The sensory system is responsible for processing sensory information from the outside world.
We shall begin this section by considering the ascending tracts. The ascending tracts refer to the neural pathways by which sensory information from the peripheral nerves is transmitted to the cerebral cortex. In some texts, ascending tracts are also known as somatosensory pathways or systems. Functionally, the ascending tracts can be divided into the type of information they transmit – conscious or unconscious. The conscious tracts are comprised of the dorsal column-medial lemniscal pathway and the anterolateral system and the unconscious tracts are comprised of the spinocerebellar tracts.
Following this we will consider the different modalities of sensation. Sensory receptors exist throughout the body and are activated by different stimuli – allowing us to process and react to different things in our environment. We possess sensory receptors to respond to chemical, mechanical, thermal and nociceptive stimuli.
Sensory acuity is how accurately we are able to locate a stimulus and its varies throughout the body depending on the function of the area. For example, the fingertips require a greater sensory acuity than the forearm. Sensory acuity is determined by three things: lateral inhibition, two point discrimination and synaptic convergence and divergence.
We will then look at some of the individual pathways responsible for sensory processing.
Pain is a somatic and emotional sensation which is unpleasant in nature and associated with actual or potential tissue damage. Physiologically it is essential for survival as behaviours causing pain are often dangerous and so unlikely to be reinforced and therefore not repeated. In this section we will consider how the classic picture of pain is processed within the nervous system.
We will then consider the visual pathway. The optic nerve (CN II) is the second cranial nerve, responsible for transmitting the special sensory information for vision. It is developed from the optic vesicle, an outpocketing of the forebrain. The optic nerve can therefore be considered part of the central nervous system. Visual stimuli are processed within the visual cortex in the occipital lobe.
The auditory pathway conveys the special sense of hearing. Information travels from the receptors in the organ of corti of the inner ear (cochlear hair cells) to the central nervous system, carried by the vestibulocochlear nerve (CN VIII). This pathway ultimately reaches the primary auditory cortex for conscious perception. In addition, unconscious processing of auditory information occurs in parallel.