Niklas Heuser - GFS Bio bilingual: The structure of the nervous system lyrics

The structure of the nervous system 1 The nervous system 1.1 Coordination of the body 1.2 The central nervous system (CNS) 1.3 The peripheral nervous system (PNS) 1.4 Example of the cooperation of CNS and PNS 2 The nerve cell / neuron 2.1 The structure of nerves 2.2 The neuron 2.2.1 The cell body 2.2.2 The axon 2.2.3 The Swann cells and the nodes of Ranvier 2.2.4 The end plates and synapses 3 Nerve impulses 3.1 What are nerve impulses? 3.2 How are they transmitted? 4 Summary 5 References 1 The nervous system 1.1 Coordination of the body In our life, coordination plays an important role. A dishwasher for example is coordinated by an automatic program; a car is directed by a steering wheel. Coordination processes can also be found in our body. What happens in a single cell is programmed in the DNA in the cell's nucleus. However, the activities of tissues and organs in our body are controlled by the brain. With the help of the brain, organs and tissues work together and carry out their various functions, like breathing, heartbeat, blinking reflex and muscle action, due to the needs of the body. To achieve this coordination, information between the brain and the organs is transmitted by two systems: the nervous system and the endocrine system. The nervous system acts very rapidly with electric impulses travelling along nerves. The action only takes seconds and is often localised to one organ. Examples are blinking reflex and the movement of arms and legs. The endocrine system is working much slower, the response can take years before completed. The information is transmitted by chemical messengers which are transported via the bloodstream. It mostly has a widespread effect on many organs and controlles processes like growth and the development of reproductive systems. The following report will explain the nervous system works. 1.2 The central nervous system (CNS) The nervous system of mammals is built up of two parts: The central nervous system and the peripheral nervous system. The central nervous system (CNS) consists of the brain and the spinal cord. Both are well protected: the brain is inside the bony skull and surrounded by brain liquid, which absorbs pushes. The spinal cord is in the vertebral column and well protected here. 1.3 The peripheral nervous system (PNS) The peripheral nervous system is also devided into two parts: the sensory nerves and the motor nerves. The sensory nerves receive information from receptors, for example in the skin, and carry it to the CNS. The motor nerves receive information from the CNS and carry it to an effector organ, like a muscle. 1.4 Example of the cooperation of CNS and PNS Here is an example of a possible flow of information through the nervous system: A man sees a gla** of water. His photo receptors in the retina receive the stimulus and give an electric impulse to a sensory nerve. This nerve carries it to the CNS, which processes the information and recognizes: a gla** of water! As the man is thirsty, it reacts and sends a message through a motor nerve to the muscles in the arm, which contract, grab the gla** and lead it to the mouth. 2 The nerve cell/ neuron 2.1 The structure of nerves Every organ and muscle in the human body is connected with the CNS by nerves. These nerves consist of several bundles of nerve cells, called neurons. The neurons in a bundle run parallel and are electrically isolated against each other by a protective sheath of myelin. The bundles are connected by a connective tissue that forms a protective coat. Nerves can be longer than 1m and as thick as a thumb. The totality of all nerves in our body can reach a length of 400.000 km. In one nerve, you can find different types of nerves side by side: sensory neurons (afferent), motor neurons (efferent), visceral neurons (connecting inner organs with the CNS) and somatic neurons (connecting skin and muscles with the CNS). Nerves with different types of neurons are called mixed nerves. 2.2 The neuron The neuron consists of a cell body with dendrites, an axon and end plates. 2.2.1 The cell body The cell body can be formed like a star. In the middle of the cell is the nucleus which contains the DNA. At the outer part of the cell, dendrites stretch out to connect with other cells and collect information. The cell body also controls the metabolism of the nerve cell. 2.2.2 The axon Every neuron has only one axon. It carries electric impulses from the cell body to the next neuron. Its diameter (less than ¼ mm) depends on the type and size of the cell. The longest axons are found in the legs. Here, they reach a length up to 1m. But there are also very small axons in the brain with a length of only micrometers. Especially the longer axons are covered with myelin. 2.2.3 The Swann cells and the nodes of Ranvier Many axons are covered with a fatty sheath of myelin. The cells forming this sheath are called Swann cells. The sheath has regular nodes every one to two mm. These nodes are called nodes of Ranvier. The myelin gives the axon an electrical insulation between other cells. It also makes impulses travel faster: it allows them to jump from one node to the next. In nerves without myelin sheath, electric impulses reach a speed of 0,5 to 2 m/s; whereas in nerves with myelin sheath they reach a speed between 3 m/s and 120 m/s. 2.2.4 The end plates and synapses At its end, the axon splits up into several axon terminals with end plates. The end plates have a shape like a bell and are also called synaptic bulbs. Between the end plates and the dendrite of the next cell, there's a very thin cleft (20nm), which is called synaptic cleft. End plate, synaptic cleft and dentritic spine together are called synapse. Impulses can only cross the synapse with the aid of chemical messengers, called neurotransmitters. These neurotransmitters are stored in tiny sacs in the end plates. 3 Nerve impulses 3.1 What are nerve impulses? The messages travelling along nerves are always electrical impulses that move very fast. They can only be transmitted in one direction, from the dendrites to the end plates. When an impulse reaches an end plate it has to cross a synapse to continue its way in the next neuron. To achieve this, the way of transmission changes from electric impulses to chemical messengers. 3.2 How are they transmitted? When a stimulus is received by a receptor in the skin, it triggers an electric impulse to the nerve. If it is a nerve without a myelin sheath, the nerve's membrane opens itself for natrium ions. They stream in so that the normally negative loaded inner surface depolarises and becomes positively charged. The outer surface then has a lower positive potential than the inside. Now, between the first part of the axon and the next, there's a difference in polarisation, so the next part depolarises too. The so built up electric impulse goes through the whole neuron like a wave. After the electric impulse, pota**ium ions cross the membrane from inside to outside and depolarise the surfaces again. Then, the membrane is impermeable for a short time so that the impulse can't change its direction. Transmission in nerves without a myelin sheath can reach a speed of 2 m/s. If the nerve has got a myelin sheath, this process goes on much faster. Because the myelin isolates against electricity, changes in polarisation can only take place in the nodes of Ranvier. Now, the impulse “jumps” over the myelin sheath and reaches a speed up to 120 m/s. This type of transmission is called saltatory conduction. When an impulse arrives at the end plate, some tiny sacs release chemical messengers, the so called neurotransmitters, into the gap. Receptors on the other side receive the neurotransmitters and restart the impulse in the next neurone. Because the tiny sacs are only found in the end plates, the impulse can only cross the synapse in one direction. 4 Summary The human nervous system is a very complex and specialized system to transmit information. It allows the coordination of the work of the organs and the possibility to react to outer stimuli. This difficult task is resolved by cooperation of CNS and PNS and the combination of electric impulses that are travelling in neurons and chemical messengers that cross synapses. 5 References GEO Themenlexikon, Teil 9-11, Medizin und Gesundheit R.Mestwerdt, W.Schulte (2000), Schule 2001, Grundstock des Wissens, eCo Verlag W.R.Pickering (2000), Complete Biology, Oxford Verlag T.H.Schiebler, W.Schmidt (1987), Anatomie (vierte Ausgabe), Springer Verlag T.H.Schiebler, U.Peiper, Fr.Schneider, L.C.Junqueira, J.Carneiro (1986), Histologie (zweite Ausgabe), Springer Verlag S.Silbernagel, A.Despopoulos (1983), Taschenatlas der Physiologie (zweite Ausgabe, Thieme Verlag U.Weber, (2001) Biologie Oberstufe Gesamtband, Cornelsen Verlag