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Nervous System Topic 14 I. Functional Units of the Nervous System • A. neuron – functional unit of the nervous system • B. neurons are specialized cells that transmit information in the form of electrochemical signals called action potentials • C. signals are generated when the neuron alters the voltage found across its plasma or cell membrane • D. neurons have an excitable membrane which allows them to carry an action potential • E. parts of the neuron – 1. cell body – contains the nucleus and most of the organelles – also the site for protein synthesis and energy production – 2. dendrites – receive chemical information from other neurons as changes in membrane potential and carry this information to the cell body – 3. axon – a long, slender projection of the neuron that transmits the action potential from the cell body to the target the neuron is to communicate with – can be short or as long as a meter • F. Resting potential – all cells have a voltage across their plasma membrane that is generated through the actions of a protein called the Na+ / K+ ATPase – using the hydrolysis of ATP for energy, this protein pumps sodium ions out of the cell and potassium into the cell – it also maintains the osmotic balance in cells – some of the potassium leaks back out of the cell through an ion channel called the potassium leak channel – with more positive ions on the outside of the cell, a net negative voltage of about -70 mV is found across the plasma membrane and is called the resting potential http://video.search.yahoo.com/search/video;_ylt=A0PDoS6BZ4NNfx8Ayy.JzbkF?ei=UTF8&p=resting%20potential%20of%20a%20neuron&vm=r&fr2=tab-img&fr=yfp-t-941 • G. Action potential – a wave of electrochemical information that moves through axons and muscle tissue creating a response in those target tissue – neurons and muscle cells have a protein in their plasma membrane that lets sodium ions through the membrane in response to a decrease in the membrane potential – this protein is called a voltagegated sodium channel and gives neurons an excitable membrane • (continued on next slide) • – if the membrane voltage becomes less negative than the resting potential, changing from -70 mV to -50 mV, then the voltage-gated sodium channels in the neuron’s plasma membrane will open – the voltage at which the voltage-gated channels open is called the threshold potential – when these channels are open, sodium will diffuse freely through the channel to cross the plasma membrane, flowing down a gradient from the outside of the cell into the cytoplasm – the opening of these channels in one region of the membrane and the entry of sodium through the channels cause membrane depolarization • (continued on the next slide) • After the voltage-gated sodium channels have opened and depolarization is complete, the channels close – potassium channels open and potassium rushes out allowing the membrane voltage to return to its normal negative voltage – this is called repolarization – the sodium/potassium pump puts the sodium and potassium back to their original positions and the axon is now ready for a new action potential Action Potential Graph of an Action Potential http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter45/animat ions.html# • H. Size and Frequency of Action Potentials – 1. every action potential in a neuron is the same size – 2. neurons do not have half of a depolarization or half of an action potential – it is all or nothing – 3. the strength of the stimulus does not change the size of the action potential or the duration – 4. the difference is in the frequency of the action potential • (a) weak – may only trigger one action potential in a second • (b) strong – may trigger many more in the same period – 5. the membrane cannot fire again immediately – it must first depolarize and then repolarize – the time before it can fire again is called the refractory period Action Potential Rules • I. Synapses – spaces between neurons and their target cells – the membrane potential is converted into a chemical signal or neurotransmitter – the gap between the neuron and the target cell is called the synapse or synaptic cleft – the synapse between a neuron and a muscle is called the neuromuscular junction Synapse Neuromuscular Junction http://www.youtube.com/watch? v=9FF6UKvDgeE • J. turning off signals is as important as turning it on – ways to remove neurotransmitters – 1. diffuse into the surround fluid – 2. an enzyme degrades the neurotransmitter – 3. cells in the synapse take the neurotransmitter up II. Nervous Regulation • A. stimuli - any change in the environment that causes a response • B. receptors and effectors - receptors are the senses which detect stimuli effectors are muscles and glands which respond to the stimuli III. Adaptations for nervous control A. protozoans - paramecium have fibrils which transmit impulses to cilia B. hydra - have a nerve net - impulses may travel in any direction Hydra – nerve net C. Earthworm - has a “brain” or mass of ganglia (group of neurons), a ventral nerve cord and nerve branches Earthworm • D. grasshopper - similar to earthworm plus sensory organs – eyes, antennae, tympannum IV. Human Nervous System A. neurons - units of the nervous system three types of neurons – 1. Sensory neurons - transmit impulses from sense organs or receptors to the brain and spinal cord – 2. Motor neurons - transmit impulses from the central nervous system(brain and spinal cord) to muscles or glands(effectors) – 3. Interneurons - transmit impulses between sensory and motor neurons • B. nerves - bundles of neurons • C. Central Nervous System – Brain and Spinal Cord – 1. Structure and location - thick nerve that is found inside the vertebrae – 2. Functions - transmits impulses between the brain and the rest of the body • D. brain – 1. Structure and location - large mass made mostly of interneurons found inside the cranium – 2. Functions • a. cerebrum - voluntary movement, memory, reasoning – also called the cerebral cortex • b. cerebellum - balance • c. medulla - involuntary movement (breathing, heartbeat, blood pressure, digestion) • D. olfactory lobe – smells • E. thalamus – relays info to the cerebral cortex • F. hypothalamus – hunger, thirst, pain, temp. reg., water balance • G. pons – relays info from cerebrum to cerebellum • H. reticular activating system – brain stem – process sensory inputs Brain • E. peripheral nervous system - nerves that extend away from the brain and spinal cord – 1. Somatic nervous system - consists of nerves that control the voluntary muscles – 2. Autonomic nervous system - consists of nerves that control heart muscles, glands, and smooth muscles • (a) sympathetic nervous system – uses norepinephrine – fight or flight – increases heart rate • (b) parasympathetic nervous system – acetylcholine – deactivate or slow down activities of muscles and glands – rest and digest response • F. human behavior – 1. Involuntary behavior • a. simple reflexes - inborn, automatic patterns of behavior - goes from receptor to sensory neuron to interneuron to a motor neuron to an effector Reflex Arc • G. some malfunctions of the nervous system – 1. Meningitis - inflammation of the meninges (membranes around the brain and spinal cord) – 2. Cerebral palsy - damage to the motor centers of the brain – 3. Stroke - damage to the nerve cells in the brain blockage of artery – 4. Polio - virus of the spinal cord V. Sensory Organs • A. Eye – parts and functions – 1. cornea – transparent – bends and focuses light rays – 2. pupil – opening – 3. iris – muscle that controls the opening and closing of the pupil – responds to the intensity of light – 4. lens – suspended behind pupil – light travels through here and is focused on the retina – 5. retina – contains photoreceptors that transduce light into action potentials – image is actually upside down but revision and interpretation in the cerebral cortex result in the perception of the image right-side up – 6. optic nerve – conduct visual information to the brain – 7. cones and rods • a. cones – sensitive to color • b. rods – night vision – light and dark • B. Ear – parts and functions – 1. outer ear a. Auricle (pinna) b. Auditory canal – 2. middle ear a. Tympanic membrane (eardrum) – vibrates at the same frequency as the incoming sound b. Three bones or ossicles – amplify the stimulus and transmit it through the oval window which leads to the fluid-filled inner ear 1. Malleus (hammer) 2. Incus (anvil) 3. Stapes (stirrup) – 3. inner ear a. Cochlea – contains the organ of Corti which has specialized sensory cells called hair cells – hair cells vibrate – transduce pressure into action potentials b. Semicircular canals – used for balance • C. Taste and Smell – 1. taste buds – chemical sensory cells located on the tongue – sour, salty, sweet, and bitter – 2. olfactory receptors – chemical sensors found in the olfactory membrane in the upper part of the nostrils – axons join to form the olfactory nerves