Somatic Nervous System and Cranial Nerves
The Nervous System is a huge subject. Covering it in a few blog posts is very difficult. Still, we will try to give an overview – particularly covering the points that are of interest from the perspective of Yoga. In this post, we will cover the “Somatic Nervous System” – a part of the nervous system. A few readers may find it too technical. If so, they can skip the initial part and go directly to “Yoga and Cranial Nerves”.
The human nervous system is divided into two components which cooperate very closely:
- Central nervous system – It includes the brain and spinal cord. They are protected by the skull and the vertebrae respectively. We will cover different parts of the brain and the influence of Yogasana and Pranayama practice in a different blog post – as it is a vast topic.
- Peripheral nervous system – It includes nerves outside the central nervous system (i.e. the brain and spinal cord). These nerves are not protected the way brain and spinal nerves are, and hence are susceptible to injury or trauma. The peripheral nervous system is further divided into:
- Autonomic nervous system – we already covered this in the post titled “Understanding Autonomic Nervous System”.
- Somatic nervous system – It is the focus of this post.
The somatic nervous system primarily contains “sensory nerves” and “motor nerves”. The sensory nerves (also called “afferent nerves”) have sensory neurons to carry sensory/status information from various body parts (muscles, joints, organs, and glands) to the brainstem (the topmost part of the spinal cord). While the motor nerves (also called “efferent nerves”) have motor neurons to send the commands from the brain directing the body parts to perform some actions. As we will see below, a few nerves carry both sensory and motor impulses.
The somatic nervous system is also responsible for a specific type of involuntary muscle response known as “Reflex”. The reflexes are controlled by a neural pathway (known as the “Reflex Arc”) that includes sensory neurons that sends a signal straight to the spinal cord. The spinal cord, in turn, generates a rapid subconscious response without involving the brain.
There are twelve pairs of cranial nerves (I to XII) in the somatic system – with each member of a pair serving its side of the body. Many of these nerves that serve the head, face, and neck areas connect directly with the brain without going through the brainstem. And hence they may remain intact in people who are otherwise severely paralyzed through severance of the spinal cord. The cranial nerves allow such handicapped people to continue to have senses – blink (vision), sniff (smell), sip (taste), hear (sound) and even speak.
- Cranial Nerve I (Olfactory – sensory) – they carry the sensations of odor from the roof of the nasal cavity to the brain (the limbic system and to the cortex).
- Cranial Nerve II (Optic – sensory) – they carry the signals from the retinas of the two eyes to the visual areas in the brain (Occipital lobes at the back of the skull).
- Cranial Nerve III (Oculomotor – motor) – they control the intrinsic muscles that adjust the sizes of the pupils according to the light level and the extrinsic muscles that move the eyeballs in their sockets.
- Cranial Nerve IV (Trochlear – motor) – they control the muscles of the eyes to pull the eyeball down and toward the nose.
- Cranial Nerve V (Trigeminal – both sensory and motor) – they are spread across areas of the face. The afferent nerves originate at the skin of the face, the corneas of the eyes, and at the teeth. Their efferent nerves control the muscles of the jaw when chewing.
- Cranial Nerve VI (Abducens – motor) – they are used in the movement of the eyeball through the lateral rectus muscle within the eye socket.
- Cranial Nerve VII (Facial – motor) – they control the muscles of the face to make the face expressive. They are also involved with the sense of taste and control the production of saliva and tears.
- Cranial Nerve VIII (Acoustic – sensory) – they carry acoustic signals from the ears to the brain. They also connect the balancing apparatus of the inner ears to the brain centers concerned with posture and balance.
- Cranial Nerve IX (Glossopharyngeal – both sensory and motor) – they send the taste sensation and sensations of pressure and pain in the throat to the brain.
- Cranial Nerve X (Vagus – both sensory and motor) – they are a very important part of the parasympathetic nervous system. They connect to pharynx, esophagus, larynx, trachea, heart, stomach and liver – and hence control many important processes such as respiration (breathing), digestion, and circulation (heart rate). They are both afferent and efferent. More than 80% of all of the parasympathetic nerve fibers in the body is found in the vagus nerve.
- Cranial Nerve XI (Spinal-accessory – motor) – they drive the contraction of the trapezius and sternocleidomastoid muscles in the neck. They are actively involved when lifting and opening the rib cage when breathing deeply in Yogasana and pranayama practice.
- Cranial Nerve XII (Hypoglossal – motor) – they are important in speech to control the muscles of the tongue and throat.
Yoga and Cranial Nerves
A few cranial nerves are of deep importance to Yoga practice:
- Cranial Nerve II (Optic) –They carry the visual signals when attempting to balance – and hence are of significant importance in Yogasana practice.
- Cranial Nerve V (Trigeminal) – In the context of Yogasana practice, they serve as the sensory nerve for Ocular Vagal reflex covered in a previous post titled “Why Cover Eyes Using a Blanket or Eye Pillow during Restorative Yoga poses”. It provides a way of quieting the mind by wrapping a soft bandage around the eyes or keeping a soft pillow.
- Cranial Nerve VIII (Acoustic) – they connect the balancing apparatus of the inner ears to the brain centers concerned with posture and balance. And hence, this is very important in Yogasana practice.
- Cranial Nerve IX (Glossopharyngeal) – they also send the signals from Baroreceptors (see post) present in the carotid artery. This is the key to the enormous relaxation and calmness we get in Yoga inversions. We covered this one of our most popular posts titled “Baroreflex – The Secret behind Inverted Yoga poses like Sirsasana”.
- Cranial Nerve X (Vagus) – The vagus nerve goes through all viscera in the body except the adrenal glands. Various Yoga poses stimulate the vagus nerve. We covered this in a previous post titled “Yoga Stimulates Vagus Nerve to Induce Relaxation Response”