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Nervous System • All animals must respond to environmental stimuli – Sensory receptors – detect stimulus – Motor effectors – respond to stimulus • The nervous system links the two • Consists of neurons (nerve cells) and supporting cells (neuroglia) Nervous System • Vertebrates have three types of neurons (nerve cell) • Sensory – carry impulses from sensory receptors to the central nervous system (CNS) • Motor – carry impulses from the CNS to effectors (muscles and glands) • Interneurons (association neurons) – located in the brain and spinal cord; provide more complex reflexes and higher associative functions such as learning and memory; “integrators” Nervous System • The central nervous system (CNS) consists of the brain and the spinal cord • The peripheral nervous system (PNS) consists of sensory and motor neurons; a network of nerves extending into different parts of the body; carries sensory input to the CNS and motor output away from the CNS Nervous System: Neurons • The basic structure of the neuron (consists of): – Cell body – an enlarged region containing nucleus – Dendrite – short cytoplasmic extensions extending from the cell body; receive stimuli – Axon – single, long extension that conducts impulses away from the cell body • The axons controlling the muscles in a person’s feet can be more than 1 meter (3 ft) long!; axons from the skull to the pelvis in a giraffe are 3 meters (9 ft) long! Neuron Nervous System: Neuroglia • Neurons are supported structurally and functionally by supporting cells called neuroglia – 1/10th size of a neuron, but 10x more abundant – Schwann cells and Oligodendrocytes – produce the myelin sheaths in the PNS and the CNS, respectively • Myelin sheaths surround and insulate the axon of many types of neurons; “myelinated” axons Nervous System: Neuroglia • Other neuroglia provide neurons with nutrients, remove wastes • Small gaps known as nodes of Ranvier interrupt the myelin sheath at intervals of 12μm; uninsulated, capable of generating electrical activity (sites of action potential) Conduction of the nerve impulse • Upon stimulation of a nerve cell, electrical changes spread or propagate from one part of the cell to another • Neuron function depends on a changeable permeability to ions; an electrical difference exists across the plasma membrane • Membrane potential – voltage measured across a membrane due to differences in electrical charge; inside of cell is negative relative to outside Conduction of the nerve impulse • When a neuron is not being stimulated, it maintains a resting potential; -70mV (average; ranges from -40 to -90; “polarized”) • The inside of the cell is negatively charged relative to the outside – Polarization is established by maintaining an excess of Na+ ions on the outside, and an excess of K+ ions on the inside • Most animal cells have a low concentration of Na+ and a high K+ relative to their surroundings Conduction of the nerve impulse • A certain amount of Na+ and K+ ions are always leaking across the membrane through leakage channels, but Na+/K+ pumps actively restore the ions to their appropriate sides – The Na+/K+ pump: brings in 2 K+ for every 3 Na+ pumped out – Ion leakage channels: allow more K+ to diffuse out than Na+ to diffuse in Conduction of the nerve impulse • Other ions, such as large, negatively-charged proteins and amino acids, reside within the cell • It is these large, negatively-charged ions that contribute to the overall negative charge on the inside of the cell membrane relative to the outside • Negative pole: Cytoplasm (inside cell) • Positive pole: Extracellular (outside cell) Remember: Cells contain relatively high [K+] inside the cell, but low [Na+] Conduction of the nerve impulse • A nerve impulse is generated when the difference in electrical charge disappears – Occurs when a stimulus contacts the tip of a dendrite and increases the permeability of the cell membrane to Na+ ions – Na+ ions rush into the cystoplasm, and the difference in electrical charge across the membrane disappears (depolarized) – Remember, the concentration of Na+ inside the cell is low relative to its surroundings Conduction of the nerve impulse • Some stimuli open K+ channels – As a result, K+ leaves the cell (remember: high [K+] inside the cell) – Membrane potential becomes more negative (more negative inside the cell) – “hyperpolarization” • Some stimuli open Na+ channels – Causes Na+ to enter the cell – Membrane potential becomes less positive – “depolarization” Conduction of the nerve impulse • When the strength of stimuli determines how many ion channels will open; graded response • Caused by the acvtivation of a gated ion channels which behave like a door that can open or close, unlike ion leakage channels that are always open • Each gated channel is selective, opening only to allow diffusion of one type of ion • Normally closed in a resting cell Graded Potentials Action Potentials • Permeability changes are measurable as depolarizations or hyperpolarizations of the membrane potential • Depolarization – makes membrane potential less negative (more positive) • Hyperpolarization – makes membrane potential more negative • Ex: -70mV -65mV = Depolarization -70mV -75mV = Hyperpolarization Action Potentials • Action potentials are rapid, reversals in voltage across the plasma membrane of axons • Once a threshold of depolarization is reached (-50 to -55 mV), an action potential will occur • An ‘all or nothing’ response, not graded • Magnitude of the action potential is independent of strength of depolarizing stimuli • Action potentials are the signals by which neurons communicate and spread messages Action Potentials • An action potential is caused by a different class of ion channels, voltage-gated ion channels • These channels open and close in response to changes in membrane potential; only open at certain membrane potentials; flow of ions controlled by these channels creates the action potential Action Potentials • Voltage-gated channels are very specific; each ion has its own channel – Voltage-gated Na+ channels – Voltage-gated K+ channels Action Potentials • When the threshold voltage is reached, Na+ channels open rapidly • Influx of Na+ causes the membrane to depolarize • K+ channels open slowly, eventually repolarizing the membrane • Action potential consists of three phases: • Rising, falling, and undershoot Action Potentials – the Spoiler • At threshold, membrane is depolarized enough that Na+ voltage-gated channels open; Na+ moves into interior of cell, becoming less negative; rapid depolarization ( 45mV), then stops – Stops because channels will close after a specific amount of time has elapsed Action Potentials II – the Spoiler • K+ voltage-gated channels will also open, before membrane potential reaches zero; K+ moves out of cell, making cell become more negative, returns cell to resting • Na+/K+ pump is also activated, moving 3 Na+ out for every 2 K+ in, makes cell more negative • Returns cell to rest (~-70mV) Action Potentials III – the Spoiler • Excess K+ diffuse out before K+ channel closes, or over-activity of the Na+/K+ pump; results in undershoot (hyperpolarization) • Entire process occurs very rapidly: 2-4ms from start to finish Action Potentials • Each action potential, in its rising, reflects a reversal in membrane polarity • Positive charges due to Na+ influx can depolarize adjacent region to threshold, causing the next region to produce an action potential of its own • The previous region then repolarizes back to its resting membrane potential 3. Top curve 2. Rising Phase Maximum voltage reached Stimulus causes above threshold voltage Potassium gate opens K+ Na+ 1. Resting Phase Equilibrium between diffusion of K+ out of cell and voltage pulling K+ into cell Voltage-gated potassium channel Membrane potential (mV) Sodium channel activation gate opens Na+ channel inactivation gate closes +50 0 –70 1 3 2 Time (ms) 4. Falling Phase Undershoot occurs as excess potassium diffuses out before potassium channel closes Potassium channel gate closes Potassium gate open Equilibrium restored Potassium channel Voltage-gated sodium channel Sodium channel activation gate closes. Inactivation gate opens. Na+ channel inactivation gate closed Action Potentials • Action potentials are localized events • They DO NOT travel down the membrane • They are generated anew in a sequence along the neuron as they propogate along axon • During undershoot, the membrane is less likely to depolarize • This keeps the action potential moving in one direction resting repolarized depolarized + + + + + + + + + – – – – – – – – – + + + + + + + + + – – – – – – – – – – – + + + + + + + + + – – – – – – – Na+ + + + + + + + – – – – – – – – – + + K+ + + – – + + + + + – – + + – – – – – Na+ + + + + + – – + + – – – – – + + – – K+ K+ + + + + – – – + + – – – – + + + – – Na+ + + – – – + + + + – – + + + – – – – K+ K+ + + + + + + + – – – – – – – – – + + Na+ – – + + + + + + + + + – – – – – – – Cytoplasm Cell membrane K+ Saltatory Conduction • Two ways to increase velocity of conduction: – Increase diameter of axon; reduces resistance to current flow; found primarily in invertebrates – Axon is myelinated; impulse jumps from node to node (Nodes of Ranvier – the only site of action potentials) = saltatory conduction – one action potential still serves as stimulus for the next one, but the impulse (depolarization at one end) spreads quickly beneath the insulating myelin to trigger the opening of voltage-gated ion channels at the next node Saltatory Conduction http://www.flickr.com/photos/photoklick/2829645922/ Saltatory Conduction