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The Nervous System

 

The human nervous system enables to control actions and sensations, as well as reflection, emotions and memory. Its role is to receive and to analyse a nervous information and then to send an appropriate response. All organs and muscles of the body depend upon this nerve impulse to work :

  • With its million of sensory receptors (located in skin, eyes etc), the nervous system detects all changes in its internal and external environnement. Then, it carries those information to the central nervous system

  • Next, it annalyses and integrates the information that it receives from the receptors (it perceives the nature's information, their origin, their intensity), compare them with 'the touchstones in its memory' and then, decides on an adapted response ;

  • Finnaly, it generates appropriate reactions by sending a 'signal' to effector organs.

 

 

The central nervous system is divided into :

  • the central nervous system which is composed of spinal cord and the brain; and

  • the peripheral nervous system, composed of sensory receptors and nerves.

 

The pheripheral nervous system is subdivided into :

  • the autonomis nervous system : composed of the sympathetic, the parasympathetic and the entericit systems. It affects predominantly involuntary actions such as heart rate, respiration rate, disgestion, perspiration, salivation, diameter of the pupils... and
  • the somatic nervous system, which controls all voluntary body movements

 

 

 

 

The nervous system controls our actions, emotions and behaviors by receiving and sending messages through the brain and the body. These signals are called « nervous messages » or « nerve impulses ».
These nerve impulses are transmitted by neurons.

 

What is a neurone?

A neuron is a cell which is composed of three basic parts : the dendrites, the cell body and the axon. Their rôle is to transmit the nerve impulse. There are about one hundred billion of neurons in the brain. Axons constitute the principal branching of the cell. Many axons bundled together form nerves. Thus, nerves are involved in the transmission of the nervous message.

 

 

 


 

Thus, nerves are fundamental in the nervous system because they are intermediaries between the central nervous system and organs : they transmit the nervous messageNerves linked to the spinal cord are the spinal nerves and those linked to the brain are the cranial nerves.

 

There are two kind of nerves :

  • The nerves that carry information from sensory receptors such as the eye, to the central nervous system are called sensory nerves or afferent nerves. (Afferent pathway)

  • those that carry nerve impulses from the central nervous system to effector organs such as the muscles are called motor nerves or efferent nerves. (Efferent pathway)


    Propagation of the nerve impulse :

     

    The nerve impulse "arises" at sensory receptors level. Indeed, the sensory receptors are sensible to a special information coming from the internal and external environnement of the organism : the stimulus. These stimuli can be auditory, gustative, olfactory, tactile or visual. When a sensory receptor detects one of these stimuli, a nerve impulse is created at sensory receptors level and is transmited along of the nerone, in an unidirectional way :

    dendrites → cell body → axon

    Thus, dendrites are responsible for the reception of the nervous message, the cell bodies are responsible for its integration and genesis, and axons have the function to propagate it.


    Axons

    Dendrites

    • Take information away from the cell body

    • Smooth Surface

    • Generally only 1 axon per cell

    • No ribosomes

    • Can have myelin

    • Branch further from the cell body

    • Bring information to the cell body

    • Rough Surface (dendritic spines)

    • Usually many dendrites per cell

    • Have ribosomes

    • No myelin insulation

    • Branch near the cell body

     

     

    More precisely, the nerve impulse is an electrochemical signal that travels along of the neurone. It is due to a the reversal in charge of the neuron's membrane. Indeed the area outside the neurone is positive and the area inside the axon is negative. An action potential is a brief reversal of that charge which moves down the axon.




    These signals are trans along of the cell body to the synapse, where they are transmitted to the next neurone by a biochemical phenomenon. the neurone frommitted

     A neurone passes an action potential to another through the synapse. The passing of this signal is mediated by a neurotransmitter which binds to receptors located in the postsynaptic cell.

     

     

    The synapse

     The link between the axon of a neuron and the dendrites of an other is the synapse. 

    Actually, there are visicules which contain neurotransmitters in the pre-synaptic neuron. These neurotransmitters will be released in the synapse and will bind into the receptors of the post-synaptic neuron. It's in this way that the the nerve impulse is transmitted.


     

     

     

    1) An action potential crosses the neuron from the dendrites to the axon

    2) Visicles in the pre-synaptic neuron will fix on the membrane of the pre-synaptic neuron

    3) These visicles will release neurotansmitters in the synaptic cleft

    4) The neurotranmitters will bind into their specific receptors on the post-synaptic neuron

    5) An new action potential will be created



    A neurotransmitter is a molecule that transmits the nerve impulse from neurons to neurons through the synapses.
    There are different kind of neurotransmitters : dopamine, serotonin, acetylcholine... It is important to know that there is a specific neurotransmitter for each kind of nervous message. For exemple, the neurotransmitter that internerves for pain is called : glutamate.

     

    It is on this process that acts cannabis.

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