There is a general rule that one cerebral hemisphere handles sensory inputs from the opposite side of the body.
Sensory inputs to the cortex arise from the thalamic nuclei, and there is a topographic map of one half of the body's surface in the contralateral thalamus and somatosensory cortex.
Areas of the body surface with greatest two-point discrimination (i.e. densest innervation) have the largest areas of somatosensory cortex.
The somatotopic map of the body surface on the cortex is therefore distorted, depending on the density of innervation of the skin, and is called the sensory homonculus.
Areas 1, 2 and 3 are strips of cortex each dealing with a different type of sensory input from the skin, muscles and joints.
Damage to the sensory cortex results in decreased sensory thresholds, an inability to discriminate the properties of tactile stimuli or to identify objects by touch.
If a region of the body is amputated (such as a finger) there is reorganization of the primary somatosensory cortex, with neurons that were previously activated by touching that finger now responding to stimulation of adjacent areas of skin.
The Second Somatosensory Area, S2
The second somatosensory area of the cortex (SII) lies on the upper side of the lateral sulcus and has a somatotopic map that includes inputs from both sides of the body.
The second somatosensory cortices of each hemisphere are interconnected.
|Somatosensory Pathways: the inputs to the somatosensory cortex|
Touch and Vibration.
Information about light touch and vibration pass through the dorsal columns to reach the dorsal column nuclei (N gracilis and N cuneatus) on the same side of the medulla.
The cuneate and gracile nuclei are a relay station containing second order neurones whose axons cross over to the opposite side of the neuraxis in the medulla to reach the thalamus. This is the medial lemniscus, and is sometimes called the 'sensory decussation'.
There are also interneuones within these nuclei whose job is to detect the edges of a tactile stimulus and emphasise their presence in the signal that is carried forward to the thalamus.
The ventrolateral nuclei of the thalamus process this sensory information and relay it to the sensory cortex; as in the dorsal column nuclei, fine discrimination is adied by edge detection as the signals are forwarded to the primary somatosensory receiving area.
The is a somatotopic map of the body surface in the thalamus and sensory cortex, in which the number of neurones processing information from each part of the body is related to the density of the peripheral innervation.
Pain and Temperature.
Information about injurious or thermal stimuli on one side of the body are relayed in the ipsilateral dorsal horn. Noxious stimuli are processed within the superficial layers of the dorsal horn, with Rexed's Lamina I being the source of the spinothalamic tract.
Within a few segments, the spinothalamic axons pass to the opposite lateral columns and project to the thalamus; this is the 'classical' pain pathway.
Other ascending tracts that also carry information about injurious stimuli pass rostrally in the antero-lateral system and other smaller pathways.
Note that visual signals also reach the superior colliculi, that are involved in regulating the direction of gaze.
The Visual Pathway
The visual pathway starts within the eye and information is carried by axons in the optic nerve through the Optic Chiasma and along the Optic Tract towards lateral geniculate nucleus at the back of the thalamus.
These structures can be seen on the surface of the ventral side of the brain.
The lateral geniculate nucleus is a layered structure (6 layers in humans) and processes information received through the optic nerves.
Signals from the two optic nerves reach different layers of the nucleus, and project to layer 4 of the calcarine cortex.
Some layers are concerned with the perception of colour - sensed by the cones of the fovea.
Other layers are concerned with movement of objects in the visual field, depth of vision and the ability to sense low light levels using the rods of the retina.
The Auditory Pathway
The physiology of the cochlea is considered elsewhere and this section follows the auditory pathway from the organ of Corti to the Auditory cortex.
Afferent neurones arising from the basilar membrane send their information to the cochlear nuclei in the medulla.
The pathways between the cochlear nuclei and the primary auditory cortex in the middle third of the superior temporal gyrus are complex, with several relay stations.
These pathways are unlike the other sensory pathways in that they are biilateral, i.e. information from each cochlea is relayed through both sides of the brainstem to both suditory cortices, left and right. This infomation is used to sense the direction of sound, by detecting small differences in the time at which sound reaches each ear.
There are some important relay stations including the superior olivary nuclei of the medulla, the inferior colliculi in the midbrain, and the medial geniculate nucleus of the thalamus.
As for the visual pathway, the colliculi of the midbrain are concerned with eye movements. The superior colliculi are conerned with directing the gaze towards object in the visual field. The inferior colliculi are concerned with directing the gaze - turning the eyes - towards objects that are identified in auditory space.
One of the factors that is involved in determining the origin of sounds is the time difference between the arrival of sounds ant the two ears, normally measured in a millisecond or two. The auditory pathway travels up both sides of the brainstem, and that allows the system to identify precise timings of activity in the two ears at an early stage of the pathway. The superior olivary nucleus seems to be concerned with detecting this interaural time difference; it is believed that the difference in the volume of sound reaching both ears may also be detected in this part of the pathway.
The medial geniculate body is concerned with relaying the frequency and intensity of sounds to the auditory cortex; information concerning the timing and intensity of information arising from both ears is also relayed to the cortex. There seem to be maps of sound frequencies within the inferior colliculi and the medial geniculate body (tonotopic maps, which correspond to the somatotopic map in the somatosensory cortex and the maps of the visual fields in the visual cortices).
The auditory cortex in each temporal lobe recieves information for both ears and analyses the frequency volume and interaural timing of sound. Neurons at one end of the auditory cortex respond best to low frequencies; neurons at the other respond best to high frequencies. These different parts of the auditory area are concerned therefore with the pitch and volume of a sound, while other sub-areas are concerned with musical sounds, composed of a fundamental frequency and its harmonics.
Understanding Words and Speech
Wernicke's area is a part of the cerebral cortex associated the the understanding of the spoken word. It is located in the posterior section of the superior temporal gyrus, close to the primary auditory area.
Destruction to Wernicke's area results in an inability to understand commands, written or spoken, known as a sensory dysphasia, or sensory aphasia.
The area receives its input from the primary auditory area, and sends its output to Broca's area, through the arcuate fasciculus.
Broca's area is concerned with speech, and lesions of this area or the arcuate fasciculus are associated with a motor dysphasia, in which the patient can understand commands, but cannot form spoken words, or has slurred speech and words that are imperfectly formed.
There is some uncertainty as to the exact anatomical borders of Wernicke's and Broca's areas, but the speech recognition area is though to be in the Sylvian fissure near the junction of the temporal and parietal lobes, and the arcuate fasciculus may project to the pre-motor cortex.
However the classical identities of these functions are still in common use as descriptions of the functions associated with each area.
Other areas of the parietal association cortex, adjacent to the visual occipital cortex are concerned with the understanding of written words.