Nervous system

Key Concepts

• Nervous system
• Nerves
• CNS, PNS

Introduction 

The nervous system is the major communicating, regulating, and controlling system present in the body. It is the centre of all mental actions such as thought, learning, and memory. The endocrine system, along with the nervous system, is responsible for regulating and maintaining homeostasis. The nervous system keeps us in touch with our internal and external environment through its receptors. The nervous system is present in almost all multicellular animals but varies in complexity. 

Nervous System 

It is an intricate network of nerves and cells that carry messages to and from the brain and spinal cord to several parts of the body. 

The nervous system is our body’s main command center. It starts from the brai  n, and it controls all body movements, thoughts, and automatic responses to the environment around us. It also controls other body systems, such as respiration, circulation, skeletal, digestion, breathing, and sexual development (puberty).  

Functions of the Nervous System 

1. It gathers information from inside and outside the body. This is a sensory    function. 

2. It transmits information to the various areas of the brain and spine. 

3. It processes the information in the brain and spine. This is an integration function. 

4. It sends information to the various muscles, glands, and organs so that they can respond appropriately.  

5. Mainly, it controls and coordinates all essential functions of the body, including all other body systems allowing the body to maintain homeostasis. 

Classification of Nervous System 

The nervous system contains two main types of cells: neurons and glial cells. 

Neuron: 

• It is the basic functional cell or unit of the nervous system. 
• It transmits impulses up to 250 mph. 

Parts of a Neuron 

• Dendrite – It receives a stimulus and carries its impulses toward the cell body. 

• Cell body – It consists of the nucleus and most of the cytoplasm. 
• Axon – It is a fiber that carries impulses away from the cell body. 
• Schwann cells- These are the cells that produce myelin (which is a fat layer) in the peripheral nervous system. 
• Myelin sheath – It is a dense lipid layer that insulates the axon and makes the axon look grey. 
• Node of Ranvier – These are gaps or nodes in the myelin sheath. 

• Path of impulse: Impulses travel from dendrite to cell body to axon. 

Three types of Neurons 

1. Sensory neurons – The function of sensory neurons is to send messages from the sense organs to the brain or spinal cord (CNS).  

2. Motor neurons – The function of motor neurons is to carry messages back from the brain or spinal cord (CNS) to all the muscles and glands in the body. 

3. Interneurons – Its function is to carry both sensory and motor nerves. 

Glial cells  

These cells do not conduct nerve impulses, but they support, nurture, and protect the neurons. They are present in large numbers and are capable of mitosis. 

Reflex action and Reflex arc 

Reflex actions are involuntary and immediate movements in response to stimuli. 

Some natural pathways called reflex arcs act in an instinct before the impulse reaches the brain, and this makes a reflex or a reflex action. 

Reflex arcs are formed in the spinal cord, and the information goes to the brain. 

Receptors:  There are specialized groups of cells in each sense organ, like the ear, eye, nose, tongue, and skin. These specialized groups of cells can detect changes or stimuli in the environment. Each organ has receptors sensitive to particular kinds of stimuli. 

Effectors: Effectors can be muscles and glands. Their job is to produce a specific response to a detected stimulus. 

The flow of signal in a reflex arc is as follows: 

Receptor → sensory neuron → relay neuron → motor neuron → effector (muscle) 

The human nervous system consists of two parts. They are: 

Peripheral Nervous System (PNS) – It includes all the nerves of the body. 

Central nervous system (CNS) – It includes the brain and spinal cord. 

Central Nervous System (CNS): 

The two parts of the central nervous system are: the brain and the spinal cord. 

Brain: 

The function of the brain is to receive information from the body and send out the required instructions to particular organs. It is the main control unit of the nervous system. 

Parts of brain: 

The brain is enclosed within the skull, which provides protection from all sides such as frontal, lateral, and back. The brain consists of four major parts: 

1. The brain stem 

2. The cerebrum 

3. The cerebellum 

4. The diencephalon 

1. The brain stem: It is also called the medulla oblongata.  

Location: It is located between the pons and the spinal cord, and its size is only about one inch long. 

2. The cerebrum: It is the largest part of the brain. It is the main part of memory and intelligence. The cerebrum is divided into two hemispheres, namely the left and right hemispheres.   

Function of the hemisphere: Left hemisphere controls the activities of the right side of the body and vice a versa. The left and right hemispheres are further divided into four lobes: 

Frontal lobe 

Temporal lobes 

Parietal lobe 

Occipital lobe 

3. The cerebellum: It coordinates muscular activities and maintains balance and posture. This is located behind and below the cerebrum. 

The medulla controls involuntary activities like blood pressure and beating of the heart.  

4. The diencephalon: It comprises of thalamus and hypothalamus. Thalamus is the part where sensory and other impulses go and combine.  the diencephalon helps in regulating consciousness, alertness, and sleep. It also helps in relaying sensory and motor signals to the cerebral cortex. 

Spinal Cord 

It extends from the medulla of the brain through the vertebral column and is protected by the vertebral column or backbone. 

 The spinal cord is a bundle of nerve fibers and connected tissues enclosed within the spine. All parts of the body are connected by the spinal cord to the brain. The spinal cord is enclosed in a vertebral column which is like a bony cage and is surrounded by membranes called meninges. The spinal cord helps in spinal reflex actions and the conduction of nerve impulses to and from the brain. 

The spinal cord is divided into 31 segments. Each segment gives rise to a pair of spinal nerves. 

Functions of the spinal cord: 

• Serve as a conduction pathway for impulses going to and from the brain.  
• Serve as a reflex center.  The reflex arc is the functional unit of the nervous system where many reflexes are facilitated in the spinal cord without going to the higher brain centers. 

White and gray matter 

The CNS can be roughly divided into two tissues namely, white and gray matter. The brain consists of an outer cortex (cerebral cortex) of gray matter, and an inner area consists of white matter. 

Both types of tissue (grey and white) contain glial cells, which protect and support neurons. White matter mostly contains axons (nerve projections) and oligodendrocytes (a type of glial cell), whereas gray matter consists largely of neurons. 

Other parts of the central nervous system 

The meninges are three layers of membranes that cover the brain and the spinal cord. It gives protection to the brain and the spinal cord. It acts as a barrier against bacteria and other microorganisms. 

The cerebrospinal fluid (CSF) is a fluid that circulates around the brain and spinal cord. It gives protection and nourishment to the brain and spinal cord. 

The Peripheral Nervous System (PNS) 

The PNS consists of the nerves that branch out from the brain and spinal cord.  

 The key role of the PNS is to join the CNS to other organs such as limbs, and skin. These nerves spread from the central nervous system to the various areas of the body. Axons or bundles of axons from neuron are the nerves that make up the peripheral nervous system. 

Ganglion is a cluster of neural bodies outside the central nervous system. It functions like a relay station where one nerve enters and the other exits. 

The peripheral system allows the brain and spinal cord to receive and send information to other parts of the body, which allows us to react to stimuli in our environment. 

The peripheral nervous system is divided into two parts:  

i. Somatic nervous system ii. Autonomic nervous system 

The Somatic Nervous System: 

The somatic system is responsible for transmitting sensory information as well as for voluntary movement. This system contains two major types of neurons: 

Motor neurons (efferent neurons, Latin name “efferre” meaning “to bring away from”): The function of these neurons is to transmit the signals originating in the central nervous system to the various organs and muscles and put them into action as per the orders received from the brain. Motor neuron allows us to take physical action in response to the stimuli in the environment. 

Sensory neurons (afferent neurons, Latin name “afferre” meaning “to bring towards”): These neurons carry information from the nerves to the central nervous system. It allows us to take in sensory information and send it to the brain and spinal cord. 

The Autonomic Nervous System (ANS) 

 It is the part of the peripheral nervous system responsible for regulating involuntary body functions, like blood flow, heartbeat, digestion, and breathing.  

The autonomic nervous system is divided into:  

Parasympathetic system: It helps to maintain normal body functions and keep physical resources.  

Sympathetic system: It prepares the body to expend energy to respond to environmental threats by regulating the flight or fight response.  When action is needed, the sympathetic system activates a response by increasing heart rate, increasing breathing rate, boosting blood flow to muscles, starting sweat secretion, and dilating the pupils. 

Two chemical messengers (neurotransmitters) are used to communicate within the autonomic nervous system. They are acetylcholine and norepinephrine. 

Disorders of the nervous system 

• Infection in the nervous system causes meningitis, encephalitis, polio, and epidural abscess. 

• Structural disorders: Such as brain or spinal cord injury, Bell’s palsy (muscular weakness causes one side of the face to droop), cervical spondylosis (age related wear and tear of bones, joints, and discs in the neck), carpal tunnel syndrome (pain in the hand and arm with numbness and tingling), brain or spinal cord tumors (formation of abnormal cells in the tissues of the brain or spinal cord), peripheral neuropathy, and Guillain-Barré syndrome (Body’s immune system attacks nerves). 
• Functional disorders: Such as headache, epilepsy, dizziness, and neuralgia. 
• Degeneration: Such as Parkinson disease (neurological movement disorder), multiple sclerosis, amyotrophic lateral sclerosis (ALS), Huntington chorea, and Alzheimer disease. 

Summary:

• The nervous system is a complex, sophisticated system that regulates and coordinates body activities.
• It is made up of two major divisions: The central nervous system (CNS) and peripheral nervous system (PNS)
• CNS consists of the brain and spinal cord.
• The peripheral nervous system consists of the nerves that branch out from the brain and spinal cord.
• The peripheral nervous system is made up of two parts: i. Somatic nervous system ii. Autonomic nervous system
• The brain consists of an outer cortex of gray matter, and an inner area is of white matter.
• The nervous system can be damaged by injury, infections, degeneration, structural defects, tumors, blood flow disruption, etc.