Post on 27-Dec-2015
Lecture 20The Nervous System
The Nervous System
The master controlling and communicating system of the body
Functions Sensory input –
monitoring stimuli occurring inside and outside the body
Integration – interpretation of sensory input
Motor output – response to stimuli by activating effector organs
Evolutionary Path to Vertebrate Nervous Systems
Cnidarians have simplest nervous system Neurons are linked to one another
through a nerve net No associative activity, just reflexes
First associative activity is seen in free-living flatworms Two nerve cords run down bodies Permit complex control of muscles
More complex animals developed: More sophisticated sensory
mechanisms Differentiation into central and
peripheral nervous systems Differentiation of sensory and motor
nerves Increased complexity of association Elaboration of the brain
Organization of the vertebrate nervous system
The nervous system links sensory receptors & motor effectors in all vertebrates (and most invertebrates)
Association neurons (or interneurons) are located in the brain and spinal cord
Central Nervous System (CNS)
Motor (or efferent) neurons carry impulses away from CNSSensory (or afferent) neurons carry impulses to CNS
Peripheral Nervous System (PNS)
Neurons Generate Nerve Impulses All neurons have the same
basic structure Cell body – Enlarged
part containing the nucleus
Dendrites – Short, slender input channels extending from end of cell body
Axon – A single, long output channel extending from other end of cell body
Most neurons require nutritional support provided by companion neuroglial cells Schwann cells (PNS) and oligodendrocytes (CNS) envelop the axon with fatty material called
myelin which act as a electrical insulator During development cells wrap themselves around each axon several times to form a myelin
sheath Uninsulated gaps are called nodes of Ranvier Nerve impulses jump from node to node Multiple sclerosis and Tay-Sachs disease result from degeneration of the myelin sheath
Three types of neurons
Graded potentials are short-lived, local changes in membrane potential Decrease in intensity with distance Their magnitude varies directly with the strength of the stimulus Sufficiently strong graded potentials can initiate nerve impulses called
action potentials
The Nerve Impulse
The potential difference (–70 mV) across the membrane of a resting neuron is generated by different concentrations of Na+, K+, and Cl
Ionic differences are the consequence of: Differential permeability of
the cell membrane to Na+ and K+
Operation of the sodium-potassium pump
PLAY Action Potential
How an Action Potential Works
An action potential forms when the membrane potential reaches -55 to -50 mV
The action potential results from ion movements in and out of voltage-gated channels
The change in membrane potential causes Na+
activation channels to open Sudden influx of Na+ into cell causes “depolarization” Local voltage change opens adjacent Na+ channels
and an action potential is produced
When the membrane potential reaches +100 mV, K+ voltage-gated channels open
K+ flows out of the cell Na+ inactivation channels snap close The negative charge in the cell is restored
The Na+ channels remain closed until the membrane potential normalizes (-70 mV), keeping the action potential from moving backward
The ion balance across the membrane is restored by the action of the sodium-potassium pump
Synapses
A junction that mediates information transfer from one neuron: To another neuron To an effector cell
Presynaptic neuron – conducts impulses toward the synapse
Postsynaptic neuron – transmits impulses away from the synapse
PLAY Transmission Across A Synapse
Excitatory synapse Receptor protein is a chemically-gated sodium
channel On binding the neurotransmitter, the
channel opens Na+ floods inwards Action potential begins
Inhibitory synapse Receptor protein is a chemically-gated
potassium or chloride channel On binding the neurotransmitter, the
channel opens K+ floods outwards or Cl– floods inwards Action potential is inhibited
An individual nerve cell can possess both kinds of synapses
Integration (Summation) Various excitatory and inhibitory electrical
effects cancel or reinforce one another Occurs at the axon hillock
Kinds of Synapses
Neurotransmitters Are chemical messengers that carry nerve impulses across synapses
Bind to receptors in the postsynaptic cell causing chemically-gated channels to open
Acetylcholine Released at the neuromuscular junction Have an excitatory effect on skeletal
muscle and inhibitory effect on cardiac muscle
Glycine and GABA Inhibitory neurotransmitters Important for neural control of brain
function Biogenic amines
Dopamine – Control of body movements Serotonin – Sleep regulation and mood
Neuromodulators are chemicals that prolong the effect of neurotransmitters by aiding their release or preventing their reabsorption
Example: Depression may be caused by a shortage of serotonin Prozac, inhibits its reabsorption
Cells that are exposed to a chemical signal for a prolonged time, lose their “sensitivity” They lose their ability to respond to the stimulus with their original intensity
Nerve cells are particularly prone to this loss of sensitivity They respond to high neurotransmitter exposure by inserting fewer receptor
proteins
Drug Addiction
Addiction occurs when chronic exposure to a drug induces the nervous system to act physiologically Cocaine is a neuromodulator
It causes large amounts of neurotransmitter to remain in synapses for long periods of time
Dopamine transmits pleasure messages in the body’s limbic system
High levels for long periods of time, cause nerve cells to lower the number of receptors
Tobacco “Nicotine receptors” normally served to bind acetylcholine Brain adjusts to prolonged exposure to nicotine by
1. Making fewer nicotine receptors 2. Altering the pattern of activation of nicotine receptors
Addiction occurs because the brain compensates for the nicotine-induced changes by making others
There is no easy way out The only way to quit is to quit!
Drug Addiction
Evolution of the Vertebrate Brain Brains of primitive fish, while small, already had the 3 divisions found in
contemporary vertebrate brains
Hindbrain (Rhombencephlon) Major component of early
fishes, as it is today An extension of the spinal
cord devoted primarily to coordinating muscle reflexes
Most coordination is done by the cerebellum
Midbrain (Mesencephlon) Composed primarily of optic
lobes that receive and process visual information
Forebrain (Proencephlon) Devoted for processing
olfactory (smell) information
Note: Brains of fishes continue growing
throughout their lives!
How the Human Brain Works
Diencephalon Thalamus – Relay center between
incoming sensory information and the cerebrum
Hypothalamus – Coordinates nervous and hormonal responses to many internal stimuli and emotions
Telencephalon Devoted largely to associative activity Cerebrum (~ 85% of the weight of the
human brain) Dominant part of the brain, receives
sensory data and issues motor commands
Cerebral cortex (Gray outer layer) Functions in language, thought, personality and other “thinking and feeling” activities
Basic Geography of the Human Brain The cerebrum is divided by a
groove into right and left halves called cerebral hemispheres Linked by bundles of neurons
called tracts that serve as information highways
In general: The left brain is associated with
language, speech and mathematical abilities
The right brain is associated with intuitive, musical, and artistic abilities
The Central Sulcus divides the front and back of the cerebrum The front is associated with
motor functions The back with sensory
Higher association functions are in the prefrontal area Stroke
A disorder caused by blood clots blocking blood vessels in the brain
The Diencephalon
Thalamus Major site of sensory
processing in the brain Controls balance
Hypothalamus Integrates internal activities:
body temperature, blood pressure, etc.
Controls pituitary gland secretions
Linked to areas of cerebral cortex via limbic system
The Brain Stem & Cerebellum
Cerebellum Extends back from the base of
the brain Coordinates muscle movement Even better developed in birds
Brain Stem Made up of midbrain, pons, and
medulla oblongata Connects rest of brain to spinal
cord Controls breathing, swallowing,
digestion, heart beat, and blood vessel diameter
Memory Processing
Memory is the storage and retrieval of information
The three principles of memory are:1. Storage – occurs in stages and
is continually changing2. Processing – accomplished by
the hippocampus and surrounding structures
3. Memory traces – chemical or structural changes that encode memory Short-term memory –appears
to be stored electrically in the form of a transient neural excitation
Long-term memory –appears to involve structural changes in certain neural connections
Types of Sleep
There are two major types of sleep: Non-rapid eye movement (NREM) Rapid eye movement (REM)
One passes through four stages of NREM during the first 30-45 minutes of sleep
REM sleep occurs after the fourth NREM stage has been achieved
Importance of Sleep
Slow-wave sleep is presumed to be the restorative stage
Those deprived of REM sleep become moody and depressed
REM sleep may be a reverse learning process where superfluous information is purged from the brain
Daily sleep requirements decline with age
Sleep Disorders Narcolepsy – lapsing abruptly into sleep from the awake
state Insomnia – chronic inability to obtain the amount or
quality of sleep needed Sleep apnea – temporary cessation of breathing during
sleep
Degenerative Brain Disorders
Alzheimer’s disease – a progressive degenerative disease of the brain that results in dementia
Parkinson’s disease – degeneration of the dopamine-releasing neurons of the substantia nigra
Huntington’s disease – a fatal hereditary disorder caused by accumulation of the protein huntingtin that leads to degeneration of the basal nuclei
The Spinal Cord
The spinal cord is a cable of neurons extending from the brain down through the backbone Neuron cell bodies in the
center Gray matter
Axons and dendrites on the outside White matter
It is surrounded and protected by the vertebrae Through them spinal nerves
pass out to the body Motor nerves from spine
control most of the muscles below the head
Major Nerves of Humans
Voluntary and Autonomic Nervous Systems
Are two subdivisions of vertebrate motor pathways
The Voluntary Nervous System
Relays commands to skeletal muscles
Can be controlled by conscious thought
Reflexes are rapid involuntary movements Are rapid because sensory
neuron passes information directly to a motor neuron
Most involve single connecting interneuron between sensory and motor neurons
The Autonomic Nervous System Stimulates glands and relays commands to smooth muscles
Cannot be controlled by conscious thought Composed of elements that act in opposition to each other
Parasympathetic nervous System Controls normal functions Conserves energy by slowing
down processes
Sympathetic nervous system Dominates in time of stress Controls the “fight-or-flight”
reaction Increases blood pressure,
heart rate, breathing