Communication is the essential function of the nervous system. This occurs both with the surrounding environment and within the organism. In reference to its functions, the nervous system is also divided into the somatic nervous system (SNS) and the autonomic nervous system (SNA).
The somatic nervous system is a complex system. It is the part of the nervous system responsible for transporting sensory information to the central nervous system. That is: the SNS is responsible for communicating the body with the external environment. While, the ANS is responsible for communication between organs in order to maintain homeostasis.
Somatic nervous system
The SNS acts through the skeletal muscles, regulating voluntary and reflex actions and capturing, through its receptors, all the changes that occur. The motor fibers found in the somatic nervous system have some characteristics that differentiate them from those of the autonomic nervous system:
- First, there is the absence of ganglia along the way.
- Neuronal bodies are located within the central nervous system. In addition, they run without interruption until they reach the effector (skeletal muscle).
- The conduction speed is much higher.
- The SNS may or may not stimulate the effector, but it does not inhibit it.
Sensory pathways of the somatic nervous system (SNS)
For a sensation to be perceived, the information must reach the cerebral cortex. The sensory path is therefore the path that information takes through a set of neurons that connect the peripheral nervous system to the central one.
To reach its destination, which is the central nervous system, a sensory stimulus must pass from the receptors to the central nervous system through the connection with three neurons. In the somatosensory cortex there is a representation proportional to the sensitivity of the different parts of the body (1). In this representation, not all areas are the same size. Areas such as the fingertips and lips occupy the main place in this representation.
There are several types of sensory pathways. Depending on the sensory perception involved, we will have:
- Discriminative or epicritic sensitivity: touch.
- Protopathic : pain
- Thermoalgesic sensitivity: thermal
- Proprioceptive : position of the body
It is also possible to name them through the origin of the stimulus:
- Exteroceptive sensitivity : information comes from the skin.
- Interoceptive sensitivity : information comes from internal organs.
- Proprioceptive sensitivity : information comes from the musculoskeletal system.
Sensory pathways and neurons
To transmit sensation, nerve impulses travel through three types of neurons:
- The first order neurons : are those that transmit nerve impulses to the periphery of the body.
- Second-order neurons : found in the spinal cord and brain stem. They transmit the nerve impulse from the trunk to the thalamus. This is where a synapse is formed with the third-order neuron.
- Third-order neurons : found in the sensory nuclei of the thalamus. Their function is to guide the nerve impulse towards the somatic (i.e. sensitive to somatic stimuli) areas in the region behind the Rolando’s sulcus in the parietal lobe.
Before reaching the cortex, where sensation is interpreted, all sensory information is processed in the thalamus (except the olfactory afferents). Then, they are integrated into the parietal cortex, where sensitivity is usually added.
When we try to grasp something, this mental process requires the contraction and relaxation of the muscles in both the arm and hand.
Motor pathways guide nerve impulses from the central nervous system to the skeletal muscles (somatic effectors). The neurons involved in this process are motor neurons, which are located in the anterior horn of the spinal cord.
The motor pathway system is divided into three vascular-nerve bundles according to the origin and purpose of each nerve:
- Geniculate beam. It ends in the motor nuclei of the cranial nerves. In other words, it is the pathway that controls the facial muscles.
- Parapyramidal. It originates in the frontal gyrus or area 4. It sends stimuli to the reticular formation at the bulbar level.
- Pyramidal way. It connects the cortex with the neurons of the anterior horn of the spinal cord. Therefore, check the muscles of the trunk, neck and extremities.
In light of the above, we can deduce that the somatic nervous system is a complex system. To produce movement, several connections are generated in the central nervous system that allow for the final action. The same happens when someone touches or rubs an object: in order for the brain to recognize it, different neurons and brain areas are involved.