PYC1501 LATEST EXAM PACK

Human nervous system IMPULSE CONDUCTION IN THE HUMAN NERVOUS SYSTEM Parts of a neuron Dendrites: look like tree roots, receive messages from other neurons and surround the cell body Cell body: also called the soma. It receives and sends messages in the form of new impulses Cell nucleus: control centre of the cell; it controls all metabolic activities Axon: carries messages from the soma. It is a thin fibre that differs in length Myelin: white fatty sheath insulating the axon. It conducts much faster than un-myelinated axons. Multiple sclerosis is the effect of axons that aren’t properly myelinated, as it attacks the myelin Axon terminals: also called telodendria. Axon terminals branch out from the axon and end in small knobs, called boutons (buttons in French). Vesicles: tiny containers in the boutons which are filled with neurotransmitters Neurotransmitters: chemical substances that play an important role in the conduction of a message from one neuron to the next Types of neurons Sensory/afferent: carry messages from the environment to the brain/spinal cord. Info is detected by the senses. This info can also come from the organs in the body Motor/efferent: conduct messages from the spinal cord/brain to the muscles and glands Nerve tract: bundle of axons in the brain/spinal cord Nerve: bundle of nerves outside the brain and spinal cord Process of impulse conduction Stimulus is a form of energy received by the senses and converted into a form of energy understandable to the nervous system. Impulse conduction is the basic form of sending info in the nervous system. Two main processes: • Electrical – nerve impulse begins in the first segment of the axon and travels down the axon to the terminals because of electrical events at the cell membrane 3 • Chemical – the passage of the nerve impulse from one axon to another. There is a small gap between the axon terminals of one neuron and the dendrites of another and the chemical process will determine whether it reaches it or not Nerve impulse: each neuron is like a tiny battery that stores potential energy. The fluid inside and outside the cell contains small chemical particles called ions that are electrically charged. Some are positive and some are negative. There are more positive ions on the outside of the cell and more negative ones on the inside and they naturally constantly move from an area of high concentration to an area of low concentration. Also, opposites attract and like repel. Resting membrane potential: condition of readiness before an impulse can fire. It is an electrical charge brought about by the difference between the positive and negative ions inside and outside the cell. The neuron is ready to receive or conduct impulses Action potential: messages arriving from other neurons alter the resting potential. If the resting potential changes enough the cell reaches a threshold or critical point. Each neuron has a different threshold and the stimulus has to be intense enough to exceed the threshold and change the resting potential into action potential. In this way the structure of the axon membrane changes: tiny openings in the cell membrane allow ions from the outside of the cell to move inside. These channels first open near the soma and then sweep along the length of the axon as the action potential (and thus the impulse) sweeps along. Refractory period: immediately after an impulse has been conducted, the neuron is not ready to send another message until the resting potential has been restored. Two types of refractory periods: • Absolute – no impulse can be generated • Relative – an impulse can be generated but only with very intense stiulus Refractory periods ensure that stimulus only travel one way and also prevent over-stimulation Characteristics of impulse conduction: ‘all or nothing’ event. The stimulus is either strong enough to result in impulse conduction or it isn’t. the strength of the stimulus does not change the strength or speed of the impulse. • Strength + speed: the strength + speed of impulse conduction is constant within a particular neuron but it can vary with nerve fibres of different sizes. The larger the fibre, the stronger and faster the impulse can be conducted. 4 • Frequency: the intensity of the stimulus does make a difference in terms of the frequency at which impulses are conducted. If the stimulus is very strong there is a shorter space between the firing of each impulse so the frequency increases. • Effect of myelination: myelin sheaths insulate the axons and make action potentials travel much faster than along un-myelinated axons. There are gaps or nodes between the sheaths and it is in these nodes that ion channels open and the impulse is conducted by jumping from node to node (saltatory conduction) Synaptic transmission of impulses: the conduction of a nerve impulse in a neuron is electrical, but between neurons it is chemical. There is a tiny gap between neurons called a synapse. When an action potential reaches the tip of the axon terminals, the vesicles attach themselves to the presynaptic membrane where the membrane opens and causes chemicals to be released into the little space between the neurons called the synaptic cleft. This is the gap between the presynaptic membrane of one neuron and the postsynaptic membrane of another. These chemicals are called neurotransmitters. These chemicals combine with the fluid outside the cells and receptors in the postsynaptic membrane. Different neurons use different chemicals as their neurotransmitters but each neuron releases the same chemical from all branches of its axon. Postsynaptic potentials: Neurons that excite can make the next neuron more likely to produce an action potential. The action potential in the next neuron is called postsynaptic potential. Other neurons release neurotransmitters than inhibit the production of an action potential in the next neuron. Once the neurotransmitter excites/inhibits a receptor in the next neuron, it can become reabsorbed by the axon that released it (re-uptake), it could diffuse away, it could be broken up by enzymes, or it could bounce around for a while and then return to the postsynaptic receptor again. The longer the neurotransmitter stays in the synaptic cleft, the more likely it is to affect the next neuron. A postsynaptic potential will only be generated if the amounts of neurotransmitters discharged into the synapse are large enough. Postsynaptic potential is a graded potential. The impulse will get weaker as it travels further from the point of stimulation. If the impulse is no reinforced or strengthened, it may disappear before it reaches the next axon and then no postsynaptic potential is generated. Even a weak impulse can be strengthened by additional neurotransmitters: • Spatial summation – action potentials from the terminals of several axons reaching the same synapse