The Nervous System Biology 12 Unit 3: Homeostasis December 11, 2015 The nervous system is an elaborate communication system that contains more than 100 billion nerve cells in the brain alone There are two main divisions of the vertebrate nervous system Central Nervous System (CNS) Peripheral Nervous System (PNS) Central Nervous System The CNS consist of the nerves of the brain and spinal cord and acts as a coordinating centre for incoming and outgoing information. The PNS consists of nerves that carry information between the organs of the body and the central nervous system Anatomy of a Nerve Cell Two different types of cells are found in the nerve: Glial cells and neurons Glial Cells(often called neuroglialcells) are nonconductingcells and are important for the structural support and metabolism of the nerve cells Neuronsare the functional units of the nervous system. These specialized nerve cells are categorized into three groups: sensory neurons interneurons motor neurons Sensory Neurons Also known as afferent neurons These sense and relay information (or stimuli) from the environment to the CNS for processing For Example: special sensory receptors in your eyes, known as photoreceptors, are sensitive to chemicals respond to light Sensory neurons are located in clusters called ganglia 1
Interneurons These special neurons do exactly what the name suggests: they link neurons within the body. These are found mostly throughout the brain and spinal cord These integrate and interpret the sensory information and connect neurons to outgoing motor neurons Motor Neuron Also known as efferent neurons Relay information to the effectors muscles, organs, and glands are classified as effectors because they produce responses Dendrites all neurons contain dendrites, cell bodies, and axons Dendrites receive information from other nerve cells All Dendrites contain a cell body with a nucleus Dendrites conduct nerve impulses toward the cell body An extension of cytoplasm, called the axon, projects nerve impulses from the cell body Many axons are converted with a glistening white coat of a fatty protein called the myelin sheath, which acts as insulation for the neurons Axons that have a myelin covering are said to be myelinated Formed by special glial cells called Schwann cells, the myelin sheath insulates by preventing loss of charged ions from the nerve cell The areas between the sections of myelin sheath are known as the nodes of Ranvier Nerve impulses jump from one node to another, thereby speeding the movement of nerve impulses. Nerve impulses travel much faster along myelinated nerve fibers than nonmyelinated ones Speed is also affected by the diameter of the axon The smaller the diameter of axonthe faster the speed of the nerve impulse Neurilemma All nerve fibers in the PNS contain a thin membrane called the neurilemma, which surrounds the axon the neurilemma promotes regeneration of damaged axons this is why when you cut your finger the feeling can gradually return 2
Nerves that contain myelinated fibers in the brain appear white in colour and therefore are called white matter Nerves that do not contain myelinated fibers in the brain appear grey in colour and are called grey matter Neural Circuits If you accidently touch a hot stove, you probably do not stop to think about how your nervous system tells you it is hot. Events of a Reflex Arc 1. Stimulus causes action potentials in the sensory receptor 2. Message travels along sensory neuron 3. Message travels along sensory dendrite 4. Message reaches interneuron dendrite 5. Message splits: one to brain, one to motor neuron dendrite 6. Message travels along motor neuron axon 7. Message causes muscle to contract Electrochemical Impulse As early as 1900 German physiologist Julius Bernstein suggested that nerve impulse were an electrochemical message created by the movement of ionsthrough the nerve cell membrane A rapid change in electric potential difference (voltage) was detected each time a nerve was excited Electrochemical Impulse Normally the cell has a potential of -70mV (millivolts) This is called resting potential When the nerve is excited the potential changes to +40mV This is called action potential How are these axons charged Under normal conditions the number of Na +1 on the outside of the cell is greater than the number inside the cell this creates a charge imbalance this is called a polarized membrane 3
The Nerve Impulse When the nerve cell is excited the cell membrane becomes more permeable to sodium so there is a change in electric potential this is called depolarization This is an all or none event; that is, it happens the same way every time it happens Video https://www.youtube.com/watch?v=ifd1yg07fb8 (a) The resting membrane is more permeable to potassium than to sodium. Potassium ions diffuse out of the nerve cell faster than sodium ions diffuse into the nerve cell. The outside of the nerve cell becomes positive relative to the inside (b) A strong electrical disturbance, shown by the darker colour, moves across the cell membrane. The disturbance opens sodium ion gates and sodium ions rush into the nerve cell. The membrane becomes depolarized (c) Depolarization causes the sodium gates to close, while the potassium gates are opened once again. Potassium follows the concentration gradient and moves out of the nerve cell by diffusion. Adjoining areas of the nerve membrane become permeable to sodium ions and the action potential moves away from the site of origin (d) The electrical disturbance moves along the nerve membrane in a wave of depolarizations. The membrane is restored, as successive areas once again become permeable to potassium. The sodiumpotassium pump restores and maintains polarizations of the membrane by pumping potassium ions in and sodium ions out of the cell Refractory Period The time required for the nerve cell to become repolarized is called the refractory period. This can be 1 to 10 ms (0.001 0.010s) 4
Movement of the Action Potential The action potential moves along the nerve cell membrane like a wave It creates a very fastcycle of depolization and repolarization Threshold Levels The threshold level is the minimum level of a stimulus required to produce a response It is important to realize that this threshold works in an all or noneway: it happens or doesn t The brain interprets the number of neurons excited and the frequency of impulses The more neurons stimulated lead to more intense responses For example, a 40 C metal bar may trigger one neuron and a 50 C may trigger two neurons. The brain interprets the more neurons as the hotter of the two Synaptic Transmission Small spaces between neurons and effectors are known as synapses A single neuron may branch many times at its end plate and join with many different neurons Synapses rarely involve just two neurons Synaptic Transmission Small vesicles containing chemicals called neurotransmittersare located in the end plates of axons The impulse moves along the axon and releases neurotransmitters from the end plate The neurotransmitters are released from the presynamptic neuron and diffuse across the synamptic clef, creating a depolarization of the dendrite of the postsynaptic neuron The space between neurons is 20nm (20 x 10-9 m) Video https://www.youtube.com/watch?v=p5zfgt4aofa Acetylcholine This is an example of a neurotransmitter found in the end plates of many nerve cells It can act as an excitatory neurotransmitter by opening sodium channels causing depolarization This excitation would continue as long as acetylcholine is present An enzyme, colinesterase, destroys acetylcholine which closes the sodium channels and allows the neuron to repolarize 5
Summation In a situation where one neuron firing is not enough to meet the threshold, two neurons can meet the threshold In this case either A or B neurons (above) will not trigger a response if fired independently If they are triggered together, they will trigger a response Neurotransmitter must trigger them at the same timeso neuron D can be activated This theory is called summation Questions Page 417 #2,5 Page 426 #4,6,8,10,14 Please Complete these on loose-leaf 6