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The Nervous System A system of the body that coordinates & regulates the activities of the body Control System Maintains homeostasis homeo/stasis (same/changing): changes in order to keep a balance Feedback ◦ Negative feedback: to reverse a current trend ◦ Positive feedback: amplifies a current trend 5 major components Stimulus receptor -highly specific -receive stimuli Sensory Pathway modulator/regulator -Selects appropriate Response (spinal cord or brain) Motor Pathway effector -carries out the response (muscle or gland) Action The Nervous System & Homeostasis ◦ Organization of the Nervous System ◦ Structure of a neuron ◦ Action Potential ◦ Synaptic Transmission ◦ Structure of the Brain ◦ Senses: The Eye & Ear Nervous System Organizational Tree! Nervous System Central Nervous System (CNS) Decision maker Brain & Spinal Cord Peripheral Nervous System (PNS) Feeds into & out of CNS Sensory Pathway Somatic Pathway (Voluntary) under conscious control Examples? NS Overview Sympathetic (Stimulatory) Speeds you up! Excites you! Motor Pathway Autonomic Pathway (Involuntary) unconscious control Examples? Parasympathetic (Restores to normal) Restores balance! Restores Homeostasis! Anatomy of a Nerve Cell Two different types of cells are found in the nervous system: ◦ Glial Cells: non-conducting; important for support and metabolism of nerve cells ◦ Neurons: function units of the nervous system (conduct nerve impulses) The Neuron: Wires within a nerve! See diagram on p.410 of text The Neuron Dendrite – receives information from receptors or other neurons and conducts nerve impulses toward the cell body. Synaptic Knob – aids in nerve impulse transmission Cell Body – contains nucleus and organelles Nodes of Ranvier – gaps within the myelin sheath Impulses jump from node-to-node therefore speeding up the impulses Neurillemma – delicate membrane that promotes regeneration of damaged neurons Only found in myelinated neurons Myelin – a fatty that Axon – conducts nerve impulses away protein from the cellcovers body the axon Composed of Schwann cells, which help regenerate damaged neurons Insulate the axon allowing nerve impulses to travel faster Myelination is only found outside the brain and spinal cord Structure of a Neuron From Nelson Biology The Job of Schwaan cells = Wrap Axons! Schwaan cells: Nourish the axon Provide insulation Repair axon damage Found mainly in PNS Neurilemma = •concentric rings around axon •created by the Schwaan cell • repair mechanism Job of Myelin! Provide insulation like the covering on speaker wire Prevent loss of signal down axon! Damaged myelin results in a loss of signal down the axon! (Results in) Multiple Sclerosis (MS) Types of Neurons – see your handout! Motor Neuron ◦ Connects the central nervous system to a muscle or a gland (also called efferent neurons) Sensory Neuron ◦ Connects a sensory receptor to the central nervous system (also called afferent neurons) Interneuron (or Association Neuron) ◦ Connects two or more neurons Types of Neurons Sensory Neurons (Afferent Neurons) – conducts nerve impulse from sense organs to the brain and spinal cord (CNS) Interneuron (Association Neuron) – found within the CNS No myelination Intergrates and interprets sensory information and relays information to outgoing neurons Motor Neuron (Efferent Neurons) – conducts nerve impulses from CNS to muscle fiber or glands (effectors) To Do: Complete Sections A thru C in your Notes Package Color and Label Neuron Diagram Action Potential how do nerves work??? In 1900 Bernstein hypothesized that nerve impulses where electrochemical in nature. ◦ Future experimentation proved this. Giant Squid Experiment: ◦ Cole and Curtis placed two tiny electrodes – one inside the large axon of a squid and the second across from the first outside the axon. Giant Squid Experiment Squid Axon •Cole and Curtis measured the electrical potential across the membrane. •The resting potential was found to be about – 70mV. When stimulated, the action potential jumped to about +40 mV. The action potential only lasted for a few milliseconds before the nerve cell returned to the resting potential. +40 threshold mV -70 1 2 ms 3 4 Definitions: Action Potential: ◦ the voltage difference across a nerve cell membrane when the nerve is excited (~40 mV) Resting Potential: ◦ Voltage difference across a nerve membrane when it is NOT transmitting a nerve impulse (almost always -70 mV) Maintaining Resting Potential Caused by an uneven distribution of positively charged ions across the membrane Set up and maintained by a Sodium-Potassium pump. 3 Na+ are pumped out of the cell, 2 K+ ions are pumped into the cell. sodium/potassium ion pump sodium/potassium pump 2 These positive ions want to move with their concentration gradient by diffusion. Sodium moves out faster than potassium moves in leaving a “relative” negative charge inside the cell. The cell is polarized. resting potential clip Action Potential A Nerve impulse is an Action Potential When a neuron receives a stimulus it becomes more permeable to sodium than potassium ◦ When stimulated the ion gates for potassium close and the ion gates for sodium open up. Positive ions flood into the cell making it positive. This rapid inflow is referred to as depolarization. After the impulse, the Na+ channels close and the K+ channels open. This is called repolarization. ◦ The flow of potassium ions out of the cell (with their concentration gradient) restores the resting potential. The potassium gates close relatively slowly which makes the inside of the neuron slightly more negative then resting potential (hyperpolarization) The Na+/K+ pump continues to pump the sodium and potassium across the membrane against the concentration gradient to restore the resting potential. Refractory Period: Repolarization takes about 0.001 seconds. Once stimulated, the membrane cannot be depolarized until after the refractory period. ◦ Recovery time required before a neuron can produce another action potential. Summary of Impulse. 1. At rest – Na+/K+ pump moving 2. Stimulation – sodium gates open 3. The flood of sodium into the cytoplasm stimulate adjacent areas 4. Refractory – potassium gates open – sodium gates close 5. At rest – Na+/K+ pump moving ions – potassium gates open Action potential overview Electropotential graph Movement of Action Potential Many action potentials are generated one after another along the cell membrane, causing a wave of depolarization (similar to falling dominos). When axons are myelinated, nerve impulses travel by saltatory conduction ◦ Gated ion channels are concentrated at the nodes of Ranvier ◦ Flow of ions across cell membrane can only happen at the nodes so action potentials “jump” from node to node ◦ This causes the signal to be transmitted down an axon much faster. myelinated vs. unmyelinated impuse To Do: Read pages 415 – 419 in your textbook Complete Section D in your notes package Nerve impulse coloring diagram http://www.mcgrawhill.ca/school/applets/abbio/ch11/actionpotential_action.swf Reflex Arc A reflex that does not require the brain Interneuron sends message back on the motor neuron at the same time as it sends the message to the brain Reflexes may be innate or acquired Reflex Arc Reflexes are autonomic responses to certain stimuli They are not under conscious control, they are involuntary They pathway that a nerve impulse takes is called a reflex arc ◦ We need to identify the stimulus, receptor, sensory neuron, motor neuron, effector, and the response. Anatomy of a Reflex Arc How the Reflex Arc Functions: 1) Sensory organs (receptors) detect dangerous stimuli! 2) Impulse is passed from the sensory organ to a sensory neuron! 3) Sensory Neuron transfers the impulse to the Association neuron in the spinal cord! How the Reflex Arc Functions… 4) The INTERNEURON links the SENSORY to the MOTOR neuron! 5) The MOTOR neuron takes the impulse to the EFFECTOR! 6) The effector (usually a muscle) reacts. 7) Simultaneously, interneurons send the signal up to the BRAIN for interpretation! Reflex Arc Video To Do: Reflex Lab Section E in your notes package ◦ P. 414 Questions # 2-6 Synaptic Transmission Neurons are not directly connected to each other. The electrochemical action potential cannot jump the synaptic cleft (or synapse). Synaptic transmission is entirely chemical in nature. Synapse movie clip Synapse At the end of axons, tiny synaptic vesicles contain neurotransmitters When an impulse reaches the end of an axon, these synaptic vesicles migrate toward the end of the axon They then release their neurotransmitter and it diffuses across the synaptic cleft Synapse Neurotransmitters attach to specific receptor sites and causes sodium channels to open resulting in a depolarization in the membrane. An action potential is created and the impulse travels down the neuron. Diffusion takes time, so the more synapses involved, the slower the response. Synapse Synaptic transmission can only occur in one direction. Since only presynaptic neurons contain synaptic vesicles, and only post synaptic neurons have receptor sites for them, the messages cant be sent in the other direction This explains why impulses can only travel from sensory neuron to interneuron to motor neuron and never in the other direction Figure 10(b), pg. 420 Synaptic vesicles in the end plate of the presynaptic neuron release neurotransmitters into the synaptic cleft. The neurotransmitters attach themselves to receptors on the postsynaptic membrane, causing it to depolarize. The action potential continues along the postsynaptic neuron. Synapse flash animation Synapse action at muscular juction Action Potential Detail To Do: ◦ Relflex/Synpase Worksheet Neurotransmitters Acetylcholine (Ach): (excitatory – passes message along) ◦ neurotransmitter produced in the presynaptic knob and stored in vesicles. ◦ when an action potential reaches the presynaptic knob the vesicles rupture releasing their contents (acetylcholine) into the synaptic cleft ◦ The acetylcholine diffuses across the synapse and binds to receptor sites on the postsynaptic knob Neurotransmitters How do we stop the message? ◦ Before another message can cross the cleft, it must be cleaned (remove the neurotransmitter) ◦ The enzyme acetyl cholinesterase removes acetylcholine from the receptor sites and breaks it into acetic acid & choline ◦ the acetic acid & choline are reabsorbed into the presynaptic knob to be reused Neurotransmitters • Not all neurons cause depolarization in the post synaptic membrane. Some neurons are inhibitory. Neurotransmitters can be: ◦ excitatory - passes along message to the next neuron, or ◦ Inhibitory – binds to the next neuron and inhibits the message from being passed on See figure 11 p. 422 Neurotransmitters Often it takes more than one neuron releasing its neurotransmitter into the synaptic cleft to elicit a response in the post synaptic neuron. ◦ This is referred to as SUMMATION Figure 11, pg. 422 Action potentials must occur simultaneously in A and B to reach the threshold in D. Types of Neurotransmitters: See handout 13.2 Summary Electrochemical Impulse • Nerves conduct electrochemical impulses from the dendrites along the axon to the end plates of the neuron. • Active transport and diffusion of sodium and potassium ions establish a polarized membrane. • An action potential is caused by the inflow of sodium ions. • Nerve cells exhibit an all-or-none response. • Neurotransmitters allow the nerve message to move across synapses. To Do: Reflex/Synapse Worksheet Complete section F in your notes package Case Study “Drugs and the Synapse” p. 423-424 of text For Extra Practice: ◦ Pg. 418 # 1-4 ◦ Pg. 420 # 5-7 ◦ Pg. 425 #3-7