Lecture 20 The Nervous System. The Nervous System The master controlling and communicating system of the body Functions Sensory input – monitoring.

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