nervous system

Nervous system Profile of a nervous system A nervous system is an intricate network of specialized nerve cells called neurons . It coordinates the activities of all but the simplest of animals. The nervous system receives sensory information, interprets it and generates appropriate responses using structures such as muscles. Most complex nervous systems can be divided into two parts: 1. The central nervous system (CNS) 2. The peripheral nervous system (PNS) The central nervous system (CNS): receives all incoming information and is the site of decision-making. In humans, it consists of the brain and spinal cord. 2. The peripheral nervous system (PNS): All the other parts of the nervous system (other than brain and spinal cord) . It relays information between the sense organs receiving stimuli, the central nervous system and the structures which bring about responses.

Fig: Nervous system Information passes along neurons in the form of electrical impulses called nerve impulses . Communication between neurons, at synapses is via chemical transmitters. In humans, neurons start to appear in the embryo when it is 10 to 14 days old. The neurons we are born with are already mature and are incapable of reproducing themselves by dividing . Those that die cannot be replaced but it is possible for their most immediate neighbors to take over the roles of damaged or dead neurons.

The following figure shows the sequence of events in the nervous system of someone who is sitting too close to a fire.

Neuron: Basic unit of a nervous system The neuron is the functional unit of a nervous system. It connects with many other neurons and receives, conducts and transmits information. This figure illustrates a neuron. Although specialized in shape, it possesses the same basic structures found in other cells: nucleus, mitochondria, cell membrane and so on. Fig: Neuron

The cytoplasm contains prominent granules called Nissl’s granules which are really groups of ribosomes concerned with protein synthesis. The nucleated part of the cell, the cell body is located in the CNS (central nervous system) and is connected with neighboring neurons by arm-like processes called axon. Axon or nerve fiber is drawn out of the cell body and enters a peripheral nerve and terminates in a muscle. Each axon is filled with axoplasm that is continuous with the cytoplasm of the cell body. It is bounded by a thin membrane continuous with the plasma membrane of the cell body. The axon is enclosed within a fatty myelin sheath. It is not part of the neuron but the membrane of another cell, the Schwann cell, which wraps itself repeatedly around the axon. The myelin sheath is interrupted at approximately one millimeter intervals by constrictions called nodes of Ranvier. Function of myelin sheath: 1. Insulates the axon 2. Speeds up the transmission of impulses.

Types of Neurons Fig: Three types of neurons Neurons can be classified according to how many processes they have: 1. Unipolar neuron 2. Bipolar neuron 3. Multipolar neuron

Unipolar neurons Have a single process coming out of the cell body which then divides into two branches, a dendrite and an axon . Unipolar neurons are very common in the peripheral nervous system of vertibrates. The sensory neurons for example has its incoming dendrite in the outer parts of the body, while the other branch, the axon extends into the spinal cord. Bipolar neurons Have two processes which come out of the cell body. One process is a dendrite and the other is an axon. All neurons in the nervous system of a human embryo are bipolar, but most of them then develop into the other two types, so there are few bipolar neurons in the nervous system of an adult. They occur in the retina of the eye, the cochlea in the inner ear and in the nerves serving cells in the nose which sense smell. Multipolar neurons Have many processes coming out of the cell body- several dendrites and one axon. Multipolar neurons are the most common type of neuron in the brain and spinal cord of mammals.

Nerve impulses Some nerve impulses originate inside the central nervous system; others come from the sense organs. Indeed, the sole function of a sense organ is to change various forms of stimulus, such as light or sound into nerve impulses which pass along sensory neuron fibers to the brain. Stimulation of a sense organ must reach what is called the threshold level before the organ sends a nerve impulse to the brain. A strong stimulus such as a flash of bright light results in the eyes sending hundreds of impulses per second to the brain, where a dim light produces only a few impulses per second, and a very dim one, below the threshold level, produce none at all. Impulses passing along a nerve fiber eventually reach the end of it, where they encounter an obstacle to their progress: a microscopic gap called a synapse between the tip of one fiber and the beginning of the next.

Synapses Neurons are not continuous with one another. The axon of one neuron does not usually make direct contact with the cell body of the next; the two cells are separated by a gap called a synapse. The cell which carries a signal towards a synapse is described as a presynaptic cell; the cell carrying the signal away from the synapse is the postsynaptic cell. Neurons usually transmit information across synapses using transmitter chemicals also called neurotransmitters. Why have synapses? Synapses are important because they allow the transfer of information in nerve networks to be controlled. They: allow information to pass from one neuron to another help ensure that a nerve impulse travels in one direction only. allow the next neuron to be excited or inhibited. can amplify a signal (make it stronger).

Synapses Fig: Main structures at a synapse Fig: The sequence of events in chemical transmission at a synapse

Structure of the synapse The axon terminal of a presynaptic neuron has a bulb-like appearance, and is often called the synaptic bulb. It meets the cell body or dendrite of the next neuron, leaving a gap or synaptic cleft. The synaptic bulb contains many mitochondria which provide energy for the manufacture of the chemical transmitters. Synaptic vesicles are the temporary vacuoles which store neurotransmitter chemicals What happens at the synapse? An action potential arrives at the synaptic bulb. This opens calcium channels in the presynaptic membrane. As the Ca 2+ ion concentration inside the membrane is lower than outside, Ca 2+ ions rush in. As their concentration increases, synaptic vesicles move towards the membrane, fuse with it and release neurotransmitter into the synaptic cleft. The neurotransmitter binds to receptors on the post synaptic cell membrane, some neurotransmitters open sodium channels in the post synaptic membrane causing an inflow of Na + ions. It makes the membrane more receptive to other incoming signals. Other neurotransmitters open chloride and potassium channels causing Cl - ions to flow into the cell and K + ions to flow out. It makes post synaptic membrane less receptive to incoming signals.

Reflex actions A reflex action is behavior which a stimulus results in a response which does not have to be learned and which occurs very quickly without conscious thought. For example, a person does not have to learn or even think what to do when his hand accidentally touches a very hot object, he automatically pulls his hand away. Such responses are built into the nervous system from birth. Withdrawal from a painful stimulus is an example of a spinal reflex. The nerve impulses involved in it pass through the spinal cord along spinal nerves. The pathway of these impulses is illustrated very simply in the following figure.

The human brain Human brain weighs about 1.5 kg and contains thousands of millions of neurons. The lowest part of the brain that is the part which merges with the spinal cord is called the medulla oblongata. Above the medulla is the cerebellum. Above the cerebellum is the cerebrum.

Medulla oblongata: is concerned with many unconscious processes including the regulation of blood pressure, body temperature, rates of heart-beat and breathing. It also contains the mass of nerve fibers which connect the brain and spinal cord. Cerebellum Receives impulses from sense organs concerned with balance. It also coordinates muscles during activities like walking, running, dancing and riding a bicycle. Cerebrum i) Is the largest part of human brain. ii) It consists of an outer neuron cell bodies (gray matter) and a much thicker inner layer of nerve fibers (white matter). The outer layer is called the cortex. iii) It is concerned with all forms of conscious activity. It is here that sensations of touch taste, hearing vision and smell are generated. iv) Reasoning takes place, decisions are taken, emotions are felt and memories are stored in the cortex.

nervous system

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nervous system