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Human Nervous System

The nervous system is the part of an animal that coordinates its actions by transmitting signals to and from different parts of its body. The nervous system detects environmental changes that impact the body, then works in tandem with the endocrine system to respond to such events.


Human nervous system
Human nervous system


The neural system provides an organized network of point-to-point connections for quick coordination between various body functions.

It is composed of highly specialized nerve cells (or neurons), nerve fibers and receptors.

The human neural system is divided into two main parts:


1. The central nervous system
2. The peripheral nervous system


The central nervous system (CNS) includes the brain and the spinal cord and is the site of information processing and control.


The peripheral nervous system (PNS) is divided into three divisions called the somatic nervous system, autonomic nervous system, and the enteric nervous system.


The somatic nervous system relays impulses from the CNS to skeletal muscles, its nerves mediate voluntary movement. While the autonomic nervous system transmits impulses from the CNS to the involuntary organs and smooth muscles of the body. The autonomic neural system is further classified into the sympathetic nervous system and parasympathetic nervous system.


The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. The enteric nervous system functions to control the gastrointestinal system. Both autonomic and enteric nervous systems function involuntarily.


Nerves that exit from the cranium are called cranial nerves while those exiting from the spinal cord are called spinal nerves.


The visceral nervous system is the part of the peripheral nervous system that comprises the whole complex of nerves, fibers, ganglia, and plexuses by which impulses travel from the central nervous system to the viscera and from the viscera to the central nervous system.

Neurons:


Neurons are the structural and functional unit of the nervous system. A neuron structurally comprises three major parts: cell body, dendrites, and axon. The cell body contains cytoplasm with typical cell organelles and certain granular bodies called Nissl’s granules. Short fibers which branch repeatedly and project out of the cell body are called dendrites. These fibers transmit impulses towards the cell body. The axon is a long fiber that arises from the cell body and its distal end is branched.


Human nervous system, Neuron
Structure of Neuron

Neurons have special structures that allow them to send signals rapidly and precisely to other cells. They send these signals in the form of electrochemical waves traveling along thin fibers called axons, which cause chemicals called neurotransmitters to be released at junctions of two neurons called synapses.


Along with neurons, the nervous system contains other specialized cells called glial cells (or simply glia), which provide structural and metabolic support. In the nervous system of a single species such as humans, hundreds of different types of neurons exist, with a wide variety of morphologies and functions. These include sensory neurons that transmute physical stimuli such as light and sound into neural signals, and motor neurons that transmute neural signals into activation of muscles or glands;


On the basis of structure, there are two types of nerve fibers, namely, myelinated and non-myelinated. The myelinated nerve fibers are enveloped with Schwann cells which form a myelin sheath around the axon. The gaps between two adjacent myelinated sheaths are called nodes of Ranvier.

Generation and conduction of nerve impulse:


When a neuron is not conducting an impulse, i.e., resting, the axonal membrane is comparatively more permeable to potassium ions (K+) and nearly impermeable to sodium ions (Na+). Similarly, the membrane is impermeable to negatively charged proteins present in the axoplasm. Consequently, the axoplasm inside the axon contains a high concentration of K+ and negatively charged proteins and a low concentration of Na+. In contrast, the fluid outside the axon contains a low concentration of K+, a high concentration of Na+ and thus forming a concentration gradient.


The ionic gradients are maintained by the sodium-potassium pump which transports 3 Na+ outwards for 2 K+ into the cell. As a result, the outer surface of the axonal membrane possesses a positive charge while its inner surface becomes negatively charged and therefore, is polarized. The electrical potential difference across the resting plasma membrane is called the resting potential. The state of the resting membrane is called a polarized state.


When a stimulus is applied, the permeability of the membrane to Na+ ions is greatly increased at the point of stimulation. It is due to the fact that Na+ channels open and K+ channels remain closed. Thus sodium channels permit the influx of Na+ ions by diffusion. This results in a positive charge inside and negative charges outside. The change in polarity across the plasma membrane is known as action potential (nerve impulse) and the membrane is said to be depolarized.


With the increase in Na+ ions inside the nerve fiber, the membrane becomes less permeable to Na+ ions and more permeable to K+ions.  Na+ influx stops and K+ outflow begins until the original resting state of ionic concentration is achieved. Thus, resting potential is restored which is called repolarization of the membrane.


Salutatory conduction: The action potential jumps from node to node and passes along the myelinated axon faster than the series of smaller local currents in a non-myelinated axon. This type of conduction is called saltatory conduction.

Conduction of nerve impulse through a chemical synapse:


Synapse: The junction between two neurons, across which the impulse passes from one neuron to the next is called the synapse.


Conduction of nerve impulse through a chemical synapse
Synapse

A typical (generalized) synapse consists of a bulbous expansion of a nerve terminal called a pre-synaptic knob lying close to the membrane of a dendrite. The cytoplasm of the synaptic knob contains numerous synaptic vesicles. Each vesicle contains neurotransmitters (chemical substances).


The membrane of the synaptic knob nearest to the synapse forms the presynaptic membrane. The membrane of the dendrite is called the postsynaptic membrane. These membranes are separated by a gap, the synaptic cleft. The postsynaptic membrane contains receptor sites for the neurotransmitter. The two main neurotransmitters in the vertebrate nervous system are acetylcholine (Ach) and noradrenaline although other neurotransmitters also exist.


When an impulse arrives at a presynaptic knob, calcium ions from the synaptic cleft enter the cytoplasm of the presynaptic knob. The calcium ions cause the movement to synaptic vesicles to the surface of the knob. The synaptic vesicles are fused with the presynaptic membrane and get ruptured to discharge their contents (neurotransmitter) into the synaptic cleft.


The neurotransmitter of the synaptic cleft binds with protein receptor molecules on the postsynaptic membrane. This binding action changes the membrane potential of the postsynaptic membrane, opening channels in the membrane and allowing sodium ions to enter the cell. This causes the depolarization and generation of the action potential in the postsynaptic membrane. Thus, the impulse is transferred to the next neuron.

Central nervous system


Brain (weighing 1220 to 1400 gms) is the central information processing organ of our body. The brain lies in the cranium of the skull. Brain and spinal cord are surrounded by connective tissue membranes called meninges. There are three meninges in humans, an outer layer called duramater, a middle layer called an arachnoid membrane and an inner thin layer called piamater.
Human Brain
Human Brain

The human brain is divided into three parts:
(i) Forebrain (prosencephalon)
(ii) Midbrain (mesencephalon)
(iii) Hindbrain (rhombencephalon)

(i) Forebrain:

The forebrain consists of olfactory lobes, cerebrum, and diencephalon. Olfactory lobes are concerned with the sense of smell. Cerebrum forms a major part of the human brain. A deep cleft divides the cerebrum longitudinally into two halves, which are termed as the left and right cerebral hemispheres. The hemispheres are connected by a tract of nerve fibres called the corpus callosum


The outer portion of the cerebrum is called the cerebral cortex that makes up the grey matter of the cerebrum. Beneath the grey matter, there are present millions of medullated nerve fibers that give an opaque white appearance, hence they are collectively called white matter.


Each cerebral hemisphere is divided into four lobes: – frontal (monitors complex thoughts, actions, and ideas and controls intellectual ability), parietal (registers sensory perception and takes in information from environment, organizes it and communicates it to rest of the brain), temporal (decodes and interprets sound, smell, memory, and emotion) and occipital lobes (decodes and interprets visual information, shape, and color).


Diencephalon is completely covered by cerebral hemispheres. Its main parts are – epithalamus, thalamus, and hypothalamus. Thalamus is a major coordinating center for sensory and motor signaling. Hypothalamus lies at the base of the thalamus and contains a number of centers that control body temperature, urge for eating and drinking, growth and sexual behavior, etc. Hypothalamus is an important link between the neural and endocrine systems (neuroendocrine role). 

(ii) Midbrain:


Midbrain is located between the thalamus/hypothalamus of the forebrain and pons of the hindbrain. The dorsal portion of the midbrain consists mainly of four round swellings (lobes) called corpora quadrigemina. Corpora quadrigemina control visual reflexes and auditory reflexes and are equivalent to optic lobes of lower animals.

(iii) Hindbrain:


Hindbrain comprises pons Varolii, cerebellum and medulla oblongata. Pons consists of fiber tracts that interconnect different regions of the brain. The cerebellum is the second largest part of the human brain. It consists of two lateral cerebellar hemispheres. The cerebellum controls rapid muscular activities, such as running, typing and even taking. The medulla oblongata contains centers that control respiration, cardiovascular reflexes, and gastric secretions.


Brain stem forms the connections between the brain and spinal cord. Three major regions make up the brain stem; midbrain, pons and medulla oblongata.

Spinal cord:


Spinal cord (42 to 45 cm long) extends from the medulla oblongata and is continuous to the level of the second lumbar vertebra. It conducts impulses to and from the brain and controls most of the reflex activities and provides a means of communication between the spinal nerves and the brain.

Spinal cord, Spinal cord structure
Spinal cord
The spinal cord is formed of two types of neural tissue: internal grey matter and outer white matter. The area within the vertebral column below the second lumbar vertebra contains spinal nerves that the collectively called, the cauda equina. The spinal cord ends at the conus medullaris from which a fine filament called filum terminate arises that anchors the spinal cord within the vertebral column.


Reflex action: It is an immediate involuntary action of any organ or part of the body in response to a particular stimulus. The nervous pathway taken by nerve impulses in a reflex action is called the reflex arc. The components that mediate a reflex, usually include a receptor, afferent pathway, integrating center, efferent pathway, and effector. Some common examples of reflexes are closing of eyes when strong light is flashed, salivation on seeing some favorite food, etc.


Reflexes are categorized into unconditioned and conditioned reflexes. Unconditioned reflexes are the inborn, unlearned response to a stimulus or any change in the environment. Whereas, conditioned reflexes are not inborn but are acquired on past experience, training or learning. Conditioned reflexes were first demonstrated by Ivan Pavlov.

Peripheral nervous system


The nerves running outside the brain and spinal cord constitute the peripheral neural system. These nerves are of two types: cranial nerves (12pairs) (nerves originating from the brain) and spinal nerves (31pairs) (nerves originating from the spinal cord).


The cranial nerves are olfactory (sensory), optic (sensory), oculomotor (motor), trochlear (motor), trigeminal (mixed), abducens (motor), facial (mixed), auditory (sensory), glossopharyngeal (mixed), vagus (mixed), spinal accessory (motor) and hypoglossal (motor).


Spinal nerves are classified into 5 groups -8 pairs of cervical nerves; 12 pairs of thoracic nerves; 5 pairs of lumbar nerves; 5 pairs of sacral nerves and one pair of coccygeal nerves.

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