The somatic sensory cortex, also referred to as the primary somatosensory cortex, is localized to the postcentral gyrus of the parietal lobes, just posterior of the central sulcus, and is vested with the job of processing information being transmitted from the periphery. The somatic sensory cortex is more commonly referred to as the primary somatosensory cortex. Neuroanatomically, the primary somatosensory cortex is localized to the postcentral gyrus of the anterior portion of the parietal lobes.
The essential function of the primary somatosensory cortex is processing peripheral sensation of the contralateral side [ 2 ]. Vice versa, sensation on the left side of the body will be processed by the Skip to main content Skip to table of contents.
This service is more advanced with JavaScript available. Encyclopedia of Child Behavior and Development Edition. This arrangement is referred to as somatotopic , and the full body is represented in this way in each of the four divisions of the somatosensory cortex. Because some areas of the body e. Thus, the somatotopic maps found in the somatosensory cortex are distorted such that the highly sensitive areas of the body take up a disproportionate amount of space in them see image to the right.
Sunderland, MA. Sinauer Associates; We experience these things every day, but how do our brains create them? Your Brain, Explained is a personal tour around your gray matter. Dingman weaves classic studies with modern research into easily digestible sections, to provide an excellent primer on the rapidly advancing field of neuroscience. As the somatosensory areas receive sensations from the entire body, the representation of all body parts is present.
However, the proportion of representation is different. In the primary somatosensory cortex, the contralateral half of the body is represented as an inverted homunculus. The pharyngeal region, tongue, and lips are represented in the most inferior part; followed by face, fingers, hands, arms, trunk, and thigh.
The leg and foot areas are represented in the posterior part of paracentral lobule, present on the medial surface of cerebral hemisphere. The anal and genital regions are present here.
The proportion of the cortex representation for a particular part of the body depends on its functional importance rather than its size. In fact, the area occupied by a particular body part is proportional to the number of sensory receptors present in it. The secondary cortex is much smaller and less important than the primary somatosensory cortex.
Here, the body is bilaterally represented with the contralateral side dominant. The leg area is present most posterior, and the face area lies most anterior. The arterial supply to most of the primary somatosensory area S1 and the secondary somatosensory area S2 is derived from the medial cerebral artery. This artery supplies the lateral surfaces of the cerebral hemispheres. The part of the primary somatosensory area present on the medial surface of the cerebral hemisphere is supplied by the anterior cerebral artery.
Blood from the somatosensory cortex finally drains into the superior sagittal sinus. As we said earlier, the somatosensory area is divided into two parts; the primary somatosensory area and the secondary somatosensory area. The reason for this division is that a distinct and separate spatial orientation of different parts of the body is found in the two areas. This orientation has been discussed in the section of Representation of Body. Primary Somatosensory area S1 has a high degree of localization whereas the secondary area S2 has very poor localization.
The functions of the secondary area S2 are not well known. We will focus on the functions of the primary somatosensory area S1. Merkel nerve endings are mechanoreceptors found in the skin and mucosa of vertebrates that provide touch information to the brain. The information they provide are those regarding pressure and texture.
Each ending consists of a Merkel cell in close apposition with an enlarged nerve terminal. This is sometimes referred to as a Merkel cell—neurite complex, or a Merkel disk receptor. A single afferent nerve fiber branches to innervate up to 90 such endings. They are classified as slowly adapting type I mechanoreceptors. Describe how propioception is the sense of the position of parts of our body in a three dimensional space.
Proprioception is the sense of the relative position of neighboring parts of the body and the strength of effort being employed in movement. It is distinguished from exteroception, perception of the outside world, and interoception, perception of pain, hunger, and the movement of internal organs, etc.
The initiation of proprioception is the activation of a proprioreceptor in the periphery. The proprioceptive sense is believed to be composed of information from sensory neurons located in the inner ear motion and orientation and in the stretch receptors located in the muscles and the joint-supporting ligaments stance. Conscious proprioception is communicated by the posterior dorsal column—medial lemniscus pathway to the cerebrum.
Unconscious proprioception is communicated primarily via the dorsal and ventral spinocerebellar tracts to the cerebellum. An unconscious reaction is seen in the human proprioceptive reflex, or Law of Righting. In the event that the body tilts in any direction, the person will cock their head back to level the eyes against the horizon.
This is seen even in infants as soon as they gain control of their neck muscles. This control comes from the cerebellum, the part of the brain that affects balance. Muscle spindles are sensory receptors within the belly of a muscle that primarily detect changes in the length of a muscle.
They convey length information to the central nervous system via sensory neurons. This information can be processed by the brain to determine the position of body parts. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles. Muscle spindle : Mammalian muscle spindle showing typical position in a muscle left , neuronal connections in spinal cord middle , and expanded schematic right.
The spindle is a stretch receptor with its own motor supply consisting of several intrafusal muscle fibers. The sensory endings of a primary group Ia afferent and a secondary group II afferent coil around the non-contractile central portions of the intrafusal fibers. The Golgi organ also called Golgi tendon organ, tendon organ, neurotendinous organ or neurotendinous spindle is a proprioceptive sensory receptor organ that is located at the insertion of skeletal muscle fibers onto the tendons of skeletal muscle.
It provides the sensory component of the Golgi tendon reflex. The Golgi organ should not be confused with the Golgi apparatus—an organelle in the eukaryotic cell —or the Golgi stain, which is a histologic stain for neuron cell bodies.
Golgi tendon organ : The Golgi tendon organ contributes to the Golgi tendon reflex and provides proprioceptive information about joint position. The Golgi tendon reflex is a normal component of the reflex arc of the peripheral nervous system. In a Golgi tendon reflex, skeletal muscle contraction causes the agonist muscle to simultaneously lengthen and relax. This reflex is also called the inverse myotatic reflex, because it is the inverse of the stretch reflex.
Although muscle tension is increasing during the contraction, alpha motor neurons in the spinal cord that supply the muscle are inhibited. However, antagonistic muscles are activated.
The somatosensory pathway is composed of three neurons located in the dorsal root ganglion, the spinal cord, and the thalamus. A somatosensory pathway will typically have three long neurons: primary, secondary, and tertiary. The first always has its cell body in the dorsal root ganglion of the spinal nerve. Dorsal root ganglion : Sensory nerves of a dorsal root ganglion are depicted entering the spinal cord.
The axons of many of these neurons terminate in the thalamus, and others terminate in the reticular activating system or the cerebellum. In the case of touch and certain types of pain, the third neuron has its cell body in the ventral posterior nucleus of the thalamus and ends in the postcentral gyrus of the parietal lobe. In the periphery, the somatosensory system detects various stimuli by sensory receptors, such as by mechanoreceptors for tactile sensation and nociceptors for pain sensation.
The sensory information touch, pain, temperature, etc. Generally, there is a correlation between the type of sensory modality detected and the type of afferent neuron involved. For example, slow, thin, unmyelinated neurons conduct pain whereas faster, thicker, myelinated neurons conduct casual touch. In the spinal cord, the somatosensory system includes ascending pathways from the body to the brain. One major target within the brain is the postcentral gyrus in the cerebral cortex.
This is the target for neurons of the dorsal column—medial lemniscal pathway and the ventral spinothalamic pathway. Note that many ascending somatosensory pathways include synapses in either the thalamus or the reticular formation before they reach the cortex.
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