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The Chemistry of the Brain What is the brain? What is it composed of? Why does it have such an important role in living things? Why does it sometimes malfunction? To answer these questions we must look at the nervous system in humans. The nervous system in humans is made up of: • The CNS - central nervous system (brain and spinal chord) • The PNS - Peripheral nervous system (nerve cells leading to and from the central system) These nerve cells are specialised cells called NEURONS which conduct messages to and form the brain. Neurons • A neuron is a nerve cell and are the basic unit of the nervous system and are specialised to carry information from one place to another. • They are elongated cells that transmit this information by means of electrical impulses. Structure of neurons Neurons consist of the following: Cell body Dendrites Axon Myelin sheath Schwann cell Neurotransmitter vesicles Synapse knobs There are three types of Neuron : Sensory neuron –carries messages from the sense organ to the central nervous system (CNS). Interneuron –connects sensory and motor neurons and so carries messages within the CNS. Motor neuron –carries impulses from the CNS to muscles and glands. Cell Body The cell body contains the nucleus and other organelles that produces neurotransmitter chemicals, the absence of which would make impulse transmission impossible! Axon The function of the axon is to conducts nerve impulses away from the cell body and towards the CNS or the desired destination. Schwann Cell Schwann cells are located along the length of the neurons. They secrete the myelin sheath, which is a fat-rich membrane that insulates the electrical impulses. In multiple sclerosis (MS), patches of myelin degenerate in the CNS. As a result, the passage of nerve impulses is impeded, and the person suffers symptoms ranging from numbness and tingling to paralysis and loss of bladder control. Myelin sheath As neurons carry electrical impulses they must be insulated so that the information is not lost as it travels to the CNS. The myelin sheath acts as an insulator for the impulse as it travels through the axon. The presence of this insulation will also speed up the transmission of the impulse. Dendrites Dendrites receive information and carry it towards the cell body How are impulses initiated? When a stimulus is applied to a neuron it causes movement of positively charged ions i.e. sodium ions. These sodium ions move from the outside to the inside of the axon. This is the action that causes an impulse to travel to the CNS. Path of the impulse The impulse travels along the axon of the neuron until it reaches the end. It then needs to travel to the next neuron so it may reach It’s desired destination. The impulse now needs to travel across the “gap” between the two neurons. How does an impulse cross this “gap”? The end of each axon breaks up into many small branches Neurotransmitters e.g acetylcholine are stored in vesicles at the end of these branches. When neurotransmitters are activated by the ions they are released into the cleft for a very short period of time, transmitting the impulse to the next neuron. After transmission, the neurotransmitter is inactivated by an enzyme and reabsorbed by the presynaptic neuron and used to make new neurotransmitter substance. Central Nervous System The Brain The brain is composed of over 100 billion neurons, each receiving messages simultaneously from thousands of other neurons. The brain is protected by the skull bones, meninges (three membranes) and cerebrospinal fluid. The cerebrum is our conscious brain, with different parts having different jobs to do. hypothalamus The hypothalmus is the centre for the regulation of the internal organs. cerebrum pituitary The pituitary ‘[master] gland secretes hormones that stimulate other glands to release their hormones. Conscious actions A conscious action is one where the brain makes a considered response. Here’s what happens: 4) The brain 3) Here another sensory neuron decides to move carries the signal to the brain away the hand 5) This impulse is sent by MOTOR NEURONS to the hand muscles (the effectors) via the spinal chord… 2) The impulse is carried by SENSORY NEURONS to the spinal chord 1) Receptors in your skin detect a stimulus Stimulus 6) Which then moves the hand away Receptor Motor Neuron Sensory Neuron Effector Coordinator Response The cerebellum co-ordinates processes that we have learned to do automatically, such as speaking. Medulla oblongata cerebellum The medulla oblongata co-ordinates involuntary, automatic processes — such as breathing, heartbeat. Nervous System Disorder Parkinson’s disease is a nervous system disorder, normally seen in older people, in which muscles become rigid and movement is slow and difficult, with persistent tremors [shaking]. It is caused by the brain reducing the normal amount of dopamine that it makes. There is at present no means of preventing it, but giving L-dopa (which the body changes into dopamine) can relieve the symptoms in many patients.