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Outcomes… Part 1… 6.5 Nerves, Hormones and Homeostasis IB Biology SL Part 1 - Nerves Imagine you are on a beach. What do you feel? What do you see? Smell? You can recreate the experience of being at the beach without any external stimuli. How is this possible? the brain, and from the brain only, arise our pleasures, joys, laughter and jests, as well as our sorrows, pains griefs and tears. Through it, in particular, we think, see, hear... Eyes, ears, tongue, hands and feet act in accordance with the discernment of the brain.” 6.5.1State that the nervous system consists of the central nervous system (CNS) and peripheral nerves, and is composed of cells called neurons that can carry rapid electrical impulses. 6.5.2Draw and label a diagram of the structure of a motor neuron. 6.5.3State that nerve impulses are conducted from receptors to the CNS by sensory neurons, within the CNS by relay neurons, and from the CNS to effectors by motor neurons. 6.5.4Define resting potential and action potential (depolarization and repolarization). 6.5.5Explain how a nerve impulse passes along a nonmyelinated neuron. 6.5.6Explain the principles of synaptic transmission. What is pain? What is pleasure? What are thoughts? We know the brain is made up of cells but how does the miracle of the mind emerge from this mass of cells? The human Nervous System is a whole that is far greater than the sum of its parts. “...from Hippocrates Much of what we know about the brain is drawn from inferences. There remain many unanswered questions... This makes neuroscience so fascinating! 1 The Central Nervous System The nervous system consists of two parts, the central nervous system and peripheral nervous system. The Central Nervous System The central nervous system (CNS) consists of the brain and spinal cord. Both structures receive sensory information from receptors all over the body and they interpret the information, process it and decide if a response is required. A response by the brain or spinal cord is known as a motor response. The The Peripheral Nervous System The peripheral nervous system includes all other nerves that connect to the CNS. Peripheral nerves are composed of sensory neurons and motor neurons. Neurons The nervous system is made up of special nerve cells called neurons. These cells are very different in shape from other eukaryotic cells and they transmit messages in the form of electrical impulses at incredible speed. Many neurons grouped together form a nerve. peripheral system has two categories of peripheral nerves. There are 31 pairs of spinal nerves which emerge directly from the spinal cord and are a mix of sensory neurons and motor neurons. It also has 12 pairs of cranial nerves which emerge from the brain stem. Structure of a Motor Neuron 2 Cell body: Contains the nucleus, rough ER and other organelles. Axon: Dendrites: Myelin Schwann Nodes much smaller cytoplasm extensions which carry impulses to the cell body. Cells: special type of glial cells that produce the myelin sheath. a very long and thin extension of cytoplasm from the cell body. Carries impulses away from the cell body. Sheath: a type of insulation that is wrapped around the axons of neurons; composed of Schwann cells. Helps reduce signal loss. of Ranvier: regularly occurring gaps between sections of myelin sheath where the axon is “naked”. 3 Relay neurons carry impulses within the CNS itself from neuron to neuron; also known as interneurons or association neurons. Nerve Impulses Sensory neurons carry impulses (information) from sensory cells in the body (receptors) to the CNS; also known as afferent neurons. Motor neurons carry impulses away from the CNS to the effectors (muscles and gland cells); also known as efferent neurons. Reflex Arc neurons, relay neurons and motor neurons are involved in a neural pathway known as a reflex arc which allows for a quick reaction. Resting Potential Sensory All cells have an electrical potential difference (voltage) across their plasma membrane that is measured in mV and the difference is known as the membrane potential. Neurons typically have a membrane potential between -60 and -80 mV (millivolts) when the cell is not transmitting a signal and this is called the resting potential. 4 Resting potential can be defined as the electrical potential across the plasma membrane of a cell that is not sending an impulse. The resting potential of all neurons depends on the ionic gradients that exist across the plasma membrane of the cell. NOTE: Sodium ions are highly concentrated outside of the cell and they have a tendency to diffuse inside the cell. Potassium ions diffuse out of the cell as sodium diffuses in but the membrane is about 50 times more permeable to potassium than sodium so the movement is unequal. The sodium-potassium pumps use active transport to control the movement of these ions. When an impulse is passing along a neuron the potassium and sodium ions are allowed to diffuse (passive transport) across the membrane through proteins known as voltage-gated ion channels. This reverses the electrical potential of the neuron but it is quickly restored and this is the action potential. The neuron is depolarized and then quickly Neurons use active transport to maintain a balance of ions across their membranes. Sodium ions are pumped out and potassium ions are pumped in. There are chloride ions, DNA and other negatively charged ions inside the neuron that are fairly large and have a tendency to stay inside which creates a net negative charge inside the neuron as compared with the more positive external environment. This creates the resting potential and the membrane is said to be polarized. Action Potential Action potential can be defined as the reversal and restoration of the electrical potential across the plasma membrane of a cell as an electrical impulse passes along it. It is also measured in mV Depolarization Depolarization is the diffusion of sodium ions into the nerve cell resulting in a charge reversal (the inside becomes more positive). repolarized. 5 Repolarization Repolarization is the process of restoring the original polarity of the nerve membrane. Nerve Impulses Understanding of how an action potential works is the key to understanding how a nerve impulse passes along the axon of a neuron. An action potential in one part of a neuron will cause the development of an action potential in the next section of the neuron. This can occur because sodium ions flow from a region with an action potential to a region with a resting potential. As the ions move the resting potential is reduced which results in the opening of the voltage-gated channels. When a neuron is excited the membrane becomes more permeable to sodium than potassium. Scientists believe this occurs because sodium gates open while potassium gates close. The membrane potential is reduced and more sodium channels open. As sodium ions flow into the neuron via diffusion and charge attraction the inside of the membrane becomes positive (charge reversal) and depolarization occurs. The membrane potential has been reversed. 6 Once depolarization occurs the sodium gates are closed and the potassium channels open. Potassium diffuses out of the neuron in the direction of the concentration gradient. The loss of the positive potassium ions causes the internal environment of the neuron to become negative once again and the potential across the membrane is restored. This is known as repolarization; the return to the original polarity of the nerve membrane. The action potential moves along the membrane of the neuron creating a wave of depolarization and repolarization. As the impulse moves along the axon of the neuron it moves from a depolarized region and initiates depolarization in the next region. Sodium-potassium pumps are used to restore the concentration gradients of the ions via active transport. Sodium is pumped back out of the neuron while potassium is pumped back in. The resting potential of the neuron is now restored and can now conduct another impulse. Interactive Action Potential Animation: http://outreach.mcb.harvard. edu/animations/actionpotenti al.swf Review of Action Potential: http://www.youtube.com/ watch?v=HnKMB11ih2o &feature=related 7 Synaptic Transmission At the end of the axons of neurons there are swollen membranous areas called terminal buttons. Inside the terminal buttons are small vesicles filled with chemicals known as neurotransmitters. Synapse A synapse is a junction between two neurons and the plasma membranes of those neurons are separated by a narrow fluid-filled gap known as the synaptic cleft. Synaptic transmission is the transmission of an action potential across the synapse from the presynaptic neuron to the postsynaptic neuron. The following sequence of events outlines the steps in synaptic These chemicals are used for synaptic transmission and they always pass in the same direction; from the presynaptic neuron to the postsynaptic neuron. 1. The nerve impulse (action potential) arrives at the end of the presynaptic neuron. transmission. 2. Depolarization of the membrane occurs and the voltage gated calcium channels open allowing calcium ions diffuse into the terminal button. 3. The movement of the calcium ions in causes the vesicles containing the neurotransmitter to fuse with the plasma membrane of the neuron and release the neurotransmitter into the synaptic cleft. 8 4. The neurotransmitter diffuses across the cleft from the presynaptic neuron and binds to receptors (transmitter-gated ion channels) in the postsynaptic neuron. neurotransmitter in the cleft is quickly broken down by enzymes to prevent continuous synaptic transmission and the calcium ions are pumped out of the presynaptic neuron and into the synaptic cleft. The ions channels close to sodium ions. 5. The binding of the neurotransmitter results in the opening of the ion channels and sodium ions along with other positively charged ions diffuse into the postsynaptic neuron. 6. The movement of the ions causes the depolarization of the postsynaptic membrane and this initiates the movement of the action potential down the neuron. The Let’s have a look… 8. The fragments of the broken down neurotransmitters diffuse back across the synaptic cleft and into the vesicles where they can be reassembled. And now for some practice IB questions… Synaptic Transmission animation http://www.sumanasinc.com/webcontent/animati ons/content/synaptictransmission.html Another one http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapte r45/animations.html# 9 Draw and label a motor neuron showing the direction of nerve impulse propagation (3) Explain how a nerve impulse travels along an unmyelinated neuron (8) You could also draw a diagram to support your answer and help you to explain! Explain the principles of synaptic transmission (8) Outline the use of four methods of membrane transport in nerves and synapses (8) 10