Nervous system ( Central nervous system , Peripheral nervous system & Autonomic nervous system )

Structure of the nervous system

Nervous system consists central nervous system ( Brain & Spinal cord ) , peripheral nervous system ( Cranial nerves & Spinal nerves ) , Autonomic nervous system ( Sympathetic nervous system & parasympathetic nervous system ) .

Central nervous system

As previously mentioned the central nervous system is divided into brain and spinal cord .

Nervous system

Nervous system

Brain

It is the major part of central nervous system and its weight is about : 350 grams at birth , 1400 grams in an adult man , It occupies a bony space called the brain case or the skull ( cranium ) , It is surrounded by three membranes called the meningies which are responsible for the protection and nutrition of brain cells , 12 pairs of cranial nerves are connected to the brain .

These membranes are :

  1. The dura mater which lines the skull .
  2. The pia mater which is in direct contact and adheres to the brain .
  3. The arachnoid which is in between the other two membranes and contains a transparent fluid to protect the brain from the mechanical trauma .

The main components of brain are Forebrain ( Brain cortex , Thalamus , Hypothalamus ) , Midbrain , Hindbrain ( Cerebellum , Pons Varolii , Medulla oblongata ) .

The structure and function of each part of the brain :

Forebrain

It represents the largest part of brain , It consists of two cerebral hemispheres and cerebral cortex ,Thalamus , Hypothalamus .

Two cerebral hemispheres and cerebral cortex

Two big lobes separated by a big fissure and attached to each other by a big bundle of nerve fibers , Each lobe is called cerebral hemisphere , The cortex of each lobe ( cerebral cortex ) is characterized by the presence of depressions of different depths called fissures and grooves and in between there are folds .

Each cerebral hemisphere is divided into many lobes , which are frontal lobe , parietal lobe , occipital lobe & temporal lobe , The 5th lobe of cerebral cortex can not be seen by the naked eye as it is covered by the fronted and partial lobes .

Functions of cerebral cortex :

  • Fronted lobe contains centers of voluntary movements ( motor centers ) and centres of memory and speech .
  • Parietal lobe controls many sensory functions and contains centres of sensation of heat , cold , pressure and touch ( somatic sensations from the skin ) .
  • Occipital lobe contains centres of vision .
  • Temporal lobe contains centres of smell and taste , and also centres of hearing .
Thalamus

It is an important centre for the coordination of different sensations ( except the smell ) that reach the cortex .

Hypothalamus

It controls the different reflex actions and contains centres of : ( Hunger , Satiety , Thirst , Body temperature regulation , Sleep ) .

Midbrain

It is the smallest part of brain , It represents a connection between the forebrain and hindbrain , It contains centers of Equilibrium ( keeping the body balance ) , Hearing and Vision , It regulates many reflexes as those related to the hearing .

Hindbrain

It consists of Cerebellum , Pons Varolii and medulla oblongata , Cerebellum is situated in the posterior region and consists of three lobes , It keeps the balance and equilibrium of the body in association with the inner ear and muscles .

Pons Varolii and medulla oblongata performs the following functions :

  • Transmission of nerve impulses between the spinal cord and different brain regions .
  • The medulla oblongata contains vital centres as those of respiration , swallowing , vomiting , cough , sneezing and blood vessels movement .

Spinal cord

It exists inside a canal in the vertebral column called the neutral canal , It extends from the medulla oblongata in the form of cylindrical cord , Its length is about 45 cm long , It is hollow from the inside due to containing a central canal .

There are two fissures ( dorsal and ventral ) extend along the midline which divide the spinal cord incompletely into two halves , It is covered by three membranes ( meningies ) , which are Dura mater , Pia mater & Arachnoid .

Structure of the spinal cord

It consists of two layers :

  1. Outer white layer ( white matter ) formed from nerve fibers .
  2. Inner grey layer ( grey matter ) is H-shaped with two dorsal horns and two ventral horns formed from nerve cells with their dendrites and neuroglia .

Its function :

  • White matter transmits impulses from the different body parts to the brain and vice versa .
  • Grey matter is the main centre of reflex action , as it contains thousands of reflex arcs .

Peripheral nervous system 

It connects the central nervous system with all parts of the body , It consists of a network of nerves distributed all over the body , which includes Cranial nerves and Spinal nerves .

Cranial nerves

Number : there are 12 pairs connected to the brain , Types : sensory ( containing sensory fibers only ) , motor ( containing motor fibers only ) or mixed nerves .

Mixed nerves are nerves carry the impulses from the receptors to the brain and from the brain to the effector organs , so , they are sensory and motor nerves together .

Spinal nerves 

Number : 31 pairs connected to the spinal cord and they are mixed nerves with both sensory and motor fibers , Spinal nerves are divided into :

  1. Cervical nerves : 8 pairs .
  2. Thoracic nerves : 12 pairs .
  3. Lumbar nerves : 5 pairs .
  4. Sacral nerves : 5 pairs .
  5. Coccigeal nerves : 1 pair .

Types : all spinal nerves are mixed ( sensory and motor nerves ) , Each spinal nerve originates from the spinal cord by two roots ( dorsal and ventral ) .

Dorsal root carries sensory nerve fibers , It transmits the impulses from the receptors to the spinal cord , then to the brain .

Ventral root carries motor nerve fibers , It transmits the impulses from the brain and spinal cord to the responding ( effector ) organs as muscles and glands .

Reflex arc ( Reflex action )

Reflex arc is the unit of nervous activity , The majority of nervous functions can be analyzed to a group of reflex actions , The reflex action consists of at least two nerve cells :

  • Sensory nerve cell ( Afferent ) .
  • Motor nerve cell ( Efferent ) .

The majority of reflex actions of five elements :

  1. Receptor ( Sense organs ) .
  2. Afferent ( Sensory ) neuron .
  3. Connector ( Intermediate ) neuron .
  4. Efferent ( Motor ) neuron .
  5. Effector ( Responding ) organ .

Types of reflex arc :

  1. Voluntary ( Somatic ) reflex arc : The effector organ is the skeletal muscle .
  2. Involuntary ( Autonomic ) reflex arc : The effector organ is an involuntary muscles or glands or heart .

Autonomic nervous system

It regulates the different involuntary activities such as contraction of the cardiac muscles and smooth ( involuntary ) muscles , secretion of the body glands , It consists of sympathetic nervous system and parasympathetic nervous system .

Sympathetic nervous system

The nerve fibers of this system are arisen from the thoracic and lumbar regions of the spinal cord , This system is important as an emergency system , as it enables the body to confront the emergency situations .

Parasympathetic nervous system

The nerve fibers of this system are arisen from the brain stem and sacral region of the spinal cord .

Most of the internal parts of the body receive nerve fibers related to both the sympathetic and parasympathetic systems , and in most cases the effect of one system antagonizes the effect of the other .

The effect of sympathetic and parasympathetic systems on some parts of the body 

Heart : sympathetic system increases the heartbeats rate and force of contraction , Parasympathetic system decreases the heartbeats rate and force of contraction .

Blood vessels : Effect of sympathetic NS is vasoconstriction ( contraction ) of blood vessels of skin , viscera , salivary glands , brain , external genetalia and lungs , Effects of Parasympathetic NS is vasodilation ( Relaxation ) of blood vessels of salivary glands and external genetalia .

Alimentary canal : Effect of sympathetic NS is relaxation of the wall of stomach , intestine and colon , Effect of Parasympathetic NS is contraction of the wall of stomach , intestine and colon .

Respiratory system : Effect of sympathetic NS is dilation ( Relaxation ) of bronchioles and decreases their secretions , Effect of Parasympathetic NS is constriction ( contraction ) of bronchioles and increases their secretions .

Urinary bladder : Effect of sympathetic NS is relaxation of the wall of urinary bladder , Effect of Parasympathetic NS is contraction of the wall of urinary bladder .

Glands : Salivary glands ( Sympathetic system stimulates little secretion of saliva , Parasympathetic system stimulates large secretion of saliva ) , Gastric juice of stomach ( Sympathetic system inhibits the secretion , Parasympathetic system stimulates the secretion ) .

Liver : Sympathetic system breaks down the glycogen and increases the glucose level in blood , Parasympathetic system decreases the glucose level in blood .

Pancreas : Sympathetic system inhibits the secretion of enzymes , Parasympathetic system stimulates the secretion of enzymes .

Adrenal medulla : Sympathetic system stimulates the secretion of a adrenaline hormone ( epinephrine ) which increases the blood pressure , heart rate and the glucose level of blood , there is no parasympathetic fibers connection .

Nervous system in man , Nerve cells types & Nature of nerve impulse

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