Nervous system in man, Nerve cells types & Nature of nerve impulse
Nervous system in man
The nervous system is highly developed in vertebrates , especially in man , The nervous system and the endocrine glands are working together to control all the functions of human body systems , They receive the information in the form of external & internal stimuli through receptor systems , and then give the proper response .
This is for keeping the human body in a continuous direct communication with his external and internal environment and for keeping the internal conditions of body in an ideal , constant and balanced state ( homeostasis ) .
The nervous system is divided into :
- Central nervous system ( CNS ) : It includes the brain and spinal cord .
- Peripheral nervous system ( PNS ) : It includes the cranial nerves and spinal nerves .
- Autonomic nervous system : It includes the nerves that control the involuntary muscles and the glands .
Autonomic nervous system is subdivided into Sympathetic nervous system and Parasympathetic nervous system .
Nerve cell ( Neuron ) is the structural unit of nervous system , Nerve cell is small in size and can’t be recognized by the naked eye , Nerve cell consists of cell body and cell processes .
Cell body of the nerve cell contains rounded nucleus , Cytoplasm surrounded the nucleus and known as neuroplasm which contains neurofilaments , all cell organelles as mitochondria and Golgi bodies , except the centrioles , so , neurons can not divide nissil’s granules .
Nissil’s granules are minute granules unique for the nerve cells and they are considered as the stored food for the nerve cell which consumes during its activity .
Cell processes : There are two types of them in the nerve cell , which are :
- Dendrites : many short processes arise from the cell body to increase the surface area to receive the nerve impulses and through which all the nerve impulses enter to the cell .
- Axon ( Nerve fiber ) : It is a long cytoplasmic extension of the cell ( may reach more than a meter in length ) .
It is surrounded by two sheaths :
- Myelin sheath : a sheath of lipid in some nerve cells that is secreted by special cells called schwann cells , Myelin sheath is not continuous around the axon , but it is interrupted at certain points by nodes of Ranvier , Ranvier’s nodes are interruptions at certain points in the myelin sheath on the axon which lack of myelin .
- Neurolemma ( Nerve sheath ) : a thin layer that represents the outer cover of axon , It ends in a group of branches called terminal arborizations .
Function of the axon : The conduction of nerve impulses from the body of nerve cell to the synapse , The conduction of nerve impulses in myelinated axons ( covered by myelin sheath ) is much more rapid than in non-myelinated nerve fibers ( axons ) , because the myelin sheath acts as an insulator .
The nerve impulse is propagated and conducted through the nerve cell in one direction only , as the nerve impulses enter the nerve cell body through the dendrites , then to the axon , while the terminal arborizations transmit these impulses away from the cell body ( to the next neuron ) through the synapse .
Types of nerve cells
According to the function , nerve cells are classified into three types
- Sensory neurons transmit impulses from the receptors to the central nervous system .
- Motor neurons transmit impulses from the central nervous system to the effector organs as muscles and glands .
- Connector ( Intermediate ) neurons relay impulses from the sensory to the motor neurons ( connect between them ) .
In addition to the cell body and the processes of nerve cells , there is another type of cells in the nervous system known as neuroglia .
Neuroglia ( Glial cells )
Another type of cells in the nervous system , have the ability to divide and perform the following functions :
- Act as a connective tissue to support the neurons .
- Act as insulators between the neurons .
- Nutrition of neurons .
- Have a role in repairing the injured parts of some neurons .
- Connect the nerve fibers ( axons ) together to form the nerve bundles which form the nerve .
Nerve consists of :
- A group of nerve bundles : Each group is formed a group of nerve fibers ( axons ) and connected by supporting neuroglial cells .
- Connective tissue sheath surrounds each nerve bundle .
- Epineurium is a connective tissue that surrounds the whole nerve and contains blood vessels .
Nerve impulse is the message that is transmitted through the nerves from the sense organs ( receptors ) to the central nervous system , then to the effector ( responding ) organs .
Nature of the nerve impulse :
The nerve impulse is an electrical phenomenon with a chemical nature ( electrochemical phenomenon ) , To understand the nature of nerve impulse and its transmission , we have to study the nerve cell during four different conditions .
- The nerve cell at rest
- The changes in the nerve cell on stimulation
- Propagation of the nerve impulse through the nerve fibers
- The return of nerve cell to its original stat
Nerve cell at rest
At rest , there is a difference in distribution and concentration of ions outside and inside the nerve cell , as the following :
The concentration of sodium ions ( Na+ ) outside the cell is 10:15 times higher than inside , The concentration of potassium ( K+ ) inside the cell is 30 times higher than outside .
The concentration of negative ions inside the cell is higher than outside , due to the presence of chloride ( Cl‾ ) and protein ions , The amount of negative ions inside the cell exceeds the positive ions , so , the inner surface of the cell carries negative charges .
The amount of positive ions outside the cell exceeds the negative ions , so , the outer surface of the cell carries positive charges , The unequal distribution of ions results in the presence of an electrical potential difference between outside and inside the cell surface , equals ( -70 millivolt ( m V ) ) .
This potential difference called resting potential , The membrane of nerve cell during this resting condition is said to be polarized and this case is called polarization , Polarization is the state of nerve cell at rest when its outer surface is positive and the inner surface is negative .
The state of polarization is a result of :
- The selective permeability of resting membrane : the membrane of nerve cell is 40 times permeable to K+ ions ( which diffuse from inside to outside ) than to Na+ ions ( which diffuse from outside to inside ) , This results in the accumulation of excess positive charges on the outer surface membrane .
- The accumulation of high molecular weight protein ions in addition to chloride ions : They are negatively charged on the inner surface of membrane .
- Sodium-potassium pump : it plays a role in maintaining this ionic distribution .
Therefore , at rest there is an accumulation of positive K+ ions outside the membrane and negative protein ( which can’t pass through the membrane due to its large size ) and chloride ( Cl‾ ) ions inside the membrane , and so the potential difference of cell at rest equals ( – 70 mV ) .
The changes in the nerve cell on stimulation
The nerve cell is stimulated only when the stimulus is sufficient , There are changes in the permeability of membrane in which the inflow of positive sodium ions ( Na+ ) exceed the outflow of potassium ions ( K+ ) through special channels in the membrane .
This leads to the accumulation of excess positive charges inside the membrane , i.e reverse the original polarity , The membrane potential becomes ( + 40 mV ) and this new state is called depolarization , Depolarization is the state of nerve cell on stimulation when its outer surface is negative and the inner surface is positive .
Propagation of the nerve impulse through the nerve fibers
The depolarized point acts as a stimulus for the neighbouring points which undergo the same previously changes as the first time and the process is repeated along the nerve fiber , The nerve impulse is propagated along the nerve fiber in the form of waves of depolarization , polarization and then depolarization again .
Return of the nerve cell to its original state ( Repolarization )
After the end of depolarization :
- The membrane becomes again permeable to potassium ions and impermeable to sodium ions .
- The continuous outflow of potassium ions leads again to the accumulation of excess positive ions outside the membrane and the membrane becomes repolarized , returns to the resting state ( – 70 mV ) .
- The occurrence of refractory period in which the membrane of nerve cell regains its physiological properties to be ready to respond to a new stimulus and transmit another nerve impulse .
- The response of nerve cell to the stimulus is called the action potential which includes a state of depolarization followed by repolarization and it equals ( 110 mV ) .
- The nerve impulse is the propagation of action potential along the nerve cell ( fiber ) .
Refractory period is the short period of time ( 0.001 : 0.003 seconds ) following the stimulation in which the nerve cell will not respond to any stimulus whatever its strength , During this period the membrane of nerve cell regions its physiological properties ( its ability to selective permeability ) to be ready to respond to new stimulus and transmit another nerve impulse .
Action potential is a potential which includes a state of depolarization ( from -70 mV to + 40 mV ) followed by repolarization ( – 70 mV ) and it equals ( 110 mV ) .
Properties of the nerve impulse
Speed of the nerve impulse : The speed of propagation of the nerve impulse along a nerve fiber depends on its diameter , as it reaches 140 m/s in thick ( myelinated ) nerve fibers , while the speed is 12 m/s in thin ( non-myelinated ) nerve fibers .
All or none law : The stimulation of nerve and muscle obeys the ” all or none ” law , which states that the nerve responds with a maximal strength or with none at all , the sufficient stimulus produces a maximum response ( generation of nerve impulse ) , Any increases in the strength of stimulus will not increase the response , The week stimuli are insufficient to produce an action potential ( nerve impulse ) .
Synapse is the site between the terminal branches ( arborizations ) of axon of one neuron and the dendrites of next neuron .
Types of Synapse
- Synapse between two neurons .
- Synapse between a neuron and a muscle fiber .
- Synapse between a neuron and a gland cells .
Structure of Synapse
The ultrastructure of synapse reveals that the synapse consists of :
- Buttons ( Synapatic knobs ) : swellings at the end of terminal arborizations of an axon and they are very close to the dendrites of next neuron .
- Synaptic cleft : narrow space separates the presynaptic membrane ( axon ) from the postsynaptic membrane ( dendrite ) .
- Synaptic vesicles : small sacs filled with chemical transmitters ( neurotransmitters ) as acetylcholine and noradrenaline which play an important role in the synaptic transmission of nerve impulse from one neuron to the next .
Chemical transmitter ( Neurotransmitter ) is a chemical substance that has a role in the nerve impulse transmission .
Mechanism of transmitting the nerve impulse across a synapse
The arrival of a nerve impulse to the buttons leads to the entrance of calcium ions by the action of calcium pump in the cell membrane , The inflow of calcium ions leads to the rupture of synaptic vesicles and the release of chemical transmitters .
The chemical transmitters cross the synaptic cleft and reach the membrane of dendrites of the next neuron , The binding of chemical transmitters to special receptors on the membrane of dendrites leads to the stimulation of these points and changes the permeability of their membrane to Na+ and K+ ions .
This results in depolarization and production of an action potential ( nerve impulse ) , This nerve impulse is propagated through the cell body and moves toward the axon of neuron , then to the next synapse and so on .
After performing its function , acetylcholine ( chemical transmitter ) is destroyed under the effect of an enzyme called cholinesterase to terminate its action , After that , the postsynaptic membrane ( dendrite ) returns to the resting state again .