Biological Psychology: Nerve Cells & Nerve Impulses

© Cengage Learning 2016 © Cengage Learning 2016

Chapter 1Nerve Cells and Nerve Impulses

© Cengage Learning 2016

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


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


How Many Neurons Do We Have?


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


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


An Electron Micrograph of the Parts of a Neuron



• 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



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

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


The Structure of a Neuron

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


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.)


A Vertebrate Motor Neuron


A Vertebrate Sensory Neuron


Components of All Neurons

• Dendrites

• Soma/cell body

• Axon

• Presynaptic terminals


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


Dendritic Spines


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


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


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


Cell Structures and Axons


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


The Diverse Shape of Neurons


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



• 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



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


Shapes of Various Glia Cells


How An Astrocyte Synchronizes Associated Axons


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


How the Blood-Brain Barrier Works


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


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


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


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


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


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


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


The Membrane of a Neuron


Methods for Recording Activity of a Neuron


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


Ion Channels in the Membrane of a Neuron


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


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


Sodium and Potassium Gradients for a Resting Membrane


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


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


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


The Movement of Sodium and Potassium Ions During an Action Potential


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


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


Blocking Sodium Channels

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


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


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


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


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


Propagation of an Action Potential, Part 2


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


An Axon Surrounded by a Myelin Sheath


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


Saltatory Conduction in a Myelinated Axon


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


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

Biological Psychology: Nerve Cells & Nerve Impulses

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Transcript of Biological Psychology: Nerve Cells & Nerve Impulses