12.3A: General Organization of the Somatosensory System
The somatosensory system is composed of the neurons that make sensing touch, temperature, and position in space possible.
- Describe how the somatosensory system is composed of neurons that make sensing touch, temperature, and position in space possible
Key Points
- The somatosensory system consists of primary, secondary, and tertiary neurons.
- Sensory receptors housed in the dorsal root ganglia project to secondary neurons of the spinal cord that decussate and project to the thalamus or cerebellum.
- Tertiary neurons project to the postcentral gyrus of the parietal lobe, forming a sensory homunculus.
- A sensory homunculus maps sub-regions of the cortical postcentral gyrus to certain parts of the body.
Key Terms
- decussate : Where nerve fibers obliquely cross from one lateral part of the body to the other.
- postcentral gyrus : A prominent structure in the parietal lobe of the human brain and an important landmark that is the location of the primary somatosensory cortex, the main sensory receptive area for the sense of touch.
- organization : The quality of being constituted of parts, each having a special function, act, office, or relation; to systematize.
- thalamus : Either of two large, ovoid structures of gray matter within the forebrain that relay sensory impulses to the cerebral cortex.
The somatosensory system is distributed throughout all major parts of our body. It is responsible for sensing touch, temperature, posture, limb position, and more. It includes both sensory receptor neurons in the periphery (eg., skin, muscle, and organs) and deeper neurons within the central nervous system.
Structure
A somatosensory pathway will typically consist of three neurons: primary, secondary, and tertiary.
- In the periphery, the primary neuron is the sensory receptor that detects sensory stimuli like touch or temperature. The cell body of the primary neuron is housed in the dorsal root ganglion of a spinal nerve or, if sensation is in the head or neck, the ganglia of the trigeminal or cranial nerves.
- The secondary neuron acts as a relay and is located in either the spinal cord or the brainstem. This neuron’s ascending axons will cross, or decussate, to the opposite side of the spinal cord or brainstem and travel up the spinal cord to the brain, where most will terminate in either the thalamus or the cerebellum.
- Tertiary neurons have cell bodies in the thalamus and project to the postcentral gyrus of the parietal lobe, forming a sensory homunculus in the case of touch. Regarding posture, the tertiary neuron is located in the cerebellum.
Processing
The primary somatosensory area of the human cortex is located in the postcentral gyrus of the parietal lobe. The postcentral gyrus is the location of the primary somatosensory area, the area of the cortex dedicated to the processing of touch information. At this location there is a map of sensory space referred to as a sensory homunculus.
A cortical homunculus is the brain’s physical representation of the human body; it is a neurological map of the anatomical divisions of the body. The surface area of cortex dedicated to a body part correlates with the amount of somatosensory input from that area.
For example, there is a large area of cortex devoted to sensation in the hands, while the back requires a much smaller area. Somatosensory information involved with proprioception and posture is processed in the cerebellum.
Homunculus : Image representing the cortical sensory homunculus. It shows how the anatomical portions of the body, such as the tongue, elbow, and hip, are mapped out on the homonculus. The surface area of cortex dedicated to a body part correlates with the amount of somatosensory input from that area.
Functions
The somatosensory system functions in the body’s periphery, spinal cord, and the brain.
- Periphery: Sensory receptors (i.e., thermoreceptors, mechanoreceptors, etc.) detect the various stimuli.
- Spinal cord: Afferent pathways in the spinal cord serve to pass information from the periphery and the rest of the body to the brain.
- Brain: The postcentral gyrus contains Brodmann areas (BA) 3a, 3b, 1, and 2 that make up the somatosensory cortex. BA3a is involved with the sense of relative position of neighboring body parts and the amount of effort being used during movement. BA3b is responsible for distributing somatosensory information to BA1 and shape and size information to BA2.