensory projections in the brain are topographically organized, meaning that a specific body region or group of receptors is linked to a localized group of neurons in the CNS. As the impulse travels along this pathway, the signal's modality is preserved, and partial processing occurs. This pathway is called the specific pathway. Along this route, information from receptors is sorted, some of it is inhibited ('filtered out'), and only the most critical part of the signal reaches the higher centers.

Alongside this, there are numerous non-specific pathways through which the signal modality is lost. Regardless of its origin, afferent input inevitably reaches the Brainstem reticular formation via axon collaterals, triggering its activation. The reticular formation is connected to non-specific thalamic nuclei, which, in turn, exert diffuse excitatory influences on the cortex (Fig. 3.19, B). As a result, cortical neurons enter an excited state, facilitating their perception of information arriving via specific pathways. The activation of the reticular formation is also accompanied by autonomic responses and motor activity.