Human Nervous System - Definition, Parts, Functions

Nervous system is a complex controlling and coordinating system of the body. It receives stimulus, processes the informations and gives response through muscles and glands.

Human nervous system is a highly complex biological network of the human body. It is responsible for coordinating body functions, receiving sensory informations and producing different body responses.

It has three basic functions. These are sensation, integration and response.

Sensation is the process by which nervous system receives information about changes in internal and external environment. These changes act as stimulus. The information is carried to the nervous system.

Integration is the process in which the received sensory input is processed. It may combine with memory and emotion. After processing, proper decision is made by the nervous system.

Response is the action produced after integration. In this process, nervous system sends commands to muscles and glands. Then the body reacts according to the stimulus.

The nervous system is built up by two fundamental types of cells. These are neurons and glial cells.

Neurons are the primary functional cells of nervous system. They are electrically active cells. They compute and communicate informations through electrical and chemical signals.

Glial cells are the supporting cells of nervous system. They maintain the chemical environment around neurons. They form protective insulation around axons, called myelin. They also provide immune defence for proper functioning of neurons.

Anatomically, nervous system is divided into two main parts. These are Central Nervous System (CNS) and Peripheral Nervous System (PNS).

Central Nervous System (CNS) consists of brain and spinal cord. It acts as the main command and processing centre of the body. Most of the information is processed here.

Peripheral Nervous System (PNS) is the network of nerves and ganglia outside brain and spinal cord. It connects the CNS with limbs, skin and internal organs. It acts as a communication pathway between central nervous system and other body parts.

Functionally, nervous system is divided into somatic nervous system and autonomic nervous system.

Somatic nervous system controls voluntary and conscious activities. It mainly controls movement of skeletal muscles. These activities occur under conscious control.

Autonomic nervous system controls involuntary activities of the body. It regulates heart, smooth muscles and glandular tissues. It works automatically without conscious thought. It helps to maintain internal balance or homeostasis.

Characteristics of Nervous System

The following are the important characteristics of nervous system–

Function- The nervous system has three basic function i.e. sensation, integration and response. Sensation refers to receiving of informations from internal and external environment. Integration is the processing of this received informations. Response is the action produced by muscles or glands after command from nervous system.

Signalling- Nervous system shows two types of signalling. These are electrical signalling and chemical signalling. Electrical signalling takes place inside the neuron as nerve impulse. Chemical signalling takes place between neurons with the help of neurotransmitters.
Speed- It is a fast acting system of the body. The nerve impulse passes very rapidly through nerve fibres. So any change is detected and response is given in a short time. The response is also precise due to fixed nerve pathway.
Cells- Nervous system is made up of two main cells. These are neurons and glial cells. Neurons are the conducting cells which carry nerve impulse. Glial cells are supporting cells which gives support, protection and insulation to neurons.

Anatomy- On the basis of structure, nervous system is divided into two parts. These are Central Nervous System (CNS) and Peripheral Nervous System (PNS). CNS includes brain and spinal cord. PNS includes nerves and ganglia outside brain and spinal cord.
Control- On the basis of function, nervous system is divided into somatic nervous system and autonomic nervous system. Somatic nervous system is related with voluntary activity. Autonomic nervous system is related with involuntary activity like heart beat, breathing and gland activity.
Homeostasis- Nervous system helps in maintaining homeostasis. It receives informations about body condition and outside changes. According to this, it regulates different organs and organ systems.

Coordination- Nervous system acts as the controlling and coordinating system of body. It controls muscles, glands and organs. Coordination between different body parts is maintained by nervous system.

Cells of the Nervous System

The cells of nervous system are mainly divided into two types-

Neurons-Neurons are the main functional cells of nervous system. They are electrically active cells which receive, process and transmit informations. The impulse is carried by electrical and chemical signals.
- Parts- A typical neuron consists of cell body, dendrites and axon. Cell body is also called soma and contains nucleus and cytoplasm. Dendrites are short branched processes which receive impulse. Axon is long fibre like process which carries impulse away from cell body.
- Types- On the basis of structure, neurons are of three types. These are unipolar, bipolar and multipolar neurons. Unipolar neurons are mostly sensory neurons. Bipolar neurons are found in retina and olfactory region. Multipolar neurons are most common type and having one axon and many dendrites.
Glial cells-Glial cells or neuroglia are supporting cells of nervous system. They do not conduct impulse like neurons. They gives support, protection, nutrition and proper environment to neurons. They also help in formation of myelin sheath.
- CNS glia- The glial cells present in Central Nervous System (CNS) are astrocytes, oligodendrocytes, microglia and ependymal cells.
  - Astrocytes- Astrocytes are star shaped glial cells. They give structural support to neurons. They regulate chemical environment around nerve cells. They also help in maintenance of blood-brain barrier.
  - Oligodendrocytes- Oligodendrocytes are glial cells of CNS. They form myelin sheath around axons. This sheath acts as insulating layer and increases speed of nerve impulse in brain and spinal cord.
  - Microglia- Microglia are immune cells of CNS. They act like macrophages. They remove damaged cells, dead materials and pathogens from nervous tissue.
  - Ependymal cells- Ependymal cells line the ventricles of brain and central canal of spinal cord. They help in production and circulation of cerebrospinal fluid (CSF).
- PNS glia- The glial cells present in Peripheral Nervous System (PNS) are Schwann cells and satellite cells.
  - Schwann cells- Schwann cells form myelin sheath around axons in peripheral nerves. They insulate nerve fibres and also help in repair of damaged peripheral nerve.
  - Satellite cells- Satellite cells surround neuron cell bodies in peripheral ganglia. They give support and protection to cell body and maintain surrounding chemical environment.

Organization of the Human Nervous System

The human nervous system is organised into different divisions. It is mainly divided on the basis of structure and function.

CNS-Central Nervous System (CNS) is the main controlling part of nervous system. It includes brain and spinal cord. It receives informations and also processes the informations. The main response is also formed from this part.
- Brain- Brain is present inside the skull. It is the chief centre of nervous system. It controls sensation, integration, response, memory and emotion.
- Spinal cord- Spinal cord is present inside vertebral column. It connects brain with body through nerves. It also acts as centre for reflex action.

PNS-Peripheral Nervous System (PNS) includes nerves and ganglia outside the brain and spinal cord. It connects CNS with different body parts like skin, limbs, muscles and internal organs.
- Afferent division- It is also called sensory division. It carries impulses from receptors to CNS. So informations from body and sense organs reach to central nervous system.
- Efferent division- It is also called motor division. It carries command from CNS to muscles and glands. These muscles and glands are called effectors.
  - Somatic nervous system- It controls voluntary activities. It is concerned with skeletal muscles. The movement which occur under conscious control are controlled by this system.
  - Autonomic nervous system- It controls involuntary activities. It is concerned with heart, smooth muscles and glands. It works automatically and helps in maintaining homeostasis.
    - Sympathetic division- It works mainly during stress and emergency condition. It produces fight or flight response. Heart rate increases and digestion becomes slow.
    - Parasympathetic division- It works mainly during resting condition. It produces rest and digest response. Heart rate becomes slow and digestion is stimulated.
  - Enteric nervous system- It is present in the wall of digestive tract. It is a semi-independent nervous system. It controls smooth muscle movement and gland secretion of digestive system.

Diagram showing the axon of a neurone in relation to the associated oligodendrocyte and myelin sheath

Central Nervous System (CNS)

Central Nervous System (CNS) is the main controlling part of nervous system. It is present in the central axis of the body. It receives informations, processes them and then sends command to the body.

The following are the important features of CNS–

Components-CNS is made up of two main organs. These are brain and spinal cord. The brain is present inside the cranial cavity and spinal cord is present inside vertebral cavity.
- Brain- Brain is the master control centre of the body. It controls sensation, motor command and higher nervous functions. These higher functions include memory, emotion, consciousness and thinking. Adult brain is divided into cerebrum, diencephalon, brain stem and cerebellum.
- Spinal cord- Spinal cord is present inside the vertebral column. It acts as main communicating pathway between brain and remaining body parts. It also works as centre for reflex action.

Retina- Retina is the neural tissue of the eye. It is structurally considered as an extension of CNS. It receives light stimulus and helps in vision.
Protection-CNS is highly protected because it is very important and delicate part. It is protected by bones, membranes and fluid.
- Bony covering- Brain is protected by skull and spinal cord is protected by vertebral column. This bony covering gives mechanical protection to CNS.
- Meninges- Meninges are three protective membranes present around brain and spinal cord. These membranes cover the CNS and protect it from injury.
- CSF- Cerebrospinal fluid (CSF) is the fluid present around brain and spinal cord. It acts as cushion. It also helps in protection and nourishment of CNS.
- BBB- Blood-brain barrier (BBB) is a selective barrier present in brain blood vessels. It prevents harmful substances, pathogens and many unwanted materials from entering into CNS.

Tissue- The tissue of CNS is mainly of two types. These are gray matter and white matter.
- Gray matter- Gray matter is formed mainly by neuron cell bodies and dendrites. It is the region where many processing activity takes place.
- White matter- White matter is formed mainly by axons. These axons are covered by fatty myelin sheath. Due to presence of myelin it appears white in colour.

Terms- In CNS, some special terms are used for arrangement of nerve cells and fibres.
- Nucleus- A nucleus is a localized group of neuron cell bodies inside CNS.
- Tract- A tract is a bundle of axons present inside CNS. It carries nerve impulses from one region to another region.

Brain: Structure and Major Divisions

Brain is the main controlling part of Central Nervous System (CNS). It is present inside the skull. Adult brain is mainly divided into four major regions.

The following are the major divisions of brain–

Cerebrum-Cerebrum is the largest part of brain. It is divided into right and left cerebral hemispheres by longitudinal fissure. The outer layer is called cerebral cortex. It is made up of gray matter and is highly folded. It controls higher nervous functions.
- Frontal lobe- Frontal lobe is mainly related with motor functions. It controls planning of movement. It is also concerned with personality, thinking and decision making.
- Parietal lobe- Parietal lobe receives and processes general body sensations. These include touch, pressure, pain and proprioception. It helps to know body position.
- Temporal lobe- Temporal lobe is mainly related with hearing. It also helps in formation of long term memory. It is present on lateral side of cerebrum.
- Occipital lobe- Occipital lobe is related with vision. It receives visual stimulus and helps in interpretation of visual informations.
- Subcortical structures- These structures are present below the cerebral cortex. It includes basal nuclei, hippocampus and amygdala. Basal nuclei helps in control of movement. Hippocampus is related with memory. Amygdala is related with emotion.
Diencephalon-Diencephalon is present deep below the cerebrum. It connects cerebrum with other parts of nervous system. The main parts are thalamus and hypothalamus.
- Thalamus- Thalamus is made up of group of nuclei. It acts as relay centre for sensory and motor informations. It sends informations to cerebral cortex from spinal cord, brain stem and periphery.
- Hypothalamus- Hypothalamus is the important controlling region. It regulates homeostasis, autonomic nervous system and endocrine system. It controls body temperature, hunger, thirst and many internal activities.
Brain stem-Brain stem is present at the base of brain. It connects brain with spinal cord. It controls important involuntary functions like respiration, heart rate and blood pressure.
- Midbrain- Midbrain is the upper part of brain stem. It coordinates visual, auditory and somatosensory reflex activities. It also helps in movement control.
- Pons- Pons is present between midbrain and medulla. It acts as connecting bridge between brain stem and cerebellum. It also helps in regulation of respiration.
- Medulla oblongata- Medulla oblongata is the lowest part of brain stem. It continues with spinal cord. It contains important centres for breathing, heart activity and blood pressure control.
Cerebellum- Cerebellum is also called little brain. It is present at the posterior side of brain stem. It compares motor command with sensory feedback. It helps in coordination of voluntary movement, posture and balance.

Spinal Cord: Structure and Functions

Spinal cord is a long tube like part of Central Nervous System (CNS). It is present inside the vertebral column. It connects brain with different parts of body.

Structure of spinal cord

Location- Spinal cord is present in the vertebral cavity of vertebral column. It is protected by bony vertebrae. It is also covered by three protective membranes called meninges.
Regions- The spinal cord has five major regions. These are cervical, thoracic, lumbar, sacral and coccygeal regions. These are named according to the region of spine from where nerves arise.

Length- The solid spinal cord does not extend through full length of vertebral column. It ends in lower back region. Below it, nerve roots pass downward and form cauda equina. It looks like horse tail.
Gray matter- In cross section, the central part contains gray matter. It is butterfly shaped or H-shaped. It contains neuron cell bodies and is divided into horns.
- Dorsal horn- Dorsal horn receives sensory informations from body.
- Ventral horn- Ventral horn contains motor neurons. These neurons send impulse to skeletal muscles.
- Lateral horn- Lateral horn contains autonomic motor neurons. It is present mainly in thoracic and upper lumbar region.
White matter-White matter surrounds the gray matter. It is mainly made up of myelinated axons. It is divided into posterior, anterior and lateral columns.
- Ascending tracts- These tracts carry sensory impulses towards brain.
- Descending tracts- These tracts carry motor commands from brain towards spinal cord.

Spinal nerves-Spinal cord gives rise to 31 pairs of spinal nerves. These nerves come out between vertebrae. Each spinal nerve is formed by dorsal root and ventral root.
- Dorsal root- Dorsal root carries sensory informations into spinal cord.
- Ventral root- Ventral root carries motor commands out from spinal cord.

Central canal- Central canal is a small canal present in the centre of spinal cord. It contains cerebrospinal fluid (CSF). It gives internal cushioning and helps in transport of materials.

Functions of spinal cord

Communication- Spinal cord acts as main communication pathway between brain and body. Sensory informations pass upward to brain. Motor commands from brain pass downward to body.
Sensory transmission- It receives sensory impulses from skin, muscles, joints and internal organs. These impulses are carried to brain for processing.

Motor transmission- It receives motor command from brain. Then it sends these commands to muscles and glands. It helps in voluntary movement and some involuntary activities.
Reflex action- Spinal cord acts as centre for reflex action. In this process response is produced quickly. It does not wait for conscious processing by brain. Sensory and motor pathways are connected locally in spinal cord.

Peripheral Nervous System (PNS)

Peripheral Nervous System (PNS) is the part of nervous system present outside the brain and spinal cord. It includes nerves and ganglia. It connects Central Nervous System (CNS) with limbs, skin, muscles and internal organs.

The following are the important parts of PNS–

Nerves- Nerves are bundles of axons present outside the CNS. They carry impulses from one part to another part. In human, there are 12 pairs of cranial nerves and 31 pairs of spinal nerves. Cranial nerves arise from brain. Spinal nerves arise from spinal cord.
Ganglia- Ganglia are small swelling like structures present along peripheral nerves. They contain group of neuron cell bodies. They act as relay points for transmission of informations.
Afferent division- It is also called sensory division. It carries sensory informations from receptors to CNS. The receptors may be present in skin, joints, muscles and internal organs.

Efferent division- It is also called motor division. It carries motor commands from CNS to effectors. The effectors are muscles and glands.
- Somatic nervous system- Somatic Nervous System (SNS) controls voluntary and conscious activities. It sends motor commands to skeletal muscles. It is involved in walking, writing, moving arm and other skeletal muscle activity.
- Autonomic nervous system-Autonomic Nervous System (ANS) controls involuntary activities. It controls cardiac muscle, smooth muscle and glands. It helps in maintaining homeostasis of body.
  - Sympathetic nervous system- Sympathetic nervous system acts during stress and emergency condition. It produces fight or flight response. It increases heart beat and slows digestion.
  - Parasympathetic nervous system- Parasympathetic nervous system acts during rest condition. It produces rest and digest response. It slows heart beat and stimulates digestion.
- Enteric nervous system- Enteric Nervous System (ENS) is a network of neurons present in wall of digestive tract. It controls movement and secretion of digestive system. It works partly independent but also connected with autonomic nervous system.

Functional Divisions of the Nervous System

The nervous system is functionally divided in two main ways. One is on the basis of basic function. Another is on the basis of body control.

Basic function- The nervous system performs three basic functions. These are sensation, integration and response.
- Sensation- Sensation is the receiving of stimulus from external and internal environment. The stimulus may come from skin, muscles, joints, sense organs and internal organs. Sensory structures detect these changes and send the informations to nervous system.
- Integration- Integration is the processing of received sensory informations. In this process the stimulus is compared with other stimulus, memory and emotional condition. After this, the nervous system decides the proper reaction.
- Response- Response is the motor function of nervous system. It is the reaction produced after integration. The response may be voluntary contraction of skeletal muscle or involuntary activity of heart, smooth muscle and glands.
Body control- On the basis of control of body response, nervous system is divided into somatic nervous system, autonomic nervous system and enteric nervous system.
- Somatic nervous system- Somatic Nervous System (SNS) is related with conscious perception and voluntary motor response. It mainly controls skeletal muscles. Walking, writing and movement of limbs are controlled by this system. Some automatic skeletal muscle response like reflex action are also controlled by it.
- Autonomic nervous system-Autonomic Nervous System (ANS) controls involuntary activities of body. It controls cardiac muscle, smooth muscle and glandular tissue. It helps in maintaining homeostasis of the body. It has two main divisions.
  - Sympathetic system- Sympathetic nervous system acts mainly during stress, fear and emergency condition. It produces fight or flight response. Heart beat increases and digestion becomes slow.
  - Parasympathetic system- Parasympathetic nervous system acts mainly during resting condition. It produces rest and digest response. It slows heart beat and stimulates digestion. It works opposite to sympathetic system in many cases.
- Enteric nervous system- Enteric Nervous System (ENS) is a special network of neurons present in digestive tract. It controls smooth muscle and glands of digestive system. It is concerned with movement, secretion and other gastrointestinal functions.

Autonomic Nervous System (ANS)

Autonomic Nervous System (ANS) is a functional division of Peripheral Nervous System (PNS). It controls the involuntary activities of the body. It works continuously without conscious effort and maintains internal balance of body.

The following are the important features of ANS–

Role- ANS is responsible for involuntary control of the body. It regulates those activities which are not under conscious control. It helps in maintaining homeostasis.
Target tissue- ANS controls mainly the visceral organs. It supplies cardiac muscle, smooth muscle and glands. Heart beat, digestion, secretion and blood vessel diameter are controlled by this system.
Neurotransmitters- ANS uses chemical messengers for communication with target organs. The main neurotransmitters are acetylcholine and norepinephrine. Norepinephrine is also called noradrenaline.

Divisions-ANS has three major divisions. These are sympathetic nervous system, parasympathetic nervous system and enteric nervous system. These divisions regulate organ functions and in many organs their actions are opposite.
- Sympathetic system- Sympathetic Nervous System is also called fight or flight system. It works during stress, fear and emergency condition. It prepares the body for sudden action. It increases heart rate. It dilates airways. It shifts more blood towards skeletal muscles. It also decreases non-essential activity like digestion for temporary period. Its nerve fibres arise from thoracic and upper lumbar region of spinal cord, so it is called thoracolumbar system.
- Parasympathetic system- Parasympathetic Nervous System is also called rest and digest system. It works mainly during resting condition. It brings the body back to normal relaxed state. It slows heart beat. It lowers blood pressure. It stimulates digestion, urination and salivation. Its nerve fibres arise from brain stem and sacral region of spinal cord, so it is called craniosacral system.
- Enteric system- Enteric Nervous System (ENS) is a special network of neurons present in wall of digestive tract. It is highly complex and contains large number of neurons. It controls digestive activities. It regulates movement of digestive tract, called peristalsis. It also controls glandular secretion of digestive system. It can work semi-independently from central nervous system.

Structure of a Neuron

Neuron is the structural and functional unit of nervous system. It is a specialised cell which receives and conducts nerve impulse. A typical neuron has cell body, dendrites and axon.

The following are the important parts of a neuron–

Cell body- Cell body is also called soma. It is the main part of neuron. It contains nucleus, cytoplasm and other cell organelles. It acts as the metabolic centre of the neuron.

Dendrites- Dendrites are short and highly branched processes of neuron. They receive incoming impulse from other neurons. The contact point between two neurons is called synapse.
Axon- Axon is a single long fibre like process. It arises from the cell body. It carries outgoing impulse away from cell body to another neuron, muscle or gland.
Axon hillock- Axon hillock is the tapering region of cell body from where axon arises. It continues into initial segment of axon. This region acts as trigger zone because action potential is started here.

Myelin sheath- Myelin sheath is a fatty insulating covering around many axons. It is formed by glial cells. It protects the axon and increases the speed of nerve impulse conduction.
Nodes of Ranvier- Nodes of Ranvier are small gaps present between two myelin segments. These parts are not covered by myelin. They help in rapid conduction of nerve impulse along the axon.
Axon terminals- Axon terminals are the terminal branches of axon. These are present at the end of axon. Each terminal ends into small swollen part called synaptic end bulb.

Synaptic end bulb- Synaptic end bulb stores chemical messengers called neurotransmitters. These neurotransmitters are released into synapse. They help to pass the impulse from one neuron to another cell.

Diagram showing some of the glial cells in relation to a neuron

Classification of Neurons

Neurons are classified mainly on the basis of structure. It depends on the shape of neuron and number of processes arise from the cell body.

The following are the structural types of neurons–

Unipolar neuron- Unipolar neuron has only one process arising from the cell body. This process divides into two branches and extends for long distance. It is also called pseudo-unipolar neuron. These neurons are sensory in function and their cell bodies are present in peripheral ganglia.

Bipolar neuron- Bipolar neuron has two processes. One is axon and another is dendrite. These two processes arise from opposite sides of the cell body. These neurons are rare and mainly found in special sensory organs like retina and olfactory epithelium.
Multipolar neuron- Multipolar neuron has one axon and two or more dendrites. It is the most common type of neuron in nervous system. Most of the motor neurons and interneurons are multipolar type.
Anaxonic neuron- Anaxonic neuron is a very small neuron. It has many processes but axon cannot be clearly distinguished from dendrites. Any process may act like axon according to the condition.

Other classifications of neurons are also used-

Function- Neurons may be classified according to their function. Example, sensory neuron, motor neuron and interneuron. Sensory neurons carry impulse to CNS. Motor neurons carry impulse from CNS to effectors. Interneurons are present between sensory and motor neurons.
Location- Neurons may be classified according to their location. Example, olfactory neurons are present in olfactory region and related with smell.
Discoverer- Some neurons are named according to scientist who discovered them. Example, Purkinje cells are present in cerebellum.

Appearance- Some neurons are classified according to their shape or appearance. Example, pyramidal cells have pyramid shaped cell body.
Neurotransmitter- Neurons may also be classified according to chemical messenger used by them. These chemicals are called neurotransmitters and are used for communication with target cells.

Protection of the Nervous System

Nervous system is protected by different protective coverings and special barriers. CNS is more protected because brain and spinal cord are very delicate organs.

Protection of CNS

Bone- The Central Nervous System (CNS) is covered by bony covering. Brain is protected inside the skull. Spinal cord is protected inside the vertebral column. This gives mechanical protection from injury.
Meninges- Meninges are three protective membranes around brain and spinal cord. These layers cover the CNS. The outer most tough layer is called dura mater. It attaches the CNS with inner side of cranial and vertebral cavity.
CSF- Cerebrospinal fluid (CSF) is a special fluid present in ventricles of brain, central canal of spinal cord and spaces of meninges. It acts as liquid cushion. It protects nervous tissue from shock. It also helps in removal of metabolic wastes.
BBB- Blood-brain barrier (BBB) is a selective barrier of CNS. It prevents many harmful substances, pathogens and some white blood cells from entering into brain tissue. It is maintained with the help of star shaped astrocytes.
Microglia- Microglia are immune cells of CNS. They work like macrophages. They ingest and destroy pathogens, diseased cells and damaged cells. This gives immune protection inside CNS.

Protection of PNS

Sheaths-Peripheral Nervous System (PNS) is not protected by bone like CNS. So peripheral nerves are protected by connective tissue sheaths. These sheaths cover the whole nerve, nerve bundles and individual axons.
- Epineurium- Epineurium is the outermost fibrous connective tissue layer. It surrounds the whole nerve.
- Perineurium- Perineurium surrounds smaller bundles of axons. These bundles are called fascicles. It protects the fascicles inside the nerve.
- Endoneurium- Endoneurium is loose connective tissue layer around each individual axon. It gives protection to single nerve fibre.

Cellular protection

Myelin- Myelin sheath is a fatty insulating layer around many axons. It is formed by oligodendrocytes in CNS and Schwann cells in PNS. It protects the nerve fibre and increases the speed of electrical impulse.

Disorders of the Human Nervous System

Disorders of nervous system are the diseases in which brain, spinal cord, nerves, neurons or myelin sheath are affected. These disorders may cause loss of movement, loss of sensation, memory loss, seizure, weakness and other nervous symptoms.

The following are some important disorders of human nervous system–

Alzheimer’s disease- Alzheimer’s disease is a neurodegenerative disease of brain. It is caused by abnormal deposition of beta-amyloid plaques and loss of neurons in basal forebrain. It causes progressive loss of memory, cognitive decline and dementia.
Parkinson’s disease- Parkinson’s disease is a hypokinetic motor disorder. It is caused by degeneration of dopamine producing neurons in substantia nigra pars compacta of basal nuclei. It is also related with toxic accumulation of alpha-synuclein protein. It causes slow movement, tremor and muscular rigidity.
Amyotrophic lateral sclerosis- Amyotrophic Lateral Sclerosis (ALS) is also called Lou Gehrig’s disease. It is a progressive motor neuron disease. In this disease, upper and lower motor neurons are destroyed. It causes muscle weakness, loss of voluntary movement, paralysis and later respiratory failure.
Huntington disease- Huntington disease is a genetic disorder. It is an autosomal dominant disease. It occurs due to accumulation of huntingtin protein which kills neurons in basal ganglia. It causes abnormal jerky movement called chorea.
Creutzfeldt-Jakob disease- Creutzfeldt-Jakob disease is a prion disease. It is related with human form of mad cow disease. In this disease, abnormal prion protein and amyloid plaques accumulate in brain. It causes rapid nervous degeneration.
Multiple sclerosis- Multiple Sclerosis (MS) is an autoimmune demyelinating disease of Central Nervous System (CNS). In this disease immune system attacks oligodendrocytes and destroys myelin sheath. Hardened scar or sclerosis is formed. Due to this nerve impulse becomes slow and motor, sensory and autonomic problems occur.
Guillain-Barré syndrome- Guillain-Barré syndrome is an autoimmune disease of Peripheral Nervous System (PNS). It causes inflammation and damage of peripheral nerves. It produces sensory symptoms, motor weakness and autonomic problems like fall in blood pressure.
Neuromyelitis optica- Neuromyelitis Optica Spectrum Disorder (NMOSD) is a rare autoimmune demyelinating disease. In this disease antibodies attack aquaporin-4 channels of astrocytes. It damages blood-brain barrier and causes demyelination.
Myasthenia gravis- Myasthenia gravis is an autoimmune neuromuscular disorder. In this disease antibodies attack the neuromuscular junction of skeletal muscles. It causes progressive muscle weakness and severe fatigue.
Charcot-Marie-Tooth disease- Charcot-Marie-Tooth (CMT) disease is a genetic neuromuscular disorder. It affects peripheral nerves. It mainly involves arms, hands, legs and feet. It causes weakness and loss of normal nerve function.
Sciatica- Sciatica is a painful condition of sciatic nerve. It occurs due to inflammation, compression or irritation of sciatic nerve or spinal nerves forming it. Pain extends from hip to posterior side of leg.
Epilepsy- Epilepsy is a neurological disorder with repeated uncontrolled seizures. Seizure occurs due to abnormal electrical activity of neurons. It may be related with genetic defects, astrocyte dysfunction or other brain abnormalities.
Cerebral palsy- Cerebral Palsy (CP) is a group of movement disorders. It occurs due to brain damage during pregnancy, childbirth or early infancy. It affects movement, posture and balance permanently.
Alexander disease- Alexander disease is a genetic astrocyte disorder. It is also called astrogliopathy. It is caused by mutation in glial fibrillary acidic protein (GFAP). It causes white matter lesions mainly in frontal lobe of brain.
Anosmia- Anosmia is the complete loss of smell. It may occur due to damage of olfactory nerve by trauma, inflammation, drugs or aging. In this condition person cannot detect smell properly.
Vascular dementia- Vascular dementia is a form of dementia due to reduced blood supply to brain. It may occur after stroke or other vascular damage. Due to lack of blood supply, neurons die and memory and thinking ability becomes affected.

Clinical Significance of the Nervous System

Nervous system has great clinical significance because different diseases of brain, spinal cord, nerves and muscles produce different clinical symptoms. These disorders may affect movement, sensation, memory, speech, reflex and autonomic function.

The following are the important clinical conditions related with nervous system–

Alzheimer’s disease- Alzheimer’s disease is a neurodegenerative disease. It is caused by abnormal accumulation of beta-amyloid plaques in brain. Due to this neurons are damaged and progressive memory loss, cognitive decline and dementia occurs.
Parkinson’s disease- Parkinson’s disease is a motor disorder. It occurs due to death of dopamine producing neurons in substantia nigra. It is also related with toxic alpha-synuclein protein deposition. L-DOPA is used in treatment to increase dopamine level.
ALS- Amyotrophic Lateral Sclerosis (ALS) is also called Lou Gehrig’s disease. It destroys upper and lower motor neurons. It causes muscle weakness, loss of voluntary movement, paralysis and finally respiratory arrest.
Huntington disease- Huntington disease is an autosomal dominant genetic disorder. In this disease neurons of basal ganglia are damaged. It causes cognitive defect and uncontrolled jerky movement called chorea.
Creutzfeldt-Jakob disease- Creutzfeldt-Jakob disease is a fatal prion disease. It is caused by abnormal prion protein accumulation in brain. It produces toxic amyloid plaques and rapid degeneration of nervous tissue.
Multiple sclerosis- Multiple Sclerosis (MS) is an autoimmune demyelinating disorder of CNS. In this disease immune system attacks oligodendrocytes and destroys myelin sheath. Due to this electrical signalling becomes slow and motor, sensory and autonomic defects are produced.
Guillain-Barré syndrome- Guillain-Barré syndrome is autoimmune demyelinating disease of PNS. It affects peripheral nerves. It causes sensory symptoms, motor weakness and autonomic failure like fall of blood pressure.
NMOSD- Neuromyelitis Optica Spectrum Disorder (NMOSD) is a rare autoimmune disease. In this disease autoantibodies attack aquaporin-4 water channels of astrocytes. It causes demyelination. If brain stem is affected then severe nausea and vomiting may occur.
Spinal cord injury- Spinal cord injury causes loss of function below the level of injury. Complete cut may cause paraplegia or tetraplegia. Half cut of spinal cord causes paralysis on injured side, loss of fine touch on injured side and loss of pain sensation on opposite side.
Stroke- Stroke occurs due to interruption of blood supply to brain. The symptoms depends on affected region of brain. Left frontal lobe damage may cause speech loss called Broca’s aphasia. Right parietal lobe damage may cause neglect of left side of body or visual field.
Motor neuron lesion- Motor neuron lesions are of two types. These are upper motor neuron lesion and lower motor neuron lesion. Upper motor neuron lesion causes spasticity, hyperreflexia and positive Babinski sign. Lower motor neuron lesion causes flaccid paralysis, hyporeflexia and muscle wasting.
Myasthenia gravis- Myasthenia gravis is an autoimmune disease of neuromuscular junction. It affects skeletal muscle. It causes progressive muscle weakness and severe fatigue.
Sciatica- Sciatica occurs due to inflammation or compression of sciatic nerve. It may also affect spinal nerves forming the sciatic nerve. Pain passes from hip to posterior side of leg.
CMT disease- Charcot-Marie-Tooth (CMT) disease is a genetic neuromuscular disorder. It affects peripheral nerves of arms, hands, legs and feet. It causes weakness and sensory problems.
Neurotoxins- Some natural toxins affect nervous system. Tetrodotoxin, saxitoxin and ciguatoxin affect voltage-gated sodium channels. They may cause paraesthesia, muscle weakness, paralysis, respiratory failure and death.
Channelopathies- Channelopathies are disorders due to mutation in ion channels or its regulators. It produces abnormal electrical activity in nervous tissue. This may cause uncontrolled action potentials and epilepsy.
Anosmia- Anosmia is complete loss of smell. It may occur due to blunt head injury, damage of olfactory nerve, some drugs or aging.
Meningitis vision loss- Meningitis is inflammation of meninges. Severe swelling around optic nerve may affect vision. It may cause visual impairment in some cases.

Functions of Nervous System

The nervous system performs three basic functions. These are sensation, integration and response.

Sensation- Sensation is the receiving of informations from external and internal environment. These changes are called stimuli. The stimulus may be taste, smell, touch, sight and hearing. Internal stimulus may be stretch of organ wall or change inside the body.
Integration- Integration is the processing of received sensory informations. These informations are carried to nervous system. Here it is compared with other sensations, memory, thinking and emotional state. After this proper reaction is selected.
Response- Response is the motor action produced by nervous system. It occurs after sensation and integration. It may be voluntary action like contraction of skeletal muscles. It may also be involuntary action like contraction of cardiac muscle, smooth muscle and secretion of glands.

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Human Nervous System – Definition, Parts, Functions