Learning Objectives Neurons Chapter 7 Identify and describe the functions of the two main divisions of the nervous system. Differentiate between a neuron and neuroglial cells in terms of structure and function. Describe the features of the three major categories of neurons. Label a generalized neuron diagram, and explain the function of each of the structures. Explain the role of the myelin sheath. Describe how a nerve cell maintains a resting potential using the sodium-potassium pump and what changes occur in the membrane upon stimulation. Explain how a nerve impulse is a bioelectrical signal, including how ions move across the membrane. Describe the events of an action potential. Compare and contrast what happens during resting and action potentials. Draw and label a synapse diagramming the movement of neurotransmitter Differentiate between excitatory and inhibitory synapses. Describe how neurotransmitters can be removed from the synapse. Exemplify the specific role of common neurotransmitters in the maintenance of stable behavioral conditions and in disease situations. Neurons (nerve cells) Excitable cells that generate and transmit messages Neuroglial cells (glial cells) Outnumber neurons by 10 to 1 Several types, each with a specific function Provide structural support, growth factors, and insulating sheaths around axons Three categories of neurons Sensory (or afferent) neurons Motor (or efferent) neurons Interneurons (or association) neurons Sensory neurons Carry information toward the CNS from sensory receptors Motor neurons Carry information away from the CNS to an effector (muscle or gland) Interneurons Found only in the brain and spinal cord (CNS) Between sensory and motor neurons Integrate and interpret sensory signals Account for more than 99% of the body s neurons 1
Figure 7.1 Neurons may be sensory neurons, interneurons, or motor neurons. Structure of Neurons The shape of a typical neuron is specialized for communicating with other cells Parts of a neuron Dendrites many short, branching projections Axon a single long extension Cell body Axons and Dendrites Dendrites Receive signals from other cells Carry information toward the cell body of a neuron Axon Carries information away from the cell body to either another neuron or an effector Cell body Contains nucleus and other organelles Functions to maintain the neuron Figure 7.2 The structure of a neuron. Axons and Dendrites Myelin Sheath Nerves Consist of parallel axons, dendrites, or both from many neurons Covered with tough connective tissue Classified as sensory, motor, or mixed (sensory and motor together) depending on the type of neurons they contain Found on most axons outside the CNS and some of those within the CNS Provides electrical insulation that increases the rate of conduction of a nerve impulse Composed of the plasma membranes of glial cells In the PNS, Schwann cells (a type of glial cell) form the myelin sheath Gaps between adjacent Schwann cells are called nodes of Ranvier: messages travel faster as they jump from one node of Ranvier to the next in a type of transmission called saltatory conduction 2
Figure 7.3 The myelin sheath. Myelin Sheath Multiple Sclerosis (MS) A disease in which the myelin sheaths in the brain and spinal cord are progressively destroyed Results from the destruction of the myelin sheath that surrounds axons in the CNS The resulting scars (scleroses) interfere with the transmission of nerve impulses Can result in paralysis and loss of sensation, including loss of vision Myelin Sheath Nerve Impulses A nerve impulse, or action potential, is an electrochemical signal involving sodium and potassium ions (Na and K ) that cross the cell membrane through ion channels Each ion channel is designed to allow only certain ions to pass through it Ion channels may be permanently open or regulated by a gate, which is a protein that changes shape and opens or closes a channel. The transport does not require any energy as the ions follow a gradient of concentration Web Activity: Myelinated Neurons and Saltatory Conduction Nerve Impulses Resting Potential Ions also are transported across the membrane by the sodium-potassium pump Special proteins in the cell membrane that actively transport sodium and potassium ions across the membrane against their concentration gradients These pumps use cellular energy to eject sodium ions from within the cell and to bring potassium ions into the cell When a neuron is not conducting a nerve impulse, it is in a resting state called the resting potential The inner surface of the membrane is about 70 mv more negative than the outer surface There are more sodium ions outside the membrane than inside There are more potassium ions inside the membrane than outside This state is maintained by the sodium-potassium pump 3
Action Potential When the neuron is stimulated, there is a sudden reversal of charge across the membrane because the sodium gates open and sodium ions enter the cell. Next, the potassium gates open and potassium ions rush out of the cell. Threshold: Minimum charge that causes the sodium gates to open Depolarization: Reduction of the charge difference across the membrane Repolarization: Restoration of the charge difference across the membrane These changes occur in a wave along the axon Figure 7.4 The resting state and the propagation of an action potential along an axon. Figure 7.5 A graphic representation of an action potential. Action Potential Does not diminish, once started Does not vary in intensity with the strength of the stimulus that triggered it Is all-or-nothing For a very brief period following an action potential, the neuron cannot be stimulated again This is called the refractory period It occurs because the sodium channels are closed and cannot be reopened Action Potential Synaptic Transmission Communication between a neuron and an adjacent cell occurs by neurotransmitters Synapse Junction between a neuron and another cell Web Activity: Nerve Impulse 4
Synaptic Transmission Figure 7.6 Structure of a synapse. Synaptic cleft Gap between two cells Neurotransmitters diffuse across the gap In the case of two neurons, the presynaptic neuron sends a message to the postsynaptic neuron Synaptic knob: swelling at the end of the axon of the presynaptic neuron Release of the Neurotransmitter and the Opening of Ion Transmitters Figure 7.7 Transmission across an excitatory synapse. The nerve impulse reaches the synaptic knob of the presynaptic neuron Calcium ions move into the synaptic knob which releases the neurotransmitter into the synaptic cleft The neurotransmitter diffuses across the synaptic cleft and binds with receptors on the membrane of the postsynaptic neuron causing an ion channel to open Release of the Neurotransmitter and the Opening of Ion Transmitters At an excitatory synapse, binding of the neurotransmitter to the receptor causes sodium channels to open increasing the likelihood that an action potential will begin At an inhibitory synapse, binding of the neurotransmitter to the receptor opens different ion channels The postsynaptic cell s interior becomes more negatively charged than usual, reducing the likelihood that an action potential will begin Summation of Input from Excitatory and Inhibitory Synapses A neuron may have as many as 10,000 synapses with other neurons at the same time Some synapses have excitatory effects and some have inhibitory effects Summation Combined effects of excitatory and inhibitory effects at any given moment Determines whether an action potential is generated This level of integration provides fine control over neuronal responses 5
Figure 7.8 A neuron may have as many as 10,000 synapses. Removal of Neurotransmitters Temporary effects Once released into a synapse, neurotransmitters are quickly removed Some are deactivated by enzymes The enzyme acetylcholinesterase removes acetylcholine from synapses Others are pumped back into the synaptic knob of the presynaptic axon Removal of Neurotransmitters Roles of Different Neurotransmitters Web Activity: The Synapse There are dozens of neurotransmitters Some neurotransmitters produce different effects on different types of cells Example: acetylcholine Acts in both the PNS and the CNS Released at every neuromuscular junction Myasthenia gravis: autoimmune disease that attacks the acetylcholine receptors at neuromuscular junctions, resulting in little muscle strength Roles of Different Neurotransmitters In the CNS, different neurotransmitters are associated with different behavioral systems Norepinephrine regulates mood, hunger, thirst, and sex drive Serotonin promotes a feeling of well-being Dopamine regulates emotions and complex movements Health Issue: Neurotransmitters and Disease Changes in the levels of neurotransmitters cause disorders Alzheimer s disease Associated with decreased levels of acetylcholine Clinical depression Associated with decreased levels of serotonin, dopamine, and norepinephrine Parkinson s disease Associated with decreased levels of dopamine 6
Health Issue: Neurotransmitters and Disease Attention Deficit Disorder (controversial RX patch) 7