Chapter 17 Nervous System. Divided into two systems Central nervous system (CNS) ▪ Brain and...

Chapter 17Nervous System

Nervous System- Overview Divided into two systems

Central nervous system (CNS)▪ Brain and spinal cord

Peripheral nervous system (PNS)▪ Sensory and motor nerves

All nervous tissue composed of 2 general cell types Neurons▪ specialized for conduction of information

Neuroglia▪ support and nourish neurons

Nervous System – Overview cont’d

Fig. 17.1

Neurons - 3 Classes

Sensory▪ Takes messages to CNS▪ May have specialized sensory receptors

Interneuron▪ Lies entirely within CNS▪ Receives input from sensory neurons and other

interneurons▪ Summarizes messages ▪ Communicates with motor neurons

Motor neurons▪ Take messages from CNS to effector organs▪ Effector organs can be muscles, glands, or organs

Neurons - Structure

Cell body Contains nucleus and most organelles

Dendrites Extensions leading toward cell body Receive signals from other neurons Send them to cell body

Axon Conducts impulses away from cell body Toward other neurons or effectors

Neurons – Structure cont’d

Myelin sheath Lipid substance- electrical insulator Covers some axons▪ Gives whitish appearance

Formed by Schwann cells in PNS▪ Neuroglia that wrap around axons ▪ Gaps between cells called Nodes of Ranvier

Important in nerve regeneration▪ Sheath serves as pathway for new axon

growth

Myelin sheath

Fig. 17.317-7

Structure of Different Neurons

Fig. 17.2

Neurons – CNS

Grey Matter Neurons with short

axons Nonmyelinated

White Matter Have myelinated

axons

Brain Surface layer of

brain is grey matter White matter lies

deep

Spinal cord Central portion is

gray matter White matter

surrounds the grey matter

Nerve Impulse – Resting Potential

Imbalance of ions between inside and outside of neurons causes a potential difference (voltage) called resting potential More sodium outside than inside, More potassium inside than outside

Nerve Impulse – Resting Potential

Caused by Sodium-Potassium pump and membrane permeability▪ Membrane is more permeable to potassium

than sodium at rest; tends to “leak” positive charges

Results in inside of axon being electronegative with respect to outside -65mV (milivolts)

Nerve Impulse – Action Potential

Occurs when a neuron sends information down an axon Stimuli alters the normal resting

potential of neurons, to a point called the threshold.

Intense stimulus causes axon to fire more often in a given time interval▪ All or none phenomenon

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Nerve Impulse – Action Potential cont’d

Rapid change in polarity across membrane as impulse occurs Change in polarity called action potential

Requires 2 types of gated channel proteins Sodium channel Potassium channel

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At threshold level, sodium gates in axomembrane open Sodium flows down gradient into axon Membrane potential changes from -

65mV up to +40mV Called depolarization because inside

changes from negative to positive Resulting positive charge causes sodium

gates to close.

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Potassium gates open Potassium flows down its gradient out of

the axon Brings potential back to -65mV Called repolarization because it returns

to original polarity Gates close

These events occur in only 1 millisecond

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Nerve Impulses: Conduction of Action Potential

Nonmyelinated axons travels down axon one small segment at a time As soon as action potential moves on, the previous

section undergoes refractory period▪ Sodium gates cannot reopen▪ Prevents retrograde transmission▪ During this time sodium-potassium pump restores ions to

original positions

Myelinated axons Gated ion channels concentrated in nodes of Ranvier Action potential travels faster▪ “Jumps” from node to node- saltatory conduction

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Nerve Impulses: Synapses Structure that allows the transmission of

information from one neuron to another neuron.

Formed where the axon terminals of one neuron lies close to the dendrite or cell body of another neuron. Synapse: Region of proximity▪ No actual contact-space is synaptic cleft

Membrane of first neuron- presynaptic membrane Membrane of second- postsynaptic membrane

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Synapse structure and function

Fig. 17.517-18

Nerve Impulses: Neurotransmitters

Chemical stored in synaptic vesicles in presynaptic neuron Allows for communication between presynaptic

and post synaptic neurons

Release of neurotransmitter Presynaptic axon depolarizes Calcium channels open and calcium moves in Causes synaptic vesicles to bind to membrane▪ Neurotransmitter released into cleft▪ Diffuses across and binds to postsynaptic receptors

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Nerve Impulses: Neurotransmitters

Response of postsynaptic membrane Depends on neurotransmitter▪ Can be excitatory and cause an action

potential▪ Can be inhibitory and prevent an action

potential

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Integration

How neurons ‘add up’ the multiple signals they receive before the generation of an action potential

Fig. 17.6 17-21

Nerve Impulses: Neurotransmitters cont’d

Acetylcholine (Ach) and norepinephrine (NE) are examples▪ Both are excitatory neurotransmitters

Once released and responses initiated, neurotransmitters are removed from cleft▪ Some removed by enzymes

▪ Ach is removed by acetylcholinesterase

▪ Others are taken back up by presynaptic neuron▪ Prevents repeated stimulation of postsynaptic membrane

Many drugs affect nervous system▪ Interfere or potentiate neurotransmitters▪ Can enhance or block release▪ Can interfere with removal from cleft

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Nervous system

Overview Note direction of

nerve impulses Fig. 17.7

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CNS: Spinal Cord

Extends from base of brain into vertebral canal

Protected by vertebrae Intervertebral disks cushion and

separate

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CNS: Spinal Cord cont’d

Structure Cross-sectional anatomy▪ Central gray matter▪ Shaped like letter “H”▪ Dorsal root- sensory fibers entering gray matter▪ Ventral root-motor fibers leaving gray matter▪ Dorsal and ventral roots join as spinal nerve▪ Interneurons in gray matter▪ Fluid-filled central canal

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Structure cont’d

Cross-sectional anatomy cont’d

▪ White matter▪ In areas around gray matter▪ Ascending and descending tracts

Ascending located dorsally Sending axons up to brain

Descending located ventrally Sending axons from brain to spinal nerves Many tracts cross over to opposite side Left side of brain controls right side of body and

vice versa17-26


Fig. 17.817-27


Functions Communication between brain and body Center for many reflex arcs▪ Sensory receptor generates impulse▪ Sensory neuron transmits impulse to cord▪ Synapses with interneurons in cord-

integration▪ Transmitted to motor neuron▪ Motor neuron carries impulse to effector

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CNS: Brain

Brain can be divided into 4 regions: Cerebrum Diencephalon Cerebellum Brain Stem

Figure 17.9 Colour codes of regions not

present in text!

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CNS: Brain cont’d

Cerebrum Connected by corpus callosum Higher thought processes, learning,

language, speech 2 Cerebral hemispheres▪ Divided by longitudinal fissure▪ Folded surface▪ Sulci (sulcus)- shallow grooves

Divide each hemisphere into 4 cerebral lobes

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CNS: Brain cont’d

Frontal lobe▪ Most ventral lobe▪ Complex thought

processes▪ Primary motor area▪ In frontal lobe ventral

to central sulcus▪ All voluntary motor

movements originate here Each body part is

controlled by a specific section

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CNS: Brain cont’d

Parietal lobe▪ Dorsal to frontal lobe▪ Primary

somatosensory area▪ dorsal to central sulcus▪ Receives sensory

information from skin and skeletal muscles

▪ Touch, temperature, taste, pressure, localization of pain

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CNS: Brain cont’d

Occipital lobe▪ Most dorsal lobe▪ Primary visual area

Temporal lobe▪ Inferior to frontal and

parietal lobes▪ Primary auditory and

olfactory areas

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Fig. 17.10

CNS: Brain cont’d

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CNS: Brain cont’d

The primary motor area and somatosensory area Fig. 17.11

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CNS: Brain cont’d

Cerebrum cont’d

Association Areas▪ Integrate signals to organize into complete message

▪ Pre-motor Area▪ Gathers signals to organize motor functions

Walking , talking and chewing gum!

▪ Somatosensory Association Area▪ Collects from skin and muscles

▪ Visual and Auditory Association Areas▪ Sorts visual and Auditory stimulus into “new” or “seen that”

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CNS: Brain cont’d

Cerebrum cont’d

Processing centers▪ Receive information from all association areas▪ Perform higher-level analytical functions▪ Integration here accounts for critical thinking abilities▪ Our ability to speak is governed by 2 processing centers▪ Wernicke’s area

dorsal part of left temporal lobe Understanding of written and spoken words

▪ Broca’s area left frontal lobe Directs motor actions for speech Grammatical refinements

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CNS: Brain cont’d

Cerebrum cont’d

Central White Matter▪ Beneath the cerebral cortex▪ Contains bundles of axons called tracts▪ Connects various regions of the brain

Basal Nuclei▪ AKA basal ganglia

▪ Masses of grey matter deep within white matter▪ Integrate motor commands

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CNS: Brain cont’d

The Diencephalon Hypothalamus▪ Homeostatic control center▪ Thermoregulation, Water balance, Hunger and satiety,

Sleep

Thalamus ▪ Sensory relay center▪ Receives incoming information and sends it to

appropriate area▪ Arousal of cerebrum▪ Memory, emotional responses

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CNS: Brain cont’d

Cerebellum Separated from brainstem by 4th

ventricle Receives both sensory and motor input Can compare actual movements with

intended movements Functions to assure smooth, coordinated

motor movements

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CNS: Brain cont’d

Brain stem Midbrain▪ relay center for tracts passing between

cerebrum, cerebellum, and breathing, reflex movements of the head

Pons▪ “ bridge” in Latin ▪ bundles of axons connecting the Cerebellum

to the rest of the CNS

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CNS: Brain cont’d

Brain Stem cont’d

Medulla oblongata▪ Autonomic control center▪ Heart rate, breathing, blood pressure, swallowing,

coughing, vomiting

EEG = Electroencephlogram▪ No electrical activity in the brain stem

indicates brain death.

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Limbic System

Involved in our emotions and higher mental functions Blends primitive emotions

(fear, aggression, pleasure) with higher mental functions (reasoning, memory)

Complex network of tracts and nuclei deep in cerebrum Incorporates many regions of

the brain.

Fig. 17.12

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Limbic System cont’d

Hippocampus Communicates with frontal lobe Learning and memory▪ May convert wrote memory to learning

Amygdala Anger, defensiveness, fear Coordinates release of epinephrine

(adrenalin)

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Memory and learning Memory is the ability to hold on to or recall a piece of

information Learning is the ability to retain and apply past memories

Types of Memory▪ Short-term memory▪ Retained for short period like a phone number you look up

▪ Long-term memory▪ Retained for long period, perhaps for life▪ Combination of semantic memory (words, numbers) and episodic memory

(people, events, etc.)

▪ Skill memory▪ Combinations of motor activities like swimming, using scissors, etc.

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Language and speech Dependent upon ▪ memory▪ primary visual and auditory center functions▪ primary motor cortex function

Left and right cerebral hemispheres have different functions▪ Broca’s and Wernicke’s areas are only in the left hemisphere▪ Broca’s- ability to speak▪ Wernicke’s- ability to comprehend speech

▪ Both hemispheres process information, but differently▪ Left is very specific, Right is very global▪ Origins of “Left Brained” or Right Brained”

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Language and speech Fig. 17.14

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Peripheral Nervous System Composed of nerves (bundles of

axons) and ganglia (swellings associated with nerves that contain cell bodies)

Cranial nerves 12 pairs, attached to the brain May be sensory, motor, or a mix of the two Mainly concerned with head, neck, and face ▪ Exception is vagus nerve which extends from and

to internal organs in the thorax and abdomen

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Peripheral Nervous System cont’d

Cranial Nerves Fig. 17.15

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Spinal nerves 31 pairs, emerge from spinal cord

between vertebrae All are mixed nerves▪ Dorsal root ▪ contains cell bodies of sensory neurons

▪ Ventral roots ▪ contain axons of motor neurons

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Spinal Nerves Fig. 17.15

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Divided into two systems Somatic nervous system▪ Serves the skin, muscles, and tendons▪ Delivers messages from receptors to the CNS and

motor responses back to periphery

▪ Many actions are reflex activities▪ A programmed response to a stimulus that is

automatic▪ Brain does not need to process information!

Can be conscious or unconscious but not mentally willed

▪ Protective functions

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Somatic Nervous System cont’d

▪ The Reflex Arc:▪ Sensory receptor, at tip of dendrites of a sensory

neuron, responds to specific stimulus and carries the stimulus to the spinal cord…

Cell body located in dorsal root ganglion, axon enters spinal cord through dorsal root

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Somatic Nervous System cont’d

▪ The Reflex Arc: cont’d▪ Interneuron, in central gray matter of cord,

integrates signal and transmits to…▪ Motor neuron, whose cell body is in ventral horn and

axon leaves via ventral root, leads to…▪ Effector organ which carries out response

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Fig. 17.16

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The Autonomic Nervous System (ANS) 2 divisions▪ Sympathetic and parasympathetic nervous systems

Features in common▪ Function automatically and generally are involuntary▪ Innervate (provide nerves to) all internal organs▪ Pathway consists of 2 motor neurons that synapse at a ganglion▪ The first is the preganglionic neuron and its cell body is in the CNS▪ The second is the postganglionic neuron and its cell body is in the ganglion

ANS regulates cardiac muscle, smooth muscle and glands▪ Important homeostatic reflexes

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Autonomic Nervous System (ANS) cont’d

Sympathetic Division▪ Mediates the “fight or flight” response▪ Increases heart rate and contractility, dilates bronchi▪ Inhibits the digestive tract▪ Stimulates release of epinephrine from adrenals

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Sympathetic Division cont’d

▪ Preganglionic Neurons▪ Cell bodies in the thoracic and lumbar regions of the spinal cord▪ short▪ Synapse in spinal ganglia which lie along the spinal cord

▪ Postganglionic neurons are long▪ Primary neurotransmitter is norepinephrine

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Parasympathetic Division▪ Preganglionic Neurons▪ Cell bodies in the brain and sacral portion of the spinal

cord▪ Long▪ Synapse in ganglia in the walls of effector organs

▪ Postganglionic neurons▪ Short ▪ Neurotransmitter is acetylcholine

▪ Mediates “rest and digest” functions▪ Promotes digestion, Decreases heart rate

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Table 17.1

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NS: Chemical Influences

Drugs are any substance that can alter the homeostasis of the body Stimulants – increase your NS function Depressants – decrease your NS function▪ Type of drug does not indicate effect on behaviors

2 main mechanisms of action on nervous system Affect the limbic system Affect neurotransmitter activity

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NS: Chemical Influences cont’d

Pain Relief Analgesics▪ stop you from feeling pain by influencing pain

pathways▪ Can be local or system wide.

Anesthetics▪ Stop ALL feeling

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Pain Relief Effect on Pain Pathways▪ Neurons: ▪ At site of injury by blocking the pain receptor’s ability to sense painful

stimuli or to transmit pain nerve impulses. Most over the counter pain medicines Local anesthetics

▪ Spinal Cord: ▪ suppress pain transmission to the brain.

Opioid based such as morphine, meperidine, oxycodone and fentanyl and

Epidural : spinal anesthetics (higher doses of local anesthetics applied directly to regions of the spinal cord

▪ Brain: ▪ suppress pain information and induce sleep.

general anesthetics, IV sedation

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Drug abuse Takes levels above therapeutic doses Generally under circumstances that increase

harmful effects

Addiction▪ Physical dependency▪ Person develops tolerance to drug so more is needed for desired

effect▪ Withdrawal symptoms when drug removed

▪ Psychological dependency▪ Person thinks about the drug constantly, preoccupied with

obtaining the drug17-64

NS: Chemical Influences cont’d Alcohol

Influences GABA (inhibitory neurotransmitter) and glutamate (excitatory neurotransmitter)

Metabolized in the liver▪ Prevents liver from breaking down fats▪ First stage of damage, it is reversible▪ Continued consumption causes fibrous scar tissue

formation, still reversible

▪ Long term consumption causes cirrhosis, cell death, irreversible

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Alcohol Poisoning▪ Extreme suppression of body functions▪ Causes hypothermia, low blood sugar causing

seizures, and ultimately cardiac arrest.

▪ Our body absorbs alcohol faster than it can metabolize it. ▪ 1 drink per hour allows liver to keep up.

▪ Binge drinking often results in consuming a lethal doses before you pass out.

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Alcoholism▪ Alcohol is a carbohydrate▪ Lacks vitamins, minerals, essential amino acids, fatty

acids

▪ Alcoholics are vitamin-deficient, undernourished, and prone to illness▪ Fetal alcohol spectrum Disorders ▪ Caused by alcohol consumption during pregnance

Quantity is unknown, only safe amount is NONE! Affects physical and mental development of fetus

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NS: Chemical Influences cont’d Nicotine

Alkaloid derived from tobacco▪ Found in cigarettes, snuff, chewing tobacco, cigars

Causes neurons to release dopamine▪ Excess dopamine causes dependence▪ Withdrawal characterized by headache, stomach pain,

irritability, insomnia

▪ Stimulates the same receptors as acetylcholine ▪ Increased skeletal muscle activity, heart rate and blood

pressure and digestive tract motility

During pregnancy greatly increases risk of low birth weight and stillbirth.

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NS: Chemical Influences cont’d Cocaine

An alkaloid derived from the Coca plant

Prevents synaptic uptake of dopamine▪ Causes extreme physical dependence▪ Accounts for state of euphoria

Two phases to response▪ Binge phase

▪ Hyperactivity, decreased appetite, increased sex drive

▪ Crash phase▪ Depression, irritability, decreased sex drive (impotency in males)

Overdose causes cardiac and respiratory arrest

Use during pregnancy causes addiction in foetus, newborns undergo withdrawal symptoms.

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Fig. 17.19

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NS: Chemical Influences cont’d Heroin

Derived from morphine which is an alkaloid of opium

Heroin binds to endorphin receptors▪ Produces euphoria, pain relief▪ Side effects include nausea, vomiting, restlessness, anxiety,

mood swings

Over time body stops producing natural endorphins▪ Tolerance develops▪ Withdrawal occurs▪ Perspiration, tremors, dilated pupils, cramps, increased blood pressure

and respiratory rate

Use during pregnancy results in foetal addiction 17-71

NS: Chemical Influences cont’d Marijuana

From dried leaves, stems, and flowers of Cannibus sativa▪ Resins contain THC- tetrahydrocannabinol

Binds to receptors for anandamide▪ Short-term memory processing, Creates feeling of

contentment

Long-term use leads to anxiety, paranoia, psychotic symptoms, and ultimately brain impairment

Use during pregnancy results in a disorder similar to FASD

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Chapter 17 Nervous System. Divided into two systems Central nervous system (CNS) ▪ Brain and...

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