HUMAN NERVOUS SYSTEM

The nervous system is a highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of the body. It is fully restricted to animals and not found in plants. It is mainly 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 neural system.
The neural system and endocrine system work together to control and coordinate various body parts to maintain physiology. The neural system involves highly specialized cells, called neurons.

NEURONS (Basic unit of nervous system)

They function 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. Neurons communicate with other cells via synapses, which are membrane-to-membrane junctions containing molecular machinery that allows rapid transmission of signals, either chemical or electrical.

Neurons or nerve cells are the structural and functional unit of neural tissues. They are known to be the longest cells present in the human body. Human neural system has about 100 billion neurons.
Structurally, neurons has two main parts:

CELL BODY

It is also known as cyton or soma that varies in sizes and forms. It may be spherical, irregular, 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., synthesizes proteins for the cell.
Some neuro-tubules are also present that helps in maintaining the shape of the neuron.

NEURITES

The processes associated with neurons are called neuritis. These are mainly of two types:
• DENDRITES

These are usually shorter, tapering and much branched structures, which contains neurofibriles, neurotubules 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 the cell body for the supply of protein.
Each branch of axon fibre terminates as a bulb like structure called synaptic knob that possess some chemicals called neurotransmitters. Axons mainly conducts nerve impulse away from the cell body and therefore the process is called as efferent process.

TYPES OF NEURONS

There are mainly three types of neurons:

SENSORY NEURONS(Afterent neurons)- They conduct impulses from receptors to central nervous system. The terminal ends of dendrites become modified to form receptor and acts as sensory neurons.
INTER NEURONS- They act as connectors and functions in connecting the sensory and motor neurons. It mainly carries stimuli in 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 muscles or glands, which will respond to the stimulus.
On the basis of number of dendrites and axons, neurons can be of other three types:

# Multipolar (with one axon and two or more dendrites).
# Bipolar (with one axon and one or more dendrites).
# Unipolar (with one axon only).

HUMAN  NERVOUS  SYSTEM

Human nervous system conducts stimuli from sensory receptors to the brain and spinal cord and conducts impulses back to other parts of the body. The human nervous system has three main parts:

CENTRAL NERVOUS SYSTEM (CNS)

It mainly comprises of brain and spinal cord.

BRAIN – It is an anterior portion of the CNS which is lodged in the cranial cavity, i.e., cranium of the skull. It weighs from 1220 to 1400 grams. Structurally, it consists of three membranes (meninges) i.e., piameter membrane, arachnoid membrane and durameter membrane.

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

Human brain structurally consists of three main parts:
1- Fore brain (prosencephalon)
2- Mid brain (Mesencephalon)
3- Hind brain (Rhombencephalon)

FORE BRAIN : It includes olfactory lobes, cerebrum and diencephalon.

 OLFACTORY LOBES – They are concerned with the sense of smell. They are the anterior part of the brain formed by the pair of short club shaped structures. They are fully covered by the cerebral hemisphere.
 CEREBRUM – It is the largest and most complex part of the brain. It mainly has two hemispheres (i.e., left and right) connected by the large bundle of myelinated fibres, the corpus callosum and small bundle of fibres. 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 the inner part is the white matter.
 DIENCEPHALON – It controls and coordinates various functions like body temperature, feeling of satisfaction, appetite, thirst etc.

MID BRAIN

It is mainly the portion located in between the thalamus or hypothalamus of the forebrain and pons of the hind brain. There is a canal known as cerebral aqueduct that passes through the mid brain.
Mid brain and Hind brain together forms the brain stem.
Functions of mid brain
1- It relays impulses back and forth between cerebrum, cerebellum, pons and medulla.
2- It is also concerned with sense of sight.

HIND BRAIN : It involves cerebellum, pons and medulla.

 CEREBELLUM – It is the second largest part of the human brain. It has convoluted surface in order to provide more space for neurons. It controls rapid muscular activities 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. It relays impulses between medulla to regulate breathing.
 MEDULLA OBLONGATA – It extends from the pons varolii in above direction and is continuous with the spinal cord in below direction. It mainly have centres that regulates heart rate, blood pressure, breathing, swallowing, salivation, sneezing, coughing, vomiting and other involuntary movements.

SPINAL CORD (MYELON) – It extends from the base of the brain and is continous to second lumbar vertebra. It mainly lies in the neural canal of the vertebral column. The three meninges, i.e., durameter, arachnoid and piameter which covers the brain, also continue over the spinal cord.

Functions of spinal cord
1- The stimuli passes from and to the brain through the spinal cord.
2- It also acts as the centre of spinal reflex action.

 PERIPHERAL NERVOUS 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 arises from brain are called cranial nerves, while the nerves originating from the spinal cord are termed as spinal nerves.

In human body there are 12 pair of cranial nerves and 31 pair of spinal nerves.

AUTONOMIC NERVOUS SYSTEM (ANS)

It transmit impulses 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, lowers the intestinal digestion activities, helps in blood clotting, increased secretion of sweat glands etc.
 Parasympathetic Nervous System – Works just analogus 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, decreases the secretion of sweat glands etc.

TERMS TO REMEMBER

 Hypothalamus – It controls the urge of eating and drinking. It is also called the thermostat of the body.
 Thalamus – It is the relay station of sensory inputs and does not produce hormones.
 Reflex arc – The reflex action is an automatic response to the stimulus. The pathway taken by nerve impulses in a reflex action is known as reflex arc.