Human Nervous System

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Nervous system is fully restricted to animals and not found in plants. It is mainly an ectodermal in origin and functions in an integrated manner for control and coordination of various body functions (responding, understanding and memorizing) through neurons and hence also called as a neural system.

The neural system and endocrine system work together to control and coordinate various body parts to maintain physiology. The neural system is basically a control system involve highly specialized cells, called neurons.

Neurons (basic unit of nervous system)

It functions by detecting and receiving information from different sense organs, i.e., receptors in the form of stimuli and transmit the stimuli to the central nervous system.

Neuron or nerve cell is a structural and functional unit of neural tissue. They are known to be the longest cells present in the human body. Human neural system has about 100 billion neurons and they are mostly present in brain.

Structurally, neuron has following two main parts

Cell body – nervous system

It is also known as cyton or soma that vary in size and form. It may be irregular, spherical, oval, round, star-shaped or pyramidal.

They have neurofibrils and Nissl’s granules that play a characteristic role in the body.

Neurofibrils play a significant role in the impulse transmission while Nissl’s granules or Nissl’s bodies are like rough endoplasmic reticulum with numerous attached free ribosomes and polysomes, i.e., synthesize proteins for the cell.

Some neuro-tubules are also present that help in maintaining the shape of the neuron.

Functions: These are mainly associated with metabolic maintenance and growth.

Neurites – nervous system

The process associated with neurons are called neurites.

These are mainly of two types

Dendrites

These are usually shorter, tapering and much-branched structures, which contains neurofibrils, neuro tubules and Nissl’s granules. They conduct nerve impulse towards the cell body and this process is called afferent process (i.e., the receiving process).

Axon

It is a very long structure of uniform thickness. The part of cyton from where the axon arises is known as axon hillock. It is mainly dependent on cell body for the supply of protein.

Each branch of axon fiber terminates as a bulb like structure called synaptic knob that possess some chemicals called neurotransmitters.

Axons mainly conduct nerve impulse away from the cell body and therefore the process is called as an efferent process.

Axons are mainly of two types

Myelinated 

Axons are nerve fibers enveloped with Schwann cells that form a myelin sheath around the axon. It mainly found in spinal and cranial nerves.

Unmyelinated

Axons are nerve fiber enveloped by a Schwann cell that does not form a myelin sheath around the axon and is commonly found in autonomous and motor neurons somatic neural system.

Types of Neurons

Sensory Neurons

They conduct impulses from receptors to central nervous system. The terminal ends of dendrites become modified to form receptor and acts as sensory.

Inter Neurons

They act as connectors and functions in connecting the sensory and motor neuron. It mainly carries stimuli in the brain and spinal cord.

Motor Neurons

They conduct impulses from central nervous system to the effectors i.e., they mainly transmit impulse from brain and spinal cord to the muscle or gland, which will respond to the stimulus.

On the basis of the number of dendrites and axons neurons can be of other three types i.e., Multipolar (with one axon and two or more dendrites), Bipolar i.e. (with one axon and one or more dendrites), Unipolar i.e. with one axon only.

Parts of Human Nervous system

Human nervous system mainly constitutes three different parts

  1. Central Nervous System
  2. Peripheral Nervous System
  3. Autonomic Nervous System

Central Nervous System (CNS)

It is mainly comprised of brain and spinal cord.

Brain – Central Nervous System

It is the anterior portion of the CNS which is lodged in the cranial cavity, i.e., cranium of the skull. It weights from 1220 to 1400 grams.

Structure – Brain

Structurally, it consists of three membranes (meninges) i.e., pia mater membrane (innermost thin, very delicate, vascular and inverts the brain closely), arachnoid membrane (outer to diameter, thin, spider, webby structure) and durameter membrane (outermost, tough fibrous membrane, adhering closely to the inside of the skull).

Cerebrospinal Fluid

The space between arachnoid membrane and piamater is known as subarachnoid space in which a fluid known as cerebrospinal fluid, serves as pad to cushion the central nervous system from shocks. It also provides a medium for exchange of food materials, waste, respiratory gases and other materials

Human brain structurally consists of three main parts

Fore Brain (Prosencephalon)

It includes olfactory lobes, cerebrum, and diencephalon.

i) Olfactory lobes are the anterior part of the brain formed by a pair of short club shaped structures. They are fully covered by the cerebral hemisphere.

Function of these olfactory lobes are concerned with the sense of smell.

ii) Cerebrum is the largest and most complex part of the brain. It mainly have two hemisphere (i.e. left and right) connected by a large bundle of myelinated fibres, the corpus callosum and small bundle of fibers.

Structure of Cerebrum

The outer portion of cerebrum is called the cerebral cortex. The surface of the cortex is greatly folded. The upward folds are known as gyri. There is a cavity present inside the cerebrum whose outer part is known as grey matter and inner part (i.e., beneath the grey matter) is the white matter.

Functions of Cerebrum 

It has sensory areas that receives impulse from receptor motor area that transmits impulse to the effectors

Diencephalon

The main parts involved in diencephalon are epithalaums, thalamus and hypothalamus.

  • Thalamus represents the lateral walls of diencephalon and mainly formed of grey matter. It is the relay station of sensory inputs and does not produce hormones.
  • Epithalamus thin membrane tissue that acts as a posterior segment of the diencephelon
  • Hypothalamus contains a number of centres that control urge for eating and drinking. It also called as thermostat of the body, because it acts as thermoregulatory centre. It also integrates and control the visceral activites.

Functions of Diencephalon

It control and coordinate various functions like body temperature, feeling of satisfaction, appetite, thirst etc.

Mid Brain

It is mainly the portion located in between the thalamus/hypothalamus of the forebrain and pons of the hind brain. There is canal known as cerebral aqueduct that passes through the mid brain.

Mid brain and hind brain together forms the brain stem.

Functions of Midbrain.
  1. It relay impulse back and forth between cerebrum, cerebellum, pons, and medulla.
  2. It is also concerned with sense of sight.
Hind brain

It involves some parts like cerebellum and pons, medulla.

Cerebellum 

It is second largest part of the human brain. It has convoluted surface in order to provide more space for many more neurons.

Functions of Cerebellum

It controls rapid muscular activites such as running typing and even talking. Nearly all activities are involuntary in nature but sometimes may involve learning in early stages.

Pons It is located below mid brain and above the medulla oblongata

Functions of Pons: It relays impulses between medulla to regulate breating

Medulla (Oblongata)

It extends from the pons varolii in above direction and is continuous with the spinal cord in below direction.

Functions of Medulla

It functions as receiver, integrates signal from spinal cord and sends resulting signal to the cerebellum and thalamus.

It mainly have centres that regulate heart rate, blood prssure, breating, swallowing salivation, sneezing, coughing, vomiting and other involuntary movements etc.

Spinal Cord (Myelon) – Central Nervous System

It extends from the base of the brain and is continuous to the second lumbar vertebra. In adult, the spinal cord ranges from 42 to 45 cm in length. It manly lies in the neural canal fo the vertebral column. It is basically the posterior part of CNS which runs mid-dorsally within the vertebral column. The three meninges, i.e., duramater, arachnoid and piameter which covers the brain, also continue over the spinal cord.

The two indentations, i.e., posterior median sulcus and the anterior median tissues separates the spinal cord into left and right halves. The inner area is the grey matter, while outside to it are white columns called the white matter.

Functions of Spinal Cord

i) The stimuli pass from and to the brain through the spinal cord.

ii) It also acts as the center of spinal reflex action.

Peripheral Neural System (PNS)

The nerves that originate from central nervous system connect either with receptor or effector organs from peripheral neural system and those nerves which arise from brain are called cranial nerves while the nerves originating from the spinal cord are termed as spinal nerves.

It relays impulse from the CNS to skeletal muscles. In human body there are 12 pairs of cranial nerves and 31 pairs of spinal nerves.

Autonomic Neural System (ANS)

It transmits impulse from the CNS to the involuntary organs and smooth muscles of the body. This system was discovered by Langly in 1921.

It is further divided into two types.

Sympathetic Nervous system

Accelerates heart beat, enlarge pupils, supply blood to muscles, contract nerves of urinary bladder, lower the intestinal digestion activities, helps in blood clotting, increased secretion of sweat glands, make breathing easier and promote liver to release sugar and decrease bile production are some activities controlled by this nervous system.

Parasympathetic Nervous System

Works just analogous to the sympathetic nervous system, i.e., slows down heart beat. dilates arteries and lower blood pressure, speeds up peristalsis, stimulate salivary gland secretion, contracts gall bladder, promotes pancreas for secretion, decreases the secretion of sweat glands etc.

 

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