Nervous System

The basic purpose of the Nervous System is to coordinate and control all the activities of the body. It enables the body to respond and adapt to the changes that occur inside as well as outside the body.
It structurally divided into 2 parts- 

1. Central Nervous System (CNS) is made up of Brain and Spinal cord.
2. Peripheral Nervous System (PNS) is made up of nerves that branch off the spinal cord and extends across all the body.

The Nervous System transmits signals between the brain and all other body parts including internal organs. The basic unit of the Nervous System is called Neuron or Nerve Cell. The human brain contains around 100 billion neurons. With Neuron, it also has other types of cells called as Glia or Glial Cells.

1. A Neuron- has a cell body, which includes the cell nucleus, and special extensions called axons and dendrites. Bundles of axons are called nerves and those are found throughout the body. Axons and dendrites allow neurons to communicate, even across long distances. Characteristics of Neuron: 
• They do not have the power of regeneration.
• They only use glucose for their energy supply.
• They cannot survive more than 3-5 seconds without O2 & glucose.



Figure 1: Neuron

2. Glia- non-neuron cell. Glia performs many important functions that keep the nervous system working properly. For example, glia:
• Help to support and hold neurons in place.
• Protect neurons.
• Create insulation called myelin, which helps move nerve impulses
• Repair neurons and help restore neuron function.
• Trim out dead neurons.
• Regulate neurotransmitters

The Central Nervous System

The Central Nervous system controls most of the functions of the body and mind. It consists of 2 major parts - The Brain and The Spinal cord.

1. The Brain:

•  The Brain is the center of our thoughts, the interpreter of our external environment, and the origin of control over body movement. Like a central computer, it interprets information from our eyes (sight), ears (sound), nose (smell), tongue (taste), and skin (touch), as well as from internal organs such as the stomach.
• The Brain is situated within the cranium or skull. It is structured in 6 main sections- Cerebrum, Cerebellum, Diencephalon, the midbrain, Pons, and the Medulla Oblongata.
• Cerebrum: This is the largest section. It is divided into two major hemispheres which are the right and left hemisphere; and the cerebrum is further divided into four Lobes. These four lobes are the frontal, the parietal, the temporal, and the occipital.
• Frontal – The frontal lobe is primarily responsible for reasoning, and thought.
• Parietal – The Parietal is primarily responsible for integrating sensory information.
• Temporal – The Temporal is primarily responsible for processing auditory information from the ears.
• Occipital – the Occipital is primarily responsible for processing visual information from the eyes.
• Cerebellum: This is the section located in the back of the head below the cerebrum and above the 1st cervical vertebra. It is responsible for muscle coordination, balance, posture, and muscle tone.
• Diencephalon: This section is found between the Cerebrum and midbrain. It contains two structures: The thalamus, and the hypothalamus
• The thalamus – behaves much like a relay station and directs sensory impulses to the Cerebrum
• The hypothalamus – controls and regulates autonomic functions, such as temperature, appetite, water balance, sleep, and blood vessel constriction and dilation. The Hypothalamus also plays a role in emotions, such as anger, fear, pleasure, pain, and affection.
• Midbrain: The midbrain section is located below the Cerebrum at the top of the brainstem. It is responsible for certain eye and auditory reflexes.
• Pons: The Pons is located below the midbrain and in the brainstem. Is responsible for certain reflex actions such as chewing, tasting, and saliva production.
• Medulla Oblongata: It’s the lowest part of the brain stem. It connects with the spinal cord and is responsible for regulating heart and blood vessel function, digestion, respiration, swallowing, coughing, sneezing, and blood pressure. It is known as the center for respiration.

2. The Spinal Cord:

• The spinal cord is the link for communication between the body and the brain. When the spinal cord is injured, the exchange of information between the brain and other parts of the body is disrupted.
• The spinal cord is divided into four different regions: the cervical, thoracic, lumbar, and afferent and efferent spinal nerves, which merge to form peripheral nerves. 
1. The afferent spinal nerves are responsible for carrying information from the body to the brain.
2. The efferent spinal nerves are responsible for carrying information from the brain to the body.

 Peripheral Nervous System

There is a system that regulates the functions of the CNS which lie outside its major components such as the brain and the spinal cord. This system is known as the Peripheral and is subdivided into two smaller systems.

• The Somatic System
• Autonomic System

1. The Somatic Nervous System 
• The Somatic System is responsible for carrying motor and sensory information both to and from the CNS. 
• This system is made up of nerves that connect to the skin, sensory organs, and all skeletal muscles.
• The somatic system is also responsible for nearly all voluntary muscle movements, as well as for processing sensory information that arrives via external stimuli including hearing, touch, and sight. 
• The structures that allow this communication to happen between the nerves throughout the body and the CNS are known as the Afferent sensory neurons and the Efferent Motor Neurons.
• Now afferent simply means “conducting inward” and efferent means “conducting outward”. So just like the spinal nerves, the Afferent neurons take information from the nerves to the CNS and the efferent neurons take information from the CNS to the muscle fibers throughout the body.
2. The Autonomic Nervous System 
• The Autonomic System is further divided into the sympathetic system and the parasympathetic system.
• The Sympathetic Nervous system is vital to our survival. Have you ever heard of the “Fight or Flight” response to danger? The sympathetic nervous system revs up the body when confronted with imminent danger to either defend yourself or to escape the threat.
• The Parasympathetic Nervous system is the counterbalance to the sympathetic response to danger, whether real or imagined. Once the threat is gone parasympathetic brings all systems of the body back to normal.

References:

• Gnosis Medical Yoga Teacher textbook.