Chapter 11: Nervous System

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Function of the Nervous System  To coordinate the actions of your body  To ensure effective behaviour  To maintain the internal environment within safe limits (homeostasis) Messages are relayed throughout the body via electrochemical messages from the brain or through chemical messengers – hormones (hormones require more time than nervous transmission but are long lasting) There are more nerve cells in the body than there are visible stars in the Milky Way!

1 cm 3 of brain tissue houses several million neurons with each connecting with several thousand others 17-2

Nervous Tissue

The nervous system is divided into a

central nervous system

(

CNS

), consisting of the brain and spinal cord, and a

peripheral nervous system

(

PNS

), consisting of nerves carrying sensory and motor information between the CNS and muscles and glands. Both systems have two types of cells:

neurons

transmit impulses and

neuroglial cells

that that support neurons.

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Organization of the nervous system

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Neurons are composed of dendrites that receive signals, a cell body with a nucleus, and an axon that conducts a nerve impulse away. Sensory neurons take information from sensory receptors to the CNS. Interneurons occur within the CNS and integrate input (nonmyelinated). Motor neurons take information from the CNS to muscles or glands.

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Types of neurons

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dendrites – receive information (either from receptor cells or other nerve cells), conducting towards the cell body (~200 dendrites/cell body) cell body – location of the nucleus, high metabolic rate (so contains mitochondria) axon– may be 1m long, very thin, conducts the impulse towards other neurons or effectors, starts at axon hillock, the smaller the neuronal diameter, the faster the neuronal transmission 17-8

nodes of Ranvier– the unmyelinated sections of a myelinated neuron, impulses “jump” between the nodes of Ranvier neurilemma– a thin layer encompassing neurons in the peripheral nervous system, promoting their regeneration 17-9

Schwann cell – responsible for the myelin synthesis, type of glial cell (supporting and nourishing cell found in the nervous system) Axon Bulb – either at a synaptic bulb or end plate to muscle, contains neurotransmitter 17-10

Myelin Sheath

Myelination covers long axons with a protective

myelin sheath

(made by neuroglial cells called

Schwann cells)

. The sheath contains lipid

myelin

which gives nerve fibers their white, glistening appearance. The sheath is interrupted by gaps called

nodes of Ranvier

.

Multiple sclerosis

is a disease of the myelin sheath.

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Myelin sheath

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FYI

Nerves are generally comprised of many neurons together (like fibre optic cable) Myelinated neurons in the brain are termed white matter (the myelin makes them look white) White matter may regenerate after injury, whereas grey matter (unprotected) will not 17-13

The Nerve Impulse

The nervous system uses the

nerve impulse

information.

to convey The nature of a nerve impulse has been studied by using excised axons and a voltmeter called an

oscilloscope

.

Voltage (in

millivolts

, mV) measures the electrical potential difference between the inside and outside of the axon.

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Membrane Polarization ( Resting Potential )

When an axon is not conducting a nerve impulse, the inside of an axon is negative (-70mV) compared to the outside(+40mV); this is the

resting potential

. To establish the –70mV potential in the cell:  Na+ is actively pumped out of the cell  K+ is actively pumped into the cell

Sodium pump

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Membrane Polarization ( Resting Potential )

 Na+ and K+ diffuse down the concentration gradient, but K+ diffuses faster due to an increased number of ion channels (gates) open to K+ ions  Since there is a net loss of positive ions to the outside of the cell, -70 mV is established inside the neuron  There are also large negative proteins inside the neuron that contribute to the negative charge 17-16

Resting potential

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Membrane Depolarization

When the nerve cell is excited, the membrane DEPOLARIZES ( Action Potential ) The membrane’s polarity changes: – Na+ channels open, Na+ rushes in, K+ gates close The positive ions flowing in causes a charge reversal to +40 mV inside the neuron ( gated channel proteins ) 17-18

Action potential

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Membrane Repolarization

Once the charge becomes positive, the Na+ gates close, K+ gates open, eventually restoring the charge inside the neuron to –70 mV (but the Na+ excess is inside and K+ excess is outside!) The Na/K Pump restores the ion concentrations inside and outside the cell 17-20

Membrane Repolarization

During the repolarization, the nerve cannot be reactivated – this is called the refractory period (1 to 10 ms) and is a recovery time for the neuron The pump requires ATP in order to operate 17-21

The Na/K Pump

To be ready for another action potential, the membrane re-establishes the proper concentration gradient for sodium and potassium Three sodium ions are actively transported across the membrane and to the ECM Two potassium ions are then carried across to the cytoplasm 17-22

Movement of the Action Potential

The action in the neuron adjacent to an area of resting membrane causes that area to depolarize, moving the action potential along (due to attraction of opposite charges) Since the area from which the action potential came is still in recovery, the action potential will only move in one direction 17-23

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Propagation of an Action Potential

The action potential travels the length of an axon, with each portion of the axon undergoing depolarization then repolarization. A

refractory period

move backwards. ensures that the action potential will not In myelinated fibers, the action potential only occurs at the nodes of Ranvier. This “jumping” from node-to-node is called

saltatory conduction

.

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Cell body Schwann cell Depolarized region (node of Ranvier) Myelin sheath Axon

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The All-or-None Response (Threshold Potential) All neurons provide an all-or-none response: - in response to a stimulus, they either activate (fire) and provide a certain level of response, or don’t fire at all A neuron will only fire if it is stimulated with an intensity of at least threshold level Every action potential for a neuron is identical in strength and duration (regardless of how much beyond threshold the stimulus is) 17-28

Threshold Potential All neurons differ in their threshold level To inform the brain of the intensity of a stimulus: - the frequency of firing is increased (not speed, which is constant for each neuron) - the number of neurons that respond to that level of stimulus can increase (neurons may have different threshold) 17-29

Transmission Across a Synapse

 The junction between neurons or neurons & effectors is called the

synapse

.  Transmission of a nerve impulse takes place when a

neurotransmitter

molecule stored in the axon bulb is released into a

synaptic vesicles synaptic cleft

between in the axon and the receiving neuron. 17-30

When a nerve impulse reaches an axon bulb, calcium channels open and Ca 2+ flow into the bulb.

This sudden rise in Ca 2+ causes synaptic vesicles to move and merge with the presynaptic membrane, releasing their neurotransmitter molecules into the synapse The binding of the neurotransmitter to receptors in the postsynaptic membrane causes either

excitation

or

inhibition

. 17-31

Synapse structure and function

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Synaptic Summation

Many synapses per single neuron is not uncommon. Excitatory signals have a depolarizing effect, and inhibitory signals have a hyperpolarizing effect on the post- synaptic membrane.

Summation

is the summing up of these excitatory and inhibitory signals.

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Summation

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Summation

Neurotransmitter Molecules

Out of 25, two well-known neurotransmitters are

acetylcholine

(

ACh

) and

norepinephrine

(

NE

). Neurotransmitters that have done their job are removed from the cleft; the enzyme

acetylcholinesterase (AChE

) breaks down acetylcholine. Neurotransmitter molecules are removed from the cleft by enzymatic breakdown or by reabsorption, thus preventing continuous stimulation or inhibition.

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FYI

 most synapses involve more than just 2 neurons (or neuron/effectors)  neurotransmitters move only by diffusion, so synaptic transmission is MUCH slower than axonal transmission.

 insecticides interfere with enzymes that break down neurotransmitters causing their hearts to remain contracted,  whereas LSD and other hallucinogens are believed to bind to the receptor sites for neurotransmitters 17-37

 Lidocaine, an anesthetic works by stabilizing the neuronal membrane so it can’t depolarize  Endorphins and enkephalins are “natural” painkillers produced in the CNS, blocking the pain transmitter that usually attaches to the injured organ allowing the perception of pain  opiates (heroin, codeine, morphine) block the production of the pain transmitter. Since they act to decrease the production of natural painkillers, the amount of opiate taken must be increased or at least maintained to maintain the effect 17-38

 Valium and other depressants are believed to enhance the action of inhibitory synapses  Alcohol acts to increase the polarization of the membrane, increasing the threshold  Since many neurons will connect to a postsynaptic neuron, it is the summation of the effects of the presynaptic neurons that determine whether or not the postsynaptic neuron or effector will depolarize 17-39

Neural Circuits – includes neuronal and synaptic transmission There are two types of neural circuits  complicated neural circuits, involving conscious thought  reflex arcs – without brain coordination  often unconscious, involuntary and faster than when thought is required (why are these useful?) 17-40

Nervous Control (in general) Stimulus  Receptor  SensoryNeuron  Interneuron  Brain  Interneuron  MotorNeuron  Effector  Response Reflex Arc (see diagram – the reflex arc) Stimulus  Receptor  SensoryNeuron  Interneuron (spinal cord)  MotorNeuron  Effector  Response When the response is made at the spinal cord level (information does not have to go to the brain to be processed), the response is quick (and always correct given the circumstances) Reflexes protect the body from injury 17-41

The Central Nervous System

The

central nervous system cord

and

brain

. (

CNS

) consists of the

spinal

Both are protected by bone, wrapped in protective membranes called

meninges

, and surrounded and cushioned with

cerebrospinal fluid

the ventricles of the brain. that is produced in 17-42

The

ventricles

are interconnecting cavities that produce and serve as a reservoir for cerebrospinal fluid.

The CNS receives and integrates sensory input and formulates motor output.

Gray matter

contains cell bodies and short, nonmyelinated fibers;

white matter

contains myelinated axons that run in

tracts

.

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The Brain

 consumes more oxygen and glucose than any other part of the body  meninges – outer layers (protection) – dura mater, arachnoid and pia mater  cerebrospinal fluid –between the inner, middle meninges & central canal of s.cord, carries nutrients, acts as a shock absorber, relays waste by diffusion & fac. diffusion, flows within ventricles – four “spaces” in the brain

The human brain

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Fig. 49-15

Frontal lobe Parietal lobe Frontal association area Speech Speech Taste Smell Hearing Auditory association area Temporal lobe Somatosensory association area Reading Visual association area Vision Occipital lobe

Fig. 49-17

Hearing words Speaking words Max Seeing words Generating words Min

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The Cerebral Cortex

The

cerebral cortex

is a thin, highly convoluted outer layer of gray matter covering both hemispheres. The

primary motor area

is in the frontal lobe; this commands skeletal muscle. The

primary somatosensory area sulcus

or groove. is dorsal to the

central

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Forebrain (cerebrum)  contains two hemispheres for coordinating sensory and motor information – speech, reasoning, memory, personality, which may be located on one side only  the outer layer is called the cerebral cortex (only 1 mm thick), deeply folded into fissures(to increase surface area)

Cerebral hemispheres

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Forebrain Continued

- the two hemispheres are connected by the corpus callosum allowing info to be shared between the hemispheres (a collection of nerve fibres) which are sometimes severed to control epilepsy leading to interesting results - the cerebrum can be subdivided into 4 lobes 1.

2.

3.

4.

Frontal (walking, speech, intellect, personality), temporal (hearing,vision, memory, interpretation), parietal (interpreting sensory info receptors, long term memory) and occipital (vision) lobes Broca’s area - a part of the left hemisphere usually where speech centre is located

The lobes of a cerebral hemisphere

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Forebrain Continued

  thalamus- below cerebrum, coordinates and interprets sensory info hypothalamus – below the thalamus, related to pituitary,  connects endocrine to the nervous system, receives sensory info, instincts, temperature control (ANS)   pituitary gland – influenced by the hypthalamus, part of the endocrine system (master gland) pineal gland – part of the endocrine system – melatonin production

 midbrain - less developed in humans than the forebrain, 4 spheres – relay centre for some eye and ear reflexes  Hindbrain - located behind the midbrain, connects brain to spinal cord  contains cerebellum (coordinates movement, balance, muscle tone), The cerebellum is involved in learning of new motor skills, such as playing the piano.

 pons (relay station between cerebellum areas, and cerebellum & medulla)  medulla oblongata (connection between peripheral and CNS, involuntary movements – heart rate, breathing (ANS), crossover of control)



FYI

 much brain research takes place during brain surgery & after people have strokes  epileptics also provide insight into brain differentiation when they undergo severing of the corpus callosum to relieve extremely serious seizures  although the brain must control the entire body, the volume of brain allocated to each part of the body is not proportional to that body part’s size – the face and hands account for the majority of the motor cortex’s attention

Fig. 49-16

Frontal lobe Parietal lobe Jaw Toes Primary motor cortex Abdominal organs Genitals Primary somatosensory cortex

Language and Speech

Language and speech are dependent upon

Broca’s area

(a motor speech area) and

Wernicke’s area

(a sensory speech area) that are involved in communication. These two areas are located only in the left hemisphere; the left hemisphere functions in language in general and not just in speech. 17-58

Language and speech

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Organization of the nervous system

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The Spinal Cord

The

spinal cord

extends from the base of the brain through the vertebral canal.

Structure of the Spinal Cord

A

central canal

holds cerebrospinal fluid. Gray matter of the spinal cord forms an “H” and contains

interneurons

and portions of sensory and motor neurons. White matter consists of

ascending tracts

taking sensory information to the brain and

descending tracts

carrying motor information from the brain. 17-61

 ventral root (towards front of body) carries motor neuron messages to muscles  dorsal root (towards back) carries sensory neuron messages from the body

Spinal cord

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Functions of the Spinal Cord

The

spinal cord

is the center for many

reflex arcs

. It also sends

sensory

information to the brain and receives

motor

output from the brain, extending communication from the brain to the peripheral nerves for both control of voluntary skeletal muscles and involuntary internal organs. Severing the spinal cord produces

paralysis

. 17-65

The Peripheral Nervous System

The

peripheral nervous system

(

PNS

) contains

nerves

(bundles of axons) and

ganglia

(cell bodies).

Sensory nerves nerves

carry information to the CNS, carry information away

motor

Humans have 12 pairs of

cranial nerves spinal nerves

. and 31 pairs of 17-66

Nerve structure

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Cranial nerves

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The

dorsal root

of a spinal nerve contains sensory fibers that conduct sensory impulses from sensory receptors toward the spinal cord.

Dorsal root ganglia

near the spinal cord contain the cell bodies of sensory neurons.

The

ventral root

that conduct impulses away from the spinal cord to effectors.

of a spinal nerve contains motor fibers 17-69

Spinal nerves

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Somatic System

The

somatic system

serves the skin, skeletal muscles, and tendons. The brain is always involved in voluntary muscle actions but somatic system reflexes are automatic and may not require involvement of the brain.

  nerves running to skeletal muscle system (under voluntary control) motor neurons  muscle) voluntary effectors (skeletal  control exists in the cerebrum & cerebellum (coordination) 17-71

Homeostasis and the Autonomic Nervous System

 All autonomic nerves are motor nerves that regulate the organs of the body without conscious control; involuntary  Control exists in the medulla  Effectors are smooth muscle (digestive system), cardiac muscle (heart) and glands (exocrine & endocrine)  Responsible for maintaining homeostasis during times of rest and during emergencies

Consists of two parts:

Sympathetic  prepares the body for stress, including “fight or flight” response  short preganglionic nerve (Ach), long postganglionic nerve (NEp)  originate in the thoracic vertebrae (ribs) or lumbar vertebrae (small of back)  Parasympathetic   restores normal balance; times of relaxation long preganglionic nerve (Ach), short postganglionic nerve (ACh)  originate in the brain (cranial nerves) or the spinal cord

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Fig. 49-8

Parasympathetic division Action on target organs: Constricts pupil of eye Stimulates salivary gland secretion Constricts bronchi in lungs Cervical Slows heart Stimulates activity of stomach and intestines Stimulates activity of pancreas Thoracic Stimulates gallbladder Lumbar Promotes emptying of bladder Promotes erection of genitals Synapse Sympathetic ganglia Sympathetic division Action on target organs: Dilates pupil of eye Inhibits salivary gland secretion Relaxes bronchi in lungs Accelerates heart Inhibits activity of stomach and intestines Inhibits activity of pancreas Stimulates glucose release from liver; inhibits gallbladder Stimulates adrenal medulla Inhibits emptying of bladder Sacral Promotes ejaculation and vaginal contractions

Autonomic nervous system

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Disorders Associated With the Nervous System

 Parkinson’s Disease: inadequate production of dopamine in the brain causes involuntary muscle contractions and tremors; can be partially alleviated with L-dopa (synthetic dopamine)

 Alzheimer’s Disease: decrease in CNS levels of acetylcholine  Multiple Sclerosis: degeneration of the Myelin sheath; Many symptoms, partial paralysis, double vision,speech problems  Amyotrophic lateral sclerosis (Lou Gehrig's disease (ALS) : genetic disease causing motor neurons to die; muscle control is lost, increased salivation, cramping, twitching

 Epilepsy: brain injury or lack of oxygen to the brain; Seizures – grand mal or petit mal – transient loss of muscle control  Spinal Cord Injuries: through injury or disease, the spinal neurons are damaged, Results in loss of motor control -degree of which depends on where the damage occurred

 Hydrocephalus: “water on the brain” – excess cerebrospinal fluid in the brain Increased pressure may lead to brain damage  Cerebral Palsy: Usually caused by oxygen deficiency before/during birth, reduced muscle coordination (cerebral damage)

Drug Abuse

depressants

decrease excitation; either can lead to

physical dependence

. Each type of drug has been found to either promote or prevent the action of a particular neurotransmitter. Medications that counter drug effects work by affecting the release, reception, or breakdown of

dopamine

, a neurotransmitter responsible for mood.

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Drug actions at a synapse

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 (a) (b) (c) (d) (e) (f) A drug can affect a neurotransmitter in these ways: cause leakage out of a synaptic vesicle into the axon bulb; prevent release of the neurotransmitter into the synaptic cleft; promote release of the neurotransmitter into the synaptic cleft; prevent reuptake by the presynaptic membrane; block the enzyme that causes breakdown of the neurotransmitter; or bind to a receptor, mimicking the action or preventing the uptake of a neurotransmitter.

Drug use

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Alcohol

Alcohol

may affect the inhibiting transmitter

GABA glutamate

, an excitatory neurotransmitter. or Alcohol is primarily metabolized in liver and heavy doses can cause liver scar tissue and cirrhosis. Alcohol is an energy source but it lacks nutrients needed for health.

Cirrhosis

of the liver and

fetal alcohol syndrome

are serious conditions associated with alcohol intake.

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Nicotine

Nicotine

is an alkaloid derived from tobacco. In the CNS, nicotine causes neurons to release

dopamine

; in the PNS, nicotine mimics the activity of acetylcholine and increases heart rate, blood pressure, and digestive tract mobility.

Nicotine induces both physiological and psychological dependence.

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Cocaine

Cocaine

is an alkaloid derived from the shrub Erythroxylum cocoa, often sold as potent extract termed “

crack

.” Cocaine prevents uptake of tolerance.

dopamine

by the presynaptic membrane, is highly likely to cause physical dependence, and requires higher doses to overcome This makes overdosing is a real possibility; overdosing can cause seizures and cardiac arrest. 17-88

Heroin

heroin

is an alkaloid of

opium

. Use of heroin causes euphoria.

Heroin alleviates pain by binding to receptors meant for the body’s own pain killers which are the

endorphins

. Tolerance rapidly develops and withdrawal symptoms are severe. 17-89

Marijuana

Marijuana

is obtained from the plant Cannabis sativa that contains a resin rich in THC (tetrahydrocannabinol).

Effects include psychosis and delirium and regular use can lead to dependence.

Long-term marijuana use may lead to brain impairment, and a fetal cannabis syndrome has been reported.

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Chapter Summary

neurons and mesoglia.

Neurons are specialized to carry nerve impulses.

A nerve impulse is an electrochemical change that travels along the length of a neuron fiber.

Transmission of signals between neurons is dependent on neurotransmitter molecules.

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The central nervous system is made up of the spinal cord and the brain.

The parts of the brain are specialized for particular functions.

The cerebral cortex contains motor areas, sensory areas, and association areas that are in communication with each other.

The cerebellum is responsible for maintaining posture; the brainstem houses reflexes for homeostasis.

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The reticular formation contains fibers that arouse the brain when active and account for sleep when they are inactive. The limbic system contains specialized areas that are involved in higher mental functions and emotional responses.

Long-term memory depends upon association areas that are in contact with the limbic system.

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There are particular areas in the left hemisphere that are involved in language and speech.

The peripheral nervous system contains nerves that conduct nerve impulses toward and away from the central nervous system.

The autonomic nervous system has sympathetic and parasympathetic divisions with counteracting activities.

Use of psychoactive drugs such as alcohol, nicotine, marijuana, cocaine, and heroin is detrimental to the body.

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