What you need to know: 1. Three Basic Functions 2. Functional Organization 3. Neuron Structure 4. Reflex Arcs 5. Nerve Impulse physiology 6. Brain Structure/Function
1. Sensory input gathering information To monitor changes (stimuli) occurring inside and outside the body 2. Integration To process and interpret sensory input and decide if action is needed 3. Motor output A response to integrated stimuli Muscles and/or glands are activated
(1) (2) Muscles, Glands (3) Figure 7.1
IMPORTANT: Direction of arrows indicates direction of nerve impulse. (maintains homeostasis) ( fight or flight response)
Central nervous system (CNS) brain spinal cord Integrate sensory information, then send response signals to muscles and glands Peripheral nervous system (PNS) nerves outside the brain and spinal cord transmits nerve impulses to/from CNS
Functional Classification of the Peripheral Nervous System 1. Sensory (afferent) division Nerve fibers that carry information to the central nervous system 2. Motor (efferent) division Nerve fibers that carry impulses away from the central nervous system
Peripheral Nervous System Muscles, Glands Figure 7.1 Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Functional Classification of the Peripheral Nervous System Motor (efferent) division Two subdivisions Somatic nervous system = voluntary Autonomic nervous system = involuntary Figure 7.1 Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Organization of the Nervous System Sympathetic nervous system allows body to function under stress fight or flight response Parasympathetic nervous system controls vegetative functions feed or breed or rest and repose constant opposition to sympathetic system Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Organization of the Nervous System Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings
IMPORTANT: Direction of arrows indicates direction of nerve impulse. (maintains homeostasis) ( fight or flight response)
Brain Cranial nerves Central Nervous System (brain and spinal cord) Peripheral Nervous System (cranial and spinal nerves) Spinal cord Spinal nerves Sensory division Sensory receptors Motor division Somatic Nervous System Skeletal muscle Autonomic Nervous System Smooth muscle Cardiac muscle Glands (a) (b) 14
Neuron Anatomy Extensions outside the cell body (processes): Dendrites conduct impulses toward the cell body Axons conduct impulses away from the cell body Figure 7.4a Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Structural Classification of Neurons Multipolar neurons many extensions from the cell body Figure 7.8a Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Structural Classification of Neurons Bipolar neurons one axon and one dendrite Figure 7.8b Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Structural Classification of Neurons Unipolar neurons have a short single process leaving the cell body Figure 7.8c Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Dendrites Peripheral process (axon) Ganglion Cell body Central process (axon) Sensory neuron Spinal cord (central nervous system) Peripheral nervous system Afferent transmission Interneuron (association neuron) Receptors Efferent transmission Motor neuron To effectors (muscles and glands) Figure 7.6
Dendrite Cell body Myelin sheath Axon Node of Ranvier Axon terminals Image References: LifeART. (1998). Super Anatomy Collection 1-9. Baltimore, MD: Williams & Wilkins, All rights reserved.
In a typical nerve cell, the largest portion is the cell body, which contains the nucleus. Dendrites extend from the cell body and receive impulses that are transmitted to the cell body and out through a long extension called an axon. Some neurons have a fatty outer layer of insulation called the myelin sheath. The myelin sheath is interrupted at intervals, causing impulses to jump the gaps (called Nodes of Ranvier) and speed the transmission rate. Impulses travel in one direction only: in through the dendrites and out through the axon. Bundles of neurons are called nerves. The connecting space between neurons is called a synapse. Neurotransmitters at the synapse must be activated for the impulse to continue to the next neuron. Electrical impulses in neurons depend on relative concentrations of ions inside and outside of the cell s membrane. Voltage-gated channels affect the membrane potential of the neuron. At resting potential (polarized state), the inside of the neuron is more negatively charged than the outside of the neuron. The reversal of polarity (or depolarization) across the membrane - the action potential - causes voltage-gated sodium channels to open and sodium ions to flow into the axon. This begins a chain reaction that causes sodium channels to open along the axon. The action potential conducts rapidly down the axon. Voltage-gated potassium channels open, allowing potassium ions to flow out of the axon, returning the membrane potential to negative. The resting potential is regained as sodium-potassium pumps restore the original concentrations of sodium and potassium inside and outside the membrane. References: Campbell, N.E. & Reece, J.B. (2002). Biology,(6 th ed.). San Francisco: Benjamin Cummings. Raven, P.H. & Johnson, G.B. (2002). Biology, (6 th ed.). McGraw-Hill.
Nerve Impulses This is simplified model of how nerves work Not all biochemical mechanisms influencing transmission of nerve impulses are included in this model This model focuses on how electrochemical gradients and potential energy are involved in the transmission of impulses Figure 7.9a c Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Nerve Impulses Electrical charges exist as ions (particles with + or charges) inside and outside the nerve s cell membrane. An electrochemical gradient is a difference in the overall electrical charges on opposite sides of a membrane. Figure 7.9a c Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Nerve Impulses The electrochemical gradient across a neuron s membranes result in potential (stored) energy. The potential energy used by nerve impulses is stored as the natural attraction between particles with opposite (+ and -) charges. Figure 7.9a c Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Nerve Impulses When a stimulus is perceived by sensory receptors, potential (stored) energy gets converted to kinetic energy (energy of movement). This change begins the action potential (nerve impulse). Figure 7.9a c Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Nerve Impulse electrochemical gradient a) Resting (polarized) membrane Energy is stored as a strong electrochemical gradient. Na + (sodium ions) outside the membrane can t move through the membrane in the resting state membrane is impermeable to Na + in the resting state Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 7.9a c
Nerve Impulse electrochemical gradient a) Resting (polarized) membrane The strength or weakness of the electrochemical gradient across the membrane can be measured and quantified. Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 7.9a c
Nerve Impulse - stimulus b) Stimulus makes membrane permeable to Na + If stimulus is strong enough, the membrane becomes permeable to Na + Na + diffuses rapidly into the nerve cell (this is a passive process and occurs simply due to existence of much higher Na + concentrations outside the cell) Figure 7.9a c Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Nerve Impulse - depolarization b) Diffusion of Na + into cell As Na + diffuses into the cell, the solution outside the cell becomes less positive (more negative) and the fluid inside the cell becomes more positive (less negative) The electrochemical gradient has been weakened. Figure 7.9a c Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Nerve Impulse action potential c) Initiation of Action Potential (nerve impulse) A slightly weakened electrochemical gradient will not result in an action potential If the electrochemical gradient has been weakened enough, then an action potential will be generated Figure 7.9a c Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings
The Action Potential If the action potential (nerve impulse) starts, it is propagated over the entire axon In other words, a nerve impulse is all or nothing There is no such thing as a weak or strong action potential (nerve impulse) or one which only goes part way along a neuron.
The Action Potential After Na + rapidly diffuses into the cell, sodium-potassium pumps rapidly pump K + (potassium ions) out of the neuron to repolarize the membrane. This action requires ATP, a molecule which carries a lot of potential energy which can be used for many cellular functions. Use of the Na + -K + pumps is expensive in that it requires energy.
Nerve Impulse Propagation The impulse continues to move toward the cell body Impulses travel faster when fibers have a myelin sheath Figure 7.9c e Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Continuation of the Nerve Impulse between Neurons Impulses are able to cross the synapse to another nerve by using neurotransmitter molecules Neurotransmitters are released from a nerve s axon terminal The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter An action potential is started in the dendrite Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings
How Neurons Communicate at Synapses Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 7.10
An impulse moves in only one direction across a synapse - from an axon to the dendrites or cell body of another neuron. http://mhln.com
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1. Why is it important that a neurotransmitter is broken down after it initiates an action potential (nerve impulse)? 2. Why is it important that a neurotransmitter is released from the receptor site after initiating the action potential?
1. Why is it important that a neurotransmitter is broken down after it initiates an action potential (nerve impulse)? Answer: If neurotransmitters were not broken down, they could continue to stimulate impulses (action potentials) in the receiving neuron. 2. Why is it important that a neurotransmitter is released from the receptor site after initiating the action potential? Answer: Release of neurotransmitters from receptor sites makes those sites available to receive new signals in the future.