The nervous system is the main controlling system of the body. It helps us experience the external world.

The main purpose of Yoga is to cleanse the consciousness. This requires controlling the mind, which is considered a seat of consciousness. The mind involves the higher-level functions of the nervous system – memory, cognition, and intelligence. The nervous system is most influenced by Yoga and hence needs to be understood well.

The nervous system is divided in two components – both of them closely connected:

• Central nervous system – It includes the brain and spinal cord. They are protected by the skull and vertebrae respectively.
• Peripheral nervous system – It includes nerves outside the central nervous system. The peripheral nervous system is further divided into the autonomic nervous system and the somatic nervous system. We already covered them in previous posts.

We will cover the central nervous system in this post.

Spinal Cord

The spinal cord is formed by neural structures and starts from the base of the brain (called the brainstem). The nerves that carry sensory information from various body parts (muscles, organs and glands) to the central nervous system are called “afferent” while those that carry command neural signals from the central nervous system to the body parts are called “efferent”.

The spinal cord serves the trunk and the limbs of the body. It receives sensory information from muscles, glands, and organs in these areas, and passes it to the brain. It also sends motor actions from the brain to these body parts. The spinal cord also handles reflex actions, which are performed directly by it without any involvement from the brain.

The spinal cord has three main functions:

1. Passing sensory information from body parts through afferent nerves.
2. Passing motor control impulses from the brain to the body parts through efferent nerves.
3. Perform reflex actions such as vomiting, breathing and walking – without involvement of the brain.

The spinal cord goes through the channel of the boney structure of the spinal vertebral column consisting of 31 vertebrae:

• 8 cervical (C1 to C8) branches serve the muscles, organs, and glands in the areas of the diaphragm, the neck, the arms, and the hands.
  • C1-C4 – breathing
  • C2 – head and neck movement
  • C4-C6 – heart rate
  • C5 – shoulder movement
  • C6-C7 – wrist and elbow movement
  • C7-T1 – hand/finger movement
• 12 thoracic (T1 to T12) thoracic branches serve areas of the trunk and intercostal muscles
  • T1-T12 – sympathetic tone
  • T2-T12 – trunk stability
  • T11-L2 – ejaculation
• 5 lumber (L1 to L5) lumbar branches serve the abdomen, legs, and feet
  • L2 – hip motion
  • L3 – knee extension
  • L4-S1 – foot motion
  • L5 – knee flexion
• 4 sacral (S1 to S4) sacral branches serving the anal and perineal regions
  • S2-S4 – penile erection/bowel
  • S2-S3 – bladder

Brain

Anatomically, the brain can be divided as (in order of location from the bottom to the top):

1. Brainstem
2. Cerebellum
3. Diencephalon
4. Cerebrum

Brainstem

The brainstem is a primitive part of the brain. It is located between the upper end of the spinal cord (C1 vertebra) and the base of the limbic system of the brain. It receives sensory information from the head, neck, face and other visceral organs and helps the brain control the muscles in these areas through the cranial nerves. The cranial nerves are covered in the post titled Somatic Nervous System and Cranial Nerves.

The brainstem contains:

• Medulla oblongata
• Pons
• Midbrain
• Reticular Formation

Medulla Oblongata

The medulla oblongata (or simply called medulla) is at the lowest end of the brainstem connected to the spinal cord. It plays an important role in the control of the functions of the autonomic nervous system such as digestion, breathing, and heart rate. We discussed the autonomic nervous system in a post earlier). Various receptors (or sensors) located in different body parts continuously signal the hypothalamus (the primary autonomic control center) about body parameters via the medulla. Based on the changes in parameters, the hypothalamus signals the medulla to either stimulate the sympathetic (fight-or-flight) or parasympathetic (rest and digest) response. The medulla, in turn, modulates the function like heart rate, breathing, and digestion. The Baroreceptor is a good example of such sensors – we covered in the post titled Baroreflex – The secret behind inverted yoga poses like Sirsasana – where parasympathetic relaxation response is triggered when the body is in an inverted pose.

The medulla also coordinates actions of swallowing, sweating, peristalsis (muscular movement in digestive tract – esophagus, stomach, and intestines), release of acid in stomach.

Also, in the region of the medulla, the motor fibers coming from the upper part of the brain towards the spinal cord cross over between the left and right sides (i.e. decussate). This means that the activation on the right side of the body is controlled by the left side of the brain and vice versa. This is called the laterality. We will cover it in another post.

Pons

The pons carry information about body movement from the cerebral hemispheres to the cerebellum. The pons also helps to regulate the autonomic nervous system by assisting the medulla oblongata. The respiratory center in the brain is located in the medulla oblongata and pons.

The pons is also involved in the control of sleep cycles and the regulation of deep sleep. The REM sleep originates in the pons. It activates inhibitory centers in the medulla to prevent body movements during sleep. We covered sleep cycles in the post titled Stages of sleep – Neuroscience and Yoga perspective.

Midbrain

The midbrain is the uppermost portion of the brainstem.

It controls reflex patterns of vision and auditory systems. A reflex action of the pupils of the eyes based on the light is controlled by the midbrain.

Reticular Formation

Another important structure of the brainstem called “Reticular Formation” runs the length of the brainstem as a thin sheet. It stretches from the medulla oblongata to the hypothalamus and beyond. It is a network of nerve pathways connecting the spinal cord, cerebrum, and cerebellum.

It regulates motor functions, sleep, consciousness, attention and concentration, respiration, and the vascular functioning of blood vessels in the brain. It is responsible for arousal of awareness and mediating the overall level of consciousness (like alertness and sleep). It helps to come to wakeful state from sleep.

Sensory signals from almost all of the sensory organs (olfactory, taste, auditory, proprioceptive, etc.) enter the reticular formation for processing before being sent to the hypothalamus. One exception to this is visual information, which bypasses the reticular formation and goes directly to the hypothalamus.

The main job of reticular formation is to control which of the sensory information is allowed to be passed to the hypothalamus. The sensory organs receive enormous information for the brain’s consumption. The reticular filters this information and only passes those to the brain that are sufficiently novel or of immediate importance. Without such filtering, the brain will get overwhelmed.

When in Savasana, the sensory signals that normally might be passed upward by the reticular formation are blocked from further upward transmission to the brain. This would be particularly relevant to the practice of Pratyahara, during which one is supposed to voluntarily block out all external stimuli from going to the hypothalamus.

Cerebellum

The cerebellum is situated posterior to the brainstem. It is responsible for balancing the body, maintain postures and movements. It works in association with the motor centers in the cerebrum (conscious movement) and the basal ganglia (natural movement).

The cerebellum is primarily a movement control center that has extensive connections with the cerebrum and the spinal cord. Just like the cerebrum, the cerebellum is divided into hemispheres; But it does not have left and right laterality like the cerebral hemispheres.

Cerebrum

The cerebrum is the largest part of the brain containing two cerebral hemispheres, the cerebral cortex (the outer layer), and the limbic system.

Cerebral Hemisphere and Cerebral Cortex

The cerebrum has got right and left cerebral hemispheres. The right hemisphere controls and processes signals from the left side of the body, while the left hemisphere controls and processes signals from the right. The cerebral hemispheres and the cerebral cortex covering these hemispheres are located in the uppermost region. They work together and form the intellectual or cognition part of the brain. The three main functions are:

• perception (knowledge)
• processing (thinking, problem-solving, analyzing, imagining and decision making)
• conscious motor action

The cerebral cortex is the outer layer of cerebral hemispheres that consists of nerve cells. The cerebral cortex constitutes the rational or thinking part of our brain. This is the part where sensations are processed and conscious decisions are made. There are different cortices each associated with one or more sensations. The cerebral cortex is divided into four lobes: Frontal, Parietal, Occipital and Temporal lobes.

Frontal Lobes

The frontal lobe includes the Prefrontal Cortex, premotor area and motor area of the brain. The frontal lobe is associated with executive functions and motor performance. Executive functions are some of the highest-order cognitive processes that humans have. Examples include planning, recognizing future consequences of current actions, choosing between good and bad actions, deciding socially unacceptable responses, determining similarities between objects or situations.

The frontal lobe is considered to be the moral center of the brain because it is responsible for advanced decision-making processes. It also plays an important role in retaining emotional memories derived from the limbic system and modifying those emotions to fit socially accepted norms. Hence for emotional balance, it is important that the prefrontal cortex has control over the limbic system with healthy communication. When this communication malfunctions, you see disorders like anxiety and depression. We will cover this in detail in future posts on nervous system disorders like anxiety and depression.

Our sensations are passed on to the prefrontal cortex for evaluation. Then they forwarded to the basal ganglia and then to the frontal lobes to initiate movement. The prefrontal cortex also controls short term memory and coordinate long term memory.

The prefrontal cortex is the logical part of the brain and is highly evolved in dealing with working memory, planning, execution, worrying, thinking about the future, and imagining scenarios. When in a state of normalcy, the thoughts and emotions are under control of the prefrontal cortex. The communication between the prefrontal cortex and the amygdala is normal.

The neuroscience experiments evidence that in a normal situation when the level of stress in the minimal, we are able to consciously control our emotions and the energizing of the amygdala (emotional part of the brain) is less and that of the prefrontal cortex (logical part of the brain) is more. The prefrontal cortex works efficiently to keep the emotions under control. It does this by controlling the action of the hypothalamus and amygdala. But when the level of stress hormones (norepinephrine, dopamine, and cortisol) due to stress, the firing of the neurons in the prefrontal cortex diminishes. Because of this, the control of the emotional response is surrendered by the prefrontal cortex, and the hypothalamus and the amygdala take the upper hand. Hence under such a situation, the response is more emotional and compulsive, and not logical. The communication between the prefrontal cortex and the amygdala malfunctions or disconnects. And so, the cognitive prefrontal cortex is out of the picture and the amygdala continues to initiate stress response through the hypothalamus directing the release of stress hormones – a vicious cycle. This results in diminished appetite, high blood pressure, elevated heart rate, and an increase in anxiety.

Following a life-changing event or consistent stressful situation covered in the previous paragraph, if the amygdala is continuously activating the stress response and the prefrontal cortex not able to extinguish it, it can result in anxiety, depression or post-traumatic stress disorder (PTSD). We will see in future posts how Yoga can help control such conditions by calming down the amygdala and inducing the parasympathetic relaxation response.

Parietal Lobes

The parietal lobes are associated with sensory skills. They are responsible for receiving and processing sensory information of touch, pain, temperature and taste. They integrate different types of sensory information and is particularly useful in spatial processing and navigation.

The parietal lobes are comprised of the somatosensory cortex and part of the visual system. The somatosensory cortex consists of a “map” of the body that processes sensory information from specific areas of the body. The left parietal lobe is involved in symbolic functions in language and mathematics, while the right parietal lobe is specialized to process images and interpretation of maps (i.e. spatial relationships).

Temporal Lobes

The temporal lobes deal with memory formation, language, speech and the sensory information of auditory and smell. The temporal lobes are associated with the retention of short- and long-term memories. They process sensory input including auditory information, language comprehension, and naming. They also create emotional responses and control biological drives such as aggression and sexuality. The left temporal lobe holds the primary auditory cortex, which is important for processing the semantics of speech.

The temporal lobe includes the amygdala and hippocampus – which actually form the limbic system – a very important function of the brain. It is also of a great importance to Yoga. Hence the limbic system is covered as a separate part of the cerebrum.

Occipital Lobes

The occipital lobes contain most of the visual cortex. They are the visual processing centers of the brain. Cells on the posterior side of the occipital lobe are arranged as a spatial map of the retinal field. The visual cortex receives raw sensory information through sensors in the retina of the eyes, which is then conveyed through the optic tracts to the visual cortex. Other areas of the occipital lobe are specialized for different visual tasks, such as visuospatial processing, color discrimination, and motion perception. Damage to the primary visual cortex (located on the surface of the posterior occipital lobe) can cause blindness, due to the holes in the visual map on the surface of the cortex caused by the lesions.

Limbic System

The limbic system is a complex set of structures found on the central underside of the cerebrum, comprising inner sections of the temporal lobes and the bottom of the frontal lobe. Though located in the temporal lobes, the limbic system has a distinct function of managing emotions and memory – and hence covered separately. It combines higher mental functions and primitive emotion into a single system often referred to as the emotional nervous system.

The limbic system is a subconscious part of the brain and lies between the brainstem (unconscious part) and the cortex (conscious and intelligent part).

The limbic system and the hypothalamus work very closely to manage emotions and psychological function. Also, anatomically, hypothalamus is located within the limbic system. And hence a few literatures include hypothalamus as a part of the limbic system. But from neuroscience perspective it has been defined as part of a separate region called the Diencephalon – along with the thalamus.

The limbic system is responsible for our emotional lives and higher mental functions, such as learning and formation of memories. The limbic system is the reason that some physical things such as eating seem so pleasurable to us, and the reason why some medical conditions (such as depression, anxiety and high blood pressure) are caused by emotional stress. There are several important structures within the limbic system: Amygdala, Hippocampus, Basal Ganglia and Cingulate Gyrus.

Hippocampus

The hippocampus is the memory center of the brain. It is mainly involved with memories having personal experiences and emotions – related to self, people, locations, time – those that provide a sense of self and self-awareness. In contrast, cognitive memories – those that are not directly related to emotions or experiences – reside primarily in the cingulate cortex (part of the cerebral hemispheres) that surrounds the hippocampus. The hippocampus plays a key role in the formation of emotion-laden, long-term memories based on input from the amygdala. The hippocampus acts as indexer – useful to store and retrieve memories from the cerebral hemisphere.

Amygdala

The amygdala is central to emotions. It is primarily associated with survival instinct i.e. fear. It learns from the experiences that are generated in negative, unpleasant, or stressful situations that are terrifying or physically challenging. The amygdala is involved in memories having strong emotional content like happiness, sadness, fear, anger, disgust. It also creates fear response resulting from anxiety. The amygdala is a subconscious part and hence difficult to control consciously. When a person is emotionally distressed, depressed or anxious the blood flow to the amygdala increases (along with right cerebral cortex). When someone is emotionally happy, upbeat, enthusiastic, energetic, the blood flow is strong in the left prefrontal cortex.

When a sensory response from vision, smell, touch, sound is perceived as a threat it is the amygdala that contributes to emotional processing. The amygdala interprets the sensory input and instantly sends a distress signal to the hypothalamus if it perceives a threat. As discussed earlier, the amygdala also participates in initiating a similar stress response if the threat is perceived by the cognitive part of the brain i.e. the prefrontal cortex. The hypothalamus then communicates with the rest of the body through the autonomic nervous system and controls involuntary body functions like breathing, blood pressure, heartbeat, and the dilation or constriction of blood vessels and airways in the lungs (called bronchioles).

We discussed the interaction between the amygdala and prefrontal cortex earlier, and how the amygdala dominates in stressful situations. Yoga therapy is helpful in calming the amygdala when the brain is heavily stressed, anxious or depressed.

Basal Ganglia

The basal ganglia help to coordinate the muscles during movement. They initiate automatic and subconscious movements – which are natural and habituated. That is the reason the muscular effects when an action is natural is different from the similar action initiated consciously by the cortex. For example, someone smiling on genuine joke looks more natural, while you see an artificial look when a smile is forced.

Originally basal ganglia were known only to be performing motor and regulatory functions. But now it is known to neuroscience that it is a key brain region associated with all reward types. For example, food is a reward when we are hungry, a primary reward. Erotic images are also considered a primary reward because mating is an essential obligation to humans. Money is a resource that affords survival in society, but it is a secondary reward because it is not a natural reward but a human creation. In any decision-making process, the brain assesses potential profit which is nothing but the size of reward with any action. And, basal ganglia play an important role in this process.

Food rewards influence the left hemisphere of the brain, while erotic rewards engage the right. Financial rewards engage the basal ganglia bilaterally (i.e. both left and right).

Cingulate Gyrus

The cingulate gyrus is located in the medial side of the brain next to the corpus callosum. There is still much to be learned about this gyrus, but it is known that its frontal part links smells and sights with pleasant memories of previous emotions. This region also participates in our emotional reaction to pain and in the regulation of aggressive behavior.

Diencephalon

The diencephalon region of the brain contains the Thalamus and Hypothalamus. It is located between the cerebral hemispheres and the limbic system. It contains elements of both the subconscious and conscious brains.

Hypothalamus

The hypothalamus is a very important part as it controls the functions of the autonomic nervous system, endocrine system and visceral organs – which are very important to maintain a healthy well-being and survival. It works subconsciously – and is outside the control of cortex. There are ways to influence the hypothalamus through the practice of Yogasana and Pranayama through direct or indirect regulation of the breath.

Also, the hypothalamus initiates autonomic response based on the reflexes like Baroreflex (based on the pressure sensed by baroreceptors, Chemoreflex (based on the precentage of oxygen and carbon dioxide in the blood sensed by chemoreceptors).

Based on the fear or emotional response from the amygdala, the hypothalamus generates a stress response by activating the sympathetic nervous system. When there is no immediate threat it generates relaxation response by activating the parasympathetic nervous system.

Thalamus

The main function of the thalamus is to relay motor and sensory signals to the cerebral cortex. It also regulates sleep, alertness, and wakefulness. The thalamus connects the cerebral cortex with the spinal cord. Sensory information coming from the spinal cord first reaches the thalamus and then forwarded to the appropriate region of the cerebrum.