Chapter 17 The Nervous System

Nervous Tissue• 2 Major Categories of Nervous System

1. Central Nervous System• The brain and spinal cord

• At midline of body

2. Peripheral Nervous System– Nerves carrying messages from CNS to muscles and glands

– Sensory nerves carrying messages to the CNS

3 Types of Nerves to KnowSensory Nerves , Interneurons & Motor

Nerves

1. Sensory Nerves - carry information to the CNS

2. Interneuron – a bridge between a sensory and motor nerve

3. Motor Nerves – carry response from the CNS to an effector – a gland or muscle that creates an action

Anatomy of a TypicalNerve CellSensory Neuron shown here

Dendtrites

Cell Body (Soma)

Nucleus

Axon Bulb

Axon

Myelin Sheath

Node of Ranvier

Myelin Sheath / Nodes of Ranvier• Formed by Schwann cells which secrete a fatty coating over the axon

• Acts like an insulation around a wire. Electrical signal jumps gap to gap because that is easier than travelling down the high resistance of the axon, itself.

• Gaps where Schwann cells do not myelinate are called the Nodes of Ranvier (click below for an animation showing Saltatory Conduction or the jumping of the impulse)

More on Myelin• Gives nerves white appearance (= WHITE MATTER)

• MS (mutliple sclerosis) is a disorder where myelin is removed from nerve. Signal passes down axon, weakening. In case of optic nerve, this can cause loss of vision. Ultimately, muscular control is lost. If the breathing and respiration centres in the medulla are affected, the result is fatal.

• Long Axons – usually myelinated to help signal make it

• Short Axons – usually non-myelinated, since signal should not degrade over short distance

• Unmyelinated nerves = GRAY MATTER

How does a Nerve Cell Pass an Electrical Signal?

• Nerve impulse carries information

• Impulse measured in mV

• Stages of Nerve Cell firing off a Signal

1. Resting Potential – electric potential before signal

2. Action Potential – potential of nerve when signal goes

3. Refractory Period – potential after signal has passed

Action potential

• Definition: an “all-or-none” change in voltage that propagates itself down the axon

• Naturally occurring action potentials begin at the axon hillock

Resting Potential

• When not conducting an electrical impulse, the potential difference across the membrane is -65mV

• Inside of axon is negative compared to outside!

• Difference between inside/outside charge of membrane is needed to start a signal

• Similar to a battery, the ends must have a potential difference for current to flow

Action potential

• Definition: an “all-or-none” change in voltage that carries on all the way down the axon, creating electrical impulse.

• Naturally occurring action potentials begin at the axon hillock = crest of the axon

• Action potentials do not occur anywhere else in a neuron – not in dendrites, not in cell bodies

• Let’s watch how a nerve fires using the A.P.

• Click on Hercules over there…

More K+ Sneaks out than Na+ in!Creates Dipole, enabling an Action Potential (Animation)

Figure 48.11 Saltatory conduction

Action Potential

• Starts ---> -70mV

– Requires Na / K gates in membrane to open

• Ends ---> +45mV

• Lots of repolarizing to do

– Na moves back outside the membrane

– K move back inside the membrane

– Na/K gates must open to allow repolarizing to happen

– Not ready to fire again until this all happens.

SODIUM POTASSIUM PUMP DEMO

How do you get from electrical signals to chemical signals and

back again?

Axon Bulbs Insert carry Signal

Dendtrites

Cell Body (Soma)

Nucleus

Axon Bulb

Axon

Myelin Sheath

Node of Ranvier

Translating signals across Synapses• The action potential moves

down the axon until it reaches the terminal (synapse)

• Electric signal converted to chemical neurotransmitters

• Neurotransmitters cross the synaptic cleft

• Signal caught on other side gets the electrical signal going again. Click on Synapse to see

Animation

Translating signals

• The action potential moves down the axon until it reaches the terminal (synapse)

• Impulse opens voltage-activated Ca2+ channels to let the neurotransmitters through

• Transmitter diffuses across synaptic cleft and binds to receptors on post-synaptic cell

Synapse Animation #2

Signal Blockers

• Curare – poisonous darts tipped in these in the Amazon

• Black widow spider venom will cause bursting of synapses.

• Smoking’s not much good either

Translating signals

• The action potential moves down the axon until it reaches the terminal (synapse)

• Its wave of depolarization opens voltage-activated Ca2+ channels

• Influx of Ca2+ causes vesicles to fuse with presynaptic cell membrane

Translating signals

• The action potential moves down the axon until it reaches the terminal (synapse)

• Its wave of depolarization opens voltage-activated Ca2+ channels

• Influx of Ca2+ causes vesicles to fuse with presynaptic cell membrane

• Transmitter diffuses across synaptic cleft and binds to receptors on post-synaptic cell

• If a transmitter depolarizes (stimulates) the post-synaptic neuron, it is said to be excitatory

Excitatory and inhibitory neurotransmitters

•If a transmitter hyperpolarizes the post-synaptic neuron, it is said to

be inhibitory

Excitation or Excitation or inhibition inhibition

depends on the depends on the receptor!!receptor!!

Some Things You Should Some Things You Should KnowKnow

1.1. AcetylcholineAcetylcholine • involved in learning and memoryinvolved in learning and memory• Reduced levels in Alzheimers patientsReduced levels in Alzheimers patients• Excitatory to musclesExcitatory to muscles

NHNH22 Amine-type Neurotransmitters Amine-type Neurotransmitters

2.2. NorEpinepherineNorEpinepherine• Released by Adrenal GlandsReleased by Adrenal Glands• Excitatory, heightens sensesExcitatory, heightens senses

3.3. DopamineDopamine• Regulates movement, balance, walkingRegulates movement, balance, walking• Generally inhibatory, evening things outGenerally inhibatory, evening things out• Reduces levels in Schizophrenics/possible Pathological GamblersReduces levels in Schizophrenics/possible Pathological Gamblers

4.4. SerotoninSerotonin– Regulates mood, emotion, thoughtRegulates mood, emotion, thought– Brain cells are bathed in this while you sleep (chemical reset)Brain cells are bathed in this while you sleep (chemical reset)– Derived from amino acid Tryptophan (warm milk, turkey). Causes drowsinessDerived from amino acid Tryptophan (warm milk, turkey). Causes drowsiness– Related to Melatonin – can get this in pill form. May help with sleep disordersRelated to Melatonin – can get this in pill form. May help with sleep disorders

The Peripheral Nervous System

– The Spinal Nerves leading away from the CNS

– Afferent axons:• Name for axons directed towardtoward the central nervous system,

conveying sensory information.

– Efferent axon:• An axon directed away fromaway from the central nervous system,

conveying motor commands to muscles and glands.

Ever have a Doctor tap your knee with a hammer? Your lef

moves without conscious thought. It’s also the reason beheaded chickens can run.

Reflex Arc Animation 1

Reflex Arc Animation 2

Gray Matter

White Matter

2 Divisions of the Peripheral Nervous System (PNS)1. Somatic nervous system

• Part of the peripheral nervous system that controls the movement of skeletal muscles or transmits sensory information (visual, sound, touch, etc.) to the central nervous system

• Quick to respond to changes in the environment

2. Autonomic nervous system (ANS)• The portion of the peripheral nervous system that controls

the body’s major systems (on autopilot usually)» Controls Cardiac, Smooth muscle and glands» Depending what the somatic nervous system picks

up, kicks the ANS 1st Gear or ReverseThese systems are complete opposites of each other

I.I. Sympathetic Nervous System - involuntarySympathetic Nervous System - involuntaryII.II. Parasympathetic Nervous System - involuntaryParasympathetic Nervous System - involuntary

Fight or Flight Reaction of ANS

• Is that a cougar on the path in front of you?!• When did it eat last? It looks skinny – gulp!• If it gets on your back each attack has a 33% chance of killing you.

After about 3 attempts, the cougar generally wins with the death bite to the back of the neck.

1. Somatic Nervous System – senses the danger2. Autonomic Nervous System kicks in

a)a) Sympathetic division alerts adrenal glands to secrete adrenalin / Sympathetic division alerts adrenal glands to secrete adrenalin / norepinepherinenorepinepherine

b)b) Senses heighten. Speed increases, pain not so easily felt, strength Senses heighten. Speed increases, pain not so easily felt, strength increases drasticallyincreases drastically

c)c) Choice now is ---> FLIGHT OR FIGHT???????Choice now is ---> FLIGHT OR FIGHT???????

Sympathetic Para-Sympathetic

Epinephrine

(+) Fatty Acid Release (-) Digestion

(+) Glycogenolysis

(+) Reaction Time

(+) Mental Alertness

(+) Muscle Contraction & Efficiency

(+) Dilates Airways

(+) Cardiac Output

ADRENAL MEDULLA

Chromaffin Cells

• The Peripheral Nervous SystemAutonomic Nervous System

Sympathetic Division of the ANS

– Adrenal medulla:• The inner portion of the adrenal gland, located atop the kidney,

controlled by sympathetic nerve fibers; secretes epinephrine and norepinephrine.

• The Peripheral Nervous SystemAutonomic Nervous System

Parasympathetic Division of the ANS

– Parasympathetic division:• Portion of the autonomic nervous system that controls

functions that occur during a relaxed state; supports activities involved with increases in the body’s supply of stored energy including salivation, gastric and intestinal motility, secretion of digestive juices, and increased blood flow to the gastrointestinal system.

• Returns body to resting state after a “FIGHT OR FLIGHT” EVENT. So, works opposite to the Sympathetic Nervous System

The Brain

• CerebrumCerebrum

• DiencephalonDiencephalon

• Brain stemBrain stem

• CerebellumCerebellum

Brain Structures

Frontal L.1 Corpus 2 Callosum

Hypothalamus 3 3 Pons 4

Medulla 5 Spinal Cord 6

12 Parietal L.

11 Thalamus

10 Occipital L.

9 Cerebellum

8 Retic. Activ Centre7 Central Canal

CerebrumCerebrum

• Thinking/Processing• Interpretation• Initiation of voluntary muscle movement• Cortex - External Grey Matter

– Processing takes place

• Basal Nuclei - Central Grey Matter – may have some voluntary muscle control, when

diseased, Parkinson may develop

Cerebrum

Frontal:Frontal: ConsciousThought &Motor Control

Temporal: Temporal: Hearing,Smelling,Perceptual Judgment,Visual/auditory Memory,

Parietal:Parietal: Sensory perception

Occipital:Occipital: Bilateral vision

Cerebrum

• Association areas connected with all lobes.

• Concerned with intellect, artistic & creative abilities, learning, memory

• Brain may be more “plastic” than once thought.

Cerebrum

• Left/Right halves communicate via Corpus Callosum

• Left brain controls right body

• Left Brain: spoken/written language, number & scientific skills, reasoning

• Right Brain: music/art awareness, 3D forms, insight, imagination

Brain StemBrain Stem

• Medulla Oblongata– Heart beat, Breathing, BP– Cough, Sneeze, Hiccup, Vomit

• Pons– Connects Cerebellum to CNS– w/ Medulla regulate breathing/head motion

w/ auditory, visual, tactile input

Brain StemBrain Stem

• Midbrain– Relay station– Reflexes: Auditory, Visual, Tactile

• Reticular Formation - Grey Matter– Controls wakefulness– Inactive: sleep– Damage: coma– Filters unnecessary stimuli

CerebellumCerebellum

• Coordinates movement

• Oversees skeletal muscle action

• Maintains muscle tone and posture

• Interprets info on body position from ear for balance

• Assists learning of new motor skills

Diencephalon

• Thalamus– Relays to/from rest of brain

• Hypothalamus– Regulates homeostasis– Link to Pituitary Gland

Reticular Activ. Centre

• The RAC acts like a filter, receiving and sending sensory and motor nerve signals.

• Controls sleep / alertness – wakes you up.

• Allows you to study while watching TV

Limbic System

• Parts of Frontal & Temporal Lobes, Thalamus, Hypothalamus, Amygdala, Hippocampus

• “Emotional Brain”: creates feelings about occurrences which influence how person acts in future.

• Memory and Learning: not fully understood, but involve Limbic. Emotionally charged = vivid memory

The Reflex Arc

Sensory stimulation – missed stair, sudden load, heat or pain sends signal to spine

– Interneuron sends signal quickly right back to skeletal muscle

– Muscle contracts: leg muscles tighen for stair / biceps flex for load / hand pulled away from heat

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Drug Abuse

Perspectives on Substance-Related

Disorders: An Overview (cont.)

Figure 11.2 (cont.)

Easy to get hooked on, hard to get off

The Depressants: Alcohol Use Disorders (cont.)

Figure 11.3

The path traveled by alcohol throughout the body

• In the United States– Most adults consider themselves light drinkers or

abstainers– Most alcohol is consumed by 11% of the U.S. population– Alcohol use is highest among Caucasian Americans– Males use and abuse alcohol more so than females– Violence is associated with alcohol, but alcohol alone

does not cause aggression

Alcohol: Some Facts and Statistics

• Facts and Statistics on Problem Drinking – 10% of Americans experience problems with alcohol– Most persons with alcoholism can moderate or cease

drinking on occasion– 20% of those with alcohol problems experience

spontaneous recovery – Anhedonia – Lack of pleasure, or indifference to

pleasurable activities– Affective flattening – Show little expressed emotion, but

may still feel emotion

Alcohol: Some Facts and Statistics (cont.)

Stimulants: An Overview

• Nature of Stimulants– Most widely consumed drugs in the United States– Such drugs increase alertness and increase energy– Examples include amphetamines, cocaine, nicotine, and

caffeine

Stimulants: An Overview

• Nature of Stimulants– Most widely consumed drug in the United

States– Such drugs increase alertness and increase

energy– Examples include amphetamines, cocaine,

nicotine, and caffeine

Stimulants: Amphetamine Use Disorders

• Effects of Amphetamines– Produce elation, vigor, reduce fatigue– Enhance the release of dopamine and norepinephrine,

while blocking reuptake– Such effects are followed by a “crash” (e.g., feeling

depressed and tired)

• Ecstasy and Ice– Produces effects similar to speed, but without the crash– 2% of college students report using Ecstasy– Both drugs can result in dependence

Stimulants: Cocaine Use Disorders

• Effects of Cocaine

– Produce short lived sensations of elation, vigor, reduce fatigue

– Effects result from blocking the reuptake of dopamine

– Cocaine is highly addictive, but addiction develops slowly

– Cocaine use in the United States has declined over the last decade

Stimulants: Nicotine Use Disorders

• Effects of Nicotine

– Stimulates the central nervous system, specifically nicotinic acetylcholine receptors

– Results in sensations of relaxation, wellness, pleasure– Nicotine is highly addictive & found in cigarettes

Opiates: ex. Codeine, Morphine, Heroin

• Nature of Opiates– Opium = Natural chemical in the opium poppy with narcotic

effects (i.e., pain relief)– a class of nature and synthetic substances with narcotic effects– Such drugs are often referred to as analgesics

• Effects of Opiates– Activate body’s enkephalins and endorphins– Low doses induce euphoria, drowsiness, and slowed breathing– High doses can result in death– Withdrawal symptoms can be lasting and severe

Stimulants: Nicotine Use Disorders

Relapse rates for nicotine compared to alcohol and heroin

An Integrative Model ofSubstance-Related Disorders

(cont.)

Stimulants: Caffeine Use Disorders

• Effects of Caffeine – The “Gentle” Stimulant

– Found in tea, coffee, cola drinks, and cocoa products– Caffeine blocks the reuptake of the neurotransmitter

adenosine– Small doses elevate mood and reduce fatigue– Used by over 90% of Americans/Canadians– Regular use can result in tolerance and dependence

Cannabis• The drugs produced from varieties of the hemp plant are, as

a group, called cannabis

– They include:

• Hashish, the solidified resin of the cannabis plant

• Marijuana, a mixture of buds, crushed leaves, and flowering tops

• The major active ingredient in cannabis is tetrahydrocannabinol (THC)– The greater the THC content, the more powerful the drug

Cannabis• When smoked, cannabis produces a mixture of hallucinogenic,

depressant, and stimulant effects

– At low doses, the user feels joy and relaxation

• May become anxious, suspicious, or irritated

• This overall “high” is technically called cannabis intoxication

– At high doses, cannabis produces odd visual experiences, changes in body image, and hallucinations

• Most of the effects of cannabis last three to six hours– Mood changes may continue longer

Cannabis

• Marijuana abuse and dependence– Marijuana was once thought not to cause abuse or

dependence

– Today many users are caught in a pattern of abuse• Some users develop tolerance and withdrawal, experiencing

flu-like symptoms when drug use is stopped

• About 1.5% of people in the U.S. displayed marijuana abuse or dependence in the past year

– About 5% will fall into these patterns at some point in their lives