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Chapter 1Nerve Cells and Nerve Impulses

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1.1 The Cells of the Nervous System

• Your mental experiences depend on the activity of a huge number of separate but interconnected cells

• We can begin to understand how this works by looking at the cells of the nervous system

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Neurons and Glia, Part 1

• The human nervous system comprises two kinds of cells– Neurons

– Glia

• The human brain contains approximately 100 billion individual neurons

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How Many Neurons Do We Have?

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Santiago Ramón y Cajal, a Pioneerof Neuroscience

• In the late 1800s, the Spanish investigator Santiago Ramon y Cajal (1852-1934) was the first to demonstrate that the individual cells comprising the nervous system remained separate– He showed that they did not merge into each

other as previously believed

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The Structures of an Animal Cell, Part 1

• Like other cells in the body, neurons contain the following structures– Membrane

– Nucleus

– Mitochondria

– Ribosomes

– Endoplasmic reticulum

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An Electron Micrograph of the Parts of a Neuron

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The Structures of an Animal Cell, Part 2

• Membrane: separates the inside of the cell from the outside environment

• Nucleus: contains the chromosomes

• Mitochondrion: performs metabolic activities and provides energy that the cells requires

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The Structures of an Animal Cell, Part 3

• Ribosomes: sites at which the cell synthesizes new protein molecules

• Endoplasmic reticulum: network of thin tubes that transports newly synthesized proteins to their location

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The Structure of a Neuron

• Neuron cells are similar to other cells of the body but have a distinctive shape

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Motor and Sensory Neurons

• A motor neuron– Has its soma in the spinal cord

– Receives excitation from other neurons

– Conducts impulses along its axon to a muscle

• A sensory neuron– Is specialized at one end to be highly sensitive

to a particular type of stimulation (touch, light, sound, etc.)

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A Vertebrate Motor Neuron

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A Vertebrate Sensory Neuron

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Components of All Neurons

• Dendrites

• Soma/cell body

• Axon

• Presynaptic terminals

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Dendrites

• Branching fibers with a surface lined with synaptic receptors responsible for bringing information into the neuron

• Some also contain dendritic spines that further branch out and increase the surface area of the dendrite

• The greater the surface area of the dendrite, the more information it can receive

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Dendritic Spines

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Cell Body/Soma

• Contains the nucleus, mitochondria, and ribosomes

• Responsible for the metabolic work of the neuron

• Covered with synapses on its surface in many neurons

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Axons

• Thin fiber of a neuron responsible for transmitting nerve impulses toward other neurons, organs, or muscles

• Maybe have a myelin sheath, an insulating material that contains interruptions in the sheath known as nodes of Ranvier

• Presynaptic terminals at the end points of an axon release chemicals to communicate with other neurons

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Afferent, Efferent, and Intrinsic

• Afferent axon: refers to bringing information into a structure

• Efferent axon: refers to carrying information away from a structure

• Interneurons or intrinsic neurons are those whose dendrites and axons are completely contained within a single structure

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Cell Structures and Axons

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Variations Among Neurons

• Neurons vary in size, shape, and function

• The shape of a neuron determines it connection with other neurons and its contribution to the nervous system

• The function is closely related to the shape of a neuron– Example: Pukinje cells of the cerebellum

branch extremely widely within a single plane

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The Diverse Shape of Neurons

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Types of Glia

• Astrocytes– Help synchronize the activity of the axon by

wrapping around the presynaptic terminal and taking up chemicals released by the axon

• Microglia – Remove waste material, viruses, and fungi

from the brain

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Neurons and Glia, Part 2

• Oligodendrocytes (in the brain and spinal cord) and Schwann cells (in the periphery of the body)– build the myelin sheath that surrounds and

insulates certain vertebrate axons

• Radial glia– Guide the migration of neurons and the growth

of their axons and dendrites during embryonic development

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Neurons and Glia, Part 3

• When embryonic development finishes, most radial glia differentiate into neurons and a smaller number differentiate into astrocytes and oligodendrocytes

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Shapes of Various Glia Cells

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How An Astrocyte Synchronizes Associated Axons

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The Blood-Brain Barrier

• A mechanism that surrounds the brain and blocks most chemicals from entering– The immune system destroys damaged or

infected cells throughout the body

– Because neurons in the brain generally do not regenerate, it is vitally important for the blood brain barrier to block incoming viruses, bacteria, or other harmful material from entering

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How the Blood-Brain Barrier Works

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Active Transport

• The protein-mediated process that expends energy to pump chemicals from the blood into the brain– Glucose, certain hormones, amino acids, and

a few vitamins are brought into the brain via active transport

• The blood-brain barrier is essential to health, but can pose a difficulty in allowing chemicals such as chemotherapy for brain cancer to pass the barrier

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Nourishment of Vertebrate Neurons

• Vertebrate neurons depend almost entirely on glucose– A sugar that is one of the few nutrients that can

pass through the blood-brain barrier

• Neurons need a steady supply of oxygen– 20% of all oxygen consumed by the body is

used by the brain

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Nourishment in Vertebrate Neurons

• The body needs a vitamin, thiamine, to use glucose

• Prolonged thiamine deficiency leads to death of neurons as seen in Korsakoff’s syndrome, a result of chronic alcoholism– Korsakoff’s syndrome is marked by severe

memory impairment

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1.2 The Nerve Impulse

• The electrical message that is transmitted down the axon of a neuron– Does not travel directly down the axon, but is

regenerated at points along the axon so that it is not weakened

• The speed of nerve impulses ranges from less than 1 meter/second to 100 meters/second– A touch on the shoulder reaches the brain

more quickly than a touch on the foot

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The Nerve Impulse

• The brain is not set up to register small differences in the time of arrival of touch messages

• However, in vision, movements must be detected as accurately as possible

• The properties of impulse control are well adapted to the exact needs for information transfer in the nervous system

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The Resting Potential of the Neuron, Part 1

• Messages in a neuron develop from disturbances of the resting potential

• At rest, the membrane maintains an electrical gradient known as polarization – A difference in the electrical charge inside and

outside of the cell

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The Resting Potential of the Neuron, Part 2

• The inside of the membrane is slightly negative with respect to the outside (approximately -70 millivolts)

• The resting potential of a neuron refers to the state of the neuron prior to the sending of a nerve impulse

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The Membrane of a Neuron

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Methods for Recording Activity of a Neuron

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Forces Acting on Sodium and Potassium Ions

• The membrane is selectively permeable, allowing some chemicals to pass more freely than others

• Sodium, potassium, calcium, and chloride pass through channels in the membrane

• When the membrane is at rest:– Sodium channels are closed

– Potassium channels are partially closed allowing the slow passage of potassium

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Ion Channels in the Membrane of a Neuron

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Ion Channels

• The sodium-potassium pump is a protein complex – Continually pumps three sodium ions out of the

cells while drawing two potassium ions into the cell

– Helps to maintain the electrical gradient

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Electrical and Concentration Gradients

• The electrical gradient and the concentration gradient – the difference in distributions of ions – work to pull sodium ions into the cell

• The electrical gradient tends to pull potassium ions into the cells– However, they slowly leak out, carrying a

positive charge with them

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Sodium and Potassium Gradients for a Resting Membrane

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The Action Potential, Part 1

• The resting potential remains stable until the neuron is stimulated– Hyperpolarization: increasing the polarization

or the difference between the electrical charge of two places

– Depolarization: decreasing the polarization towards zero

– The threshold of excitation: a level above which any stimulation produces a massive depolarization

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The Action Potential, Part 2

• A rapid depolarization of the neuron

• The action potential threshold varies from one neuron to another, but is consistent for each neuron

• Stimulation of the neuron past the threshold of excitation triggers a nerve impulse or action potential

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Voltage-Activated Channels

• Membrane channels whose permeability depends upon the voltage difference across the membrane– Sodium and potassium channels

• When sodium channels are opened, positively charged sodium ions rush in and a subsequent nerve impulse occurs

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The Movement of Sodium and Potassium Ions During an Action Potential

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The Movement of Sodium and Potassium

• After an action potential occurs, sodium channels are quickly closed

• The neuron is returned to its resting state by the opening of potassium channels– Potassium ions flow out due to the

concentration gradient and take with them their positive charge

• The sodium-potassium pump later restores the original distribution of ions

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Restoring the Sodium-Potassium Pump

• The process of restoring the sodium-potassium pump to its original distribution of ions takes time

• An unusually rapid series of action potentials can lead to a buildup of sodium within the axon– Can be toxic to a cell, but only in rare

instances such as stroke and after the use of certain drugs

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Blocking Sodium Channels

• Local anesthetic drugs block sodium channels and therefore prevent action potentials from occurring– Example: Novocain and Xylocaine

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The All-or-None Law, Part 1

• Action potentials back-propagate into the cell body and dendrites– Dendrites become more susceptible to

structural changes responsible for learning

• The all-or-none law – States that the amplitude and velocity of an

action potential are independent of the intensity of the stimulus that initiated it

– Action potentials are equal in intensity and speed within a given neuron

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The All-or-None Law, Part 2

• Action potentials vary from one neuron to another in terms of amplitude, velocity, and shape

• Studies of mammalian axons show that there is much variation in the types of protein channels and therefore in the characteristics of the action potentials

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Refractory Periods

• After an action potential, a neuron has a refractory period during which time the neuron resists the production of another action potential– The absolute refractory period: the first part of

the period in which the membrane cannot produce an action potential

– The relative refractory period: the second part, in which it takes a stronger than usual stimulus to trigger an action potential

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Propagation of an Action Potential, Part 1

• In a motor neuron, the action potential begins at the axon hillock (a swelling where the axon exits the soma)

• Propagation of the action potential: the transmission of the action potential down the axon– The action potential does not directly travel

down the axon

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Propagation of an Action Potential, Part 2

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The Myelin Sheath

• The myelin sheath of axons are interrupted by short unmyelinated sections called nodes of Ranvier– Myelin is an insulating material composed of

fats and proteins

– At each node of Ranvier, the action potential is regenerated by a chain of positively charged ions pushed along by the previous segment

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An Axon Surrounded by a Myelin Sheath

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Saltatory Conduction

• The “jumping” of the action potential from node to node– Provides rapid conduction of impulses

– Conserves energy for the cell

• Multiple sclerosis: disease in which the myelin sheath is destroyed– Associated with poor muscle coordination and

sometimes visual impairments

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Saltatory Conduction in a Myelinated Axon

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Local Neurons, Part 1

• Have short axons, exchange information with only close neighbors, and do not produce action potentials

• When stimulated, produce graded potentials – membrane potentials that vary in magnitude and do not follow the all-or-none law

• Depolarize or hyperpolarize in proportion to the stimulation

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Local Neurons, Part 2

• Difficult to study due to their small size

• Most of our knowledge has come from the study of large neurons

• Myth– Only 10 percent of neurons are active at any

given moment

• Truth– You use all of your brain, even at times when

you might not be using it very well