The Nervous System

NERVOUS SYSTEM - CONCEPTS

1. Homeostasis is maintained in the human body by various parts of the nervous system

2. Neural transmission occurs along axons, due to an action potential that causes depolarization of the neuron

3. Electrochemical communication occurs between cells at the synapse

4. The central nervous system is the body’s control center. It consists of the brain and spinal cord

NS - CONCEPTS (CONT.)

5. The brain includes centers that control involuntary responses and voluntary responses

6. The cerebrum is the largest part of the brain. It contains four pairs of lobes, each of which is associated with particular functions

7. The peripheral nervous system is composed of the somatic (voluntary) and autonomic (involuntary) system

8. The autonomic nervous system is divided into the sympathetic and parasympathetic nervous systems

STRUCTURES AND PROCESSES OF THE NERVOUS SYSTEM The nervous system regulates the human body

It coordinates with the endocrine system to maintain homeostasis

DIVISIONS OF VERTEBRATE NERVOUS SYSTEMS

Nervous System

CNS PNS

CELLS IN THE NERVOUS SYSTEM

Cells within the nervous system are either:

Neurons

Glial Cells

NERVE FIBRES

Neurons and glial cells are packed together to form nerve fibres that extend throughout the nervous system

Neurons come in three types – sensory, interneurons, and motor neurons

NEURAL CIRCUITS

Messages from sensory neurons sometimes will not travel to the brain before action is taken

This is because we have reflex arcs that are used for quick responses to stimuli

THE REFLEX ARC

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THE PURPOSE OF REFLEX ARCS

The purpose of a reflex arc is to prevent serious injury

For example, if you touch a hot object, you will often move your finger before feeling pain

This is because the reflex arc sends the pain message to the spinal cord interneurons, which redirect the message instantly to the motor neurons

Without this reflex arc, we would have to receive the pain signal, send it to the brain, have it interpreted, and then formulate the correct response

Within this time, a relatively minor burn would become a very serious one

THE NEURON

COMPONENTS OF THE NEURON

Dendrites: Receive information from adjoining cells or receptors and pass the information along the neuron

Cell Body: Contains organelles and processes the input from dendrites

Axon: Extension of the cytoplasm through which nerve impulses move

Myelin Sheath: Insulating covering surrounding the axon

COMPONENTS OF THE NEURON

Schwann Cells: Structures that produce the myelin sheath. These are a type of glial cell

Nodes of Ranvier: Junctions between myelin sections

Axon Terminal: Passes nerve impulse on to the next neuron in line

FACTORS AFFECTING NERVE IMPULSE SPEED

The diameter of the axon – in general, the smaller it is, the faster the impulse

Presence of myelin sheath – unmyelinated neurons transmit much slower than myelinated ones

MULTIPLE SCLEROSIS (MS)

Caused by destruction of the myelin sheath

Myelinated neurons are destroyed as the sheath turns into scar tissue

Produces a “short circuit” within the neuron

Symptoms include double-vision, speech difficulty, jerky limb movements, and partial paralysis of voluntary muscles

THE NEURILEMMA

This is a special membrane found in the cells of the PNS

It surrounds the axon and promotes regeneration of damaged tissue

WHITE & GREY MATTER

White matter consists of myelinated neurons It is these neurons that contain the

neurilemma as well Grey matter is unmyelinated Therefore, damage to these neurons is

permanent

A CROSS-SECTION OF THE SPINAL CORD

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ELECTROCHEMICAL IMPULSES The nerve impulses produced by neurons

differ from conventional electricity in several ways:

1. It moves much slower than conventional current

2. Cells would provide a high resistance to conventional current

3. The strength of electrical currents diminish as they move along a circuit

4. Conventional current requires an external source of energy

PRODUCTION OF THE IMPULSE

1. Sodium-potassium exchange pumps use ATP to move Na+ out of the cytoplasm of the cell and K+ into the cytoplasm. For every 2 K+ that move into the cell, 3 Na+ move out. This creates high concentration gradients across the cell membrane.

Sodium-potassium Pump Animation

2. As a result of the concentration gradients, K+ begins to diffuse out of the cytoplasm and Na+ diffuses in. However, there are more available K+ ion channels in the resting membrane, so this produces a positively charged region outside the membrane. This is called a polarized membrane or a resting membrane. There is a charge difference of about -70 mV inside the axon (there are more negative charges inside the axon than outside)

3. As an impulse is triggered, the nerve cell becomes more permeable to sodium than potassium, and the sodium rushes into the neuron. This causes a rapid reversal of charge known as depolarization. Once the charge inside the axon is positive, the sodium gates close.

Depolarization Animation – Sodium & Potassium Channels

4. The potassium gates open again and K+ begins to move back out of the nerve cell. When this occurs, the Na+ and K+ are on the opposite side of the membrane when compared to their position before depolarization. However, an excess of K+ move outside of the membrane, causing brief hyperpolarization.

5. The sodium & potassium pumps reactivate and transport Na+ out of the cytoplasm and K+ into the cytoplasm to return to the resting membrane state. This return to the original polarity is known as repolarization.

Because a neuron cannot fire again before it is repolarized, there is a time known as the refractory period where the nerve is unable to act

This refractory period takes 1 to 10 ms Action potentials in myelinated neurons only

occur at the Nodes of Ranvier

THE ENTIRE PROCESS:

MOVEMENT OF AN IMPULSE

The nerve impulse must move along the axon

This is achieved through the attraction of positive and negative charges along the nerve membrane

The positively charged ions moving into the cell when an action potential is produced are attracted to the negative ions in the neighboring regions of the cytoplasm

These positive ions begin to migrate, triggering the opening of sodium channels in that next region, causing depolarization

As a wave of depolarization moves along the membrane, it causes the potassium gates behind it to open, creating repolarization

THE MOVEMENT OF AN IMPULSE

Action Potential Propagation Animation

ENERGY AND IMPULSES

Because active transport is used to create the concentration gradients needed for a resting membrane to form, ATP must be used

THRESHOLD LEVELS

Early studies with nerve cells using electrical currents indicated that neurons will not produce a signal if a stimulus is below a certain level

This lowest level that produces a response is known as the threshold level

Therefore stimuli below threshold levels will not produce a response

As well, these experiments indicated that the response is often an all-or-none response

In other words, either the response (such as muscle contraction) would either not be present (when the threshold level had not been reached) or at maximum intensity (at any level above the threshold level)

DETECTING INTENSITY OF STIMULI

This information seems to contradict what we know from experience – stimuli can be experienced from low to very high intensities

For instance, we can distinguish very cold objects from very hot objects, but we also can feel a range of temperatures in between

This occurs because our brain interprets the intensity of a stimulus based on the frequency of the impulses it produces

Attached to each receptor are a number of neurons, each with a different threshold level

A low intensity message would be produced when only the most sensitive neurons fire, while high intensity messages occur as most or all of the neurons are actively sending impulses

THE SYNAPSE

A synapse or synaptic cleft is the space that exists between the axon terminal of one neuron and the dendrites of another neuron

Neurotransmitter chemicals leave the axon terminals through vesicles in the presynaptic neuron and travel to receptors in the postsynaptic neuron

The distance across the synapse is small (about 20 nm), but neurotransmitters must move via diffusion

This becomes the slowest part of the transmission of a nerve impulse (again, this explains the quickness of a reflex arc when compared to the message being sent to the brain)

THE SYNAPSE

http://kvhs.nbed.nb.ca Synapse Animation

TRANSMISSION AT THE SYNAPSE Excitatory transmitters trigger a nerve

impulse in a neuron These neurotransmitters are released

from vesicles within the axon endplate and diffuse across the synapse

As the neurotransmitter attaches to its receptor site, it opens sodium channels on the postsynaptic neuron

This initiates an action potential in the neuron

There are also neurotransmitters that are inhibitory – they prevent the production of a nerve impulse in the postsynaptic neuron

These most often open potassium gates, allowing the neuron to become hyperpolarized

As a result, the postsynaptic neuron cannot produce the action potential required for an impulse to occur

BREAKDOWN OF NEUROTRANSMITTERS

If a neurotransmitter remains in place on a receptor, it will prevent repolarization of the neuron

Therefore, these neurotransmitters must be broken down

This is often accomplished through the action of enzymes

ACETYLCHOLINE A good example of a neurotransmitter and its

enzyme are acetylcholine and cholinesterase Acetylcholine is an excitatory

neurotransmitter Just after acetylcholine is released, the

cholinesterase enzyme is released into the synapse

The cholinesterase enzymes seek out acetylcholine molecules and break them down

As a result, there is no more acetylcholine present and the postsynaptic neuron can repolarize

Of course, like most enzymes, inhibitors can be used to block their function

A number of insecticides and the nerve gas sarin are cholinesterase inhibitors which bind with cholinesterase and prevent it from breaking down acetylcholine

As a result, the muscles of the insect’s heart remain contracted and will not relax (which prevents it from beating)

Cholinesterase inhibitors have also been considered as treatments for Alzheimer’s Disease

Alzheimer’s Disease is related to a lowered production of acetylcholine

In patients with the disease, the cholinesterase often breaks down the low levels of acetylcholine before it has time to act

Cholinesterase inhibitors would then prevent the premature breakdown of acetylcholine by inhibiting the action of the enzymes

COMMON NEUROTRANSMITTERSNeurotransmittNeurotransmitterer

FunctionFunction Effects of Abnormal Effects of Abnormal ProductionProduction

AcetylcholineAcetylcholine ExcitatoryExcitatory Inadequate – Inadequate – Alzheimer’s DiseaseAlzheimer’s Disease

DopamineDopamine Control of body Control of body movements and movements and sensations of sensations of pleasurepleasure

Excessive – Excessive – schizophreniaschizophrenia

Inadequate – Inadequate – Parkinson’s DiseaseParkinson’s Disease

SerotoninSerotonin Temperature Temperature control, sensory control, sensory perception & perception & moodmood

Inadequate - Inadequate - depressiondepression

NorepinephrinNorepinephrinee

Prepares body for Prepares body for stressstress

Excessive – anxiety, Excessive – anxiety, insomniainsomnia

Inadequate – Inadequate – hunger, exhaustionhunger, exhaustion

SUMMATION

In many cases, a number of neurons come together at a junction

Often, when this occurs, more than one of the neurons bringing a message into the junction must be active to produce an action potential in the neuron leaving the junction

Summation is the effect produced by the accumulation of neurotransmitters from two or more neurons

As you can see here, both neurons A and B must fire at the same time to exceed the threshold level to activate D (A and B are not able to exceed the threshold levels individually)

Neuron C in this case is producing an inhibitory neurotransmitter

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THE CENTRAL NERVOUS SYSTEM The brain and spinal cord make up the CNS The brain itself is supported by three layers

of membranes known as meninges Between the inner and middle meninges

exists a layer of fluid known as cerebralspinal fluid (CSF)

This fluid is also found in the central canal of the spinal cord

This fluid acts as a shock absorber and as a transport medium for nutrients and waste to and from the brain cells

CSF AND ILLNESSES

The CSF can carry bacteria and viruses These may cause inflammations of the

meninges or areas of the spinal cord The typical method of diagnosis for diseases

such as meningitis is to remove CSF from the spinal cord and check it for pathogens

THE SPINAL CORD The spinal cord consists of neurons and is

approximately the diameter of a pencil The grey matter of the spinal cord contains

unmyelinated neurons and the cell bodies of motor neurons

The white matter consists of myelinated interneurons

THE SPINAL CORD

The dorsal nerve tract brings sensory information back into the spinal cord, while the ventral nerve carries motor information to peripheral muscles and organs

THE BRAIN

The human brain has a far more advanced forebrain than other animal species

The brain consists three sections – the forebrain, the midbrain, and the hindbrain

BRAIN STRUCTURES

THE HINDBRAIN

The hindbrain is located posterior to the midbrain and connects to the spinal cord

It consists of three main regions: the cerebellum, the pons, and the medulla oblongata

THE CEREBELLUM

This is the largest portion of the hindbrain

It controls limb movements, balance, and muscle tone

The cerebellum also receives information from proprioceptors that keep track of the location and position of the body’s limbs

This is the part of the brain that ultimately controls excitatory and inhibitory nerve impulses

THE PONS

The Pons serves as a relay station that connects the two halves of the cerebellum, and the cerebellum to the medulla oblongata

THE MEDULLA OBLONGATA This is the lowest part of the hindbrain It acts as a connection between the CNS and

the PNS It regulates involuntary muscle action (heart

rate, breathing, swallowing, coughing, etc.) The medulla oblongata also acts as a

coordinating center for the ANS

THE MIDBRAIN

The midbrain consists of four small spheres of grey matter

It relays visual and auditory information between areas of the forebrain and the hindbrain

It also plays a role in eye movement and the control of skeletal muscles

THE FOREBRAIN The forebrain contains a number of different

parts The olfactory lobes, which detect smell are

part of the forebrain The majority of the forebrain consists of the

cerebrum, which stores and interprets sensory information and initiates voluntary motor activities

SUPPLYING THE BRAIN Blood is separated from the brain by a blood-

brain barrier The blood that travels to the brain never

enters the nervous tissue itself The capillaries in the brain are made up of

tightly-fused cells This blocks the passage of many toxins and

infectious agents

TRANSPORT & THE BLOOD-BRAIN BARRIER

Substances such as glucose and oxygen are supplied to the brain through special transport mechanisms

However, lipid-based molecules move across the lipid bilayer of the capillary cells

Therefore, lipid-soluble materials (caffeine, nicotine, alcohol, heroin) have rapid effects on brain function

PARTS OF THE FOREBRAIN

PARTS AND FUNCTIONSLobe Function

Frontal Lobe

Associated with conscious thought, intelligence, memory, personality; controls voluntary muscle movement

Temporal Lobe

Involved in auditory reception

Parietal Lobe

Receive sensory information from the skin, processes information about body position

Occipital Lobe

Processes visual information

Mirror Neurons

HEMISPHERES OF THE BRAIN

The brain consists of a right and left hemisphere

These two hemispheres are connected by a bundle of nerves known as the corpus callosum

RIGHT VS. LEFT BRAIN…

Left Left BrainBrain

uses logic, detail oriented, facts rule, words uses logic, detail oriented, facts rule, words and language, present and past, math and and language, present and past, math and science, can comprehend, knowing, science, can comprehend, knowing, acknowledges, order/pattern perception, acknowledges, order/pattern perception, knows object name, reality based, forms knows object name, reality based, forms strategies, practical, safe.strategies, practical, safe.

Right Right BrainBrain

uses feeling, "big picture" oriented, uses feeling, "big picture" oriented, imagination rules, symbols and images, imagination rules, symbols and images, present and future, philosophy & religion, can present and future, philosophy & religion, can "get it" (i.e. meaning), believes, appreciates, "get it" (i.e. meaning), believes, appreciates, spatial perception, knows object function, spatial perception, knows object function, fantasy based, presents possibilities, fantasy based, presents possibilities, impetuous, risk taking.impetuous, risk taking.

The right side of the brain is associated with visual patterns and spatial awareness, while the left side is associated with verbal skills

The ability of a person to learn, and the learning style that suits them, may be partially dictated by which side of the brain is dominant

However, not all people have a dominant hemisphere of their brain

BROCA’S AREA & WERNICKE’S AREA On the left side of the cerebral cortex are

found Broca’s area (Frontal lobe) and Wernicke’s area (Temporal lobe)

Broca’s area coordinates the muscles for speaking and translates thought into speech

Wernicke’s area stores the information involved in language comprehension

Speech in Birds & Humans

OTHER PARTS OF THE FOREBRAIN

The forebrain also contains the thalamus and the hypothalamus

The thalamus, which is directly below the cerebrum, coordinates and interprets sensory information

The hypothalamus is connected to the pituitary and regulates a number of the body’s responses such as blood pressure, heart rate, temperature, basic drives (thirst & hunger) and emotions

Damage to the hypothalamus can lead to a person demonstrating unusual or violent behaviour

MAPPING BRAIN FUNCTIONS

Early information on the function of various parts of the brain was gathered from patients who recevied brain injuries or diseases

Later, Canadian Nobel Prize winner Wilder Penfield mapped the motor areas of the cerebral cortex by stimulating different parts of the brain through probing

NON-INTRUSIVE MAPPING

PET (positron-emission tomography) and MRI (magnetic resonance imaging) are now used to study and map the brain

The PET can track glucose consumption in a brain during particular activities

MRIs can produce high-detail images of the brain structure in three dimensions

THE PERIPHERAL NERVOUS SYSTEM

The peripheral nervous system includes all nerves outside of the central nervous system

The somatic nervous system, which is mostly under voluntary control, controls movement and receives information about the environment

This system contains 12 pairs of cranial nerves and 31 pairs of spinal nerves, all of which are myelinated

CRANIAL NERVES

Some nerves exit the brain itself – these are known as cranial nerves

One of the most important of these cranial nerves is the Vagus nerve

This nerve regulates the heart, the bronchi of the lungs, the liver, pancreas and digestive tract

THE AUTONOMIC NERVOUS SYSTEM

The Autonomic Nervous System controls involuntary functions within our body

This system helps to maintain homeostasis despite a changing internal environment

It consists of sympathetic and parasympathetic nerves, which are controlled by the hypothalamus and the medulla oblongata

SYMPATHETIC VS. PARASYMPATHETIC NERVES

Sympathetic nerves prepare the body for stress, while parasympathetic nerves return the body to its normal state

Sympathetic nerves use norepinephrine as an excitatory neurotransmitter which activates muscles

A number of different organs and organ systems are involved in ANS responses:

EFFECTS OF THE ANSOrgan Sympathetic ParasympatheticHeart Increases heart rate Decreases heart rate

Digestive Decreases peristalsis

Increases peristalsis

Liver Increases release of glucose

Stores glucose

Eye Dilates pupil Constricts pupil

Bladder Relaxes sphincter Contracts sphincter

Skin Increases blood flow Decreases blood flow

Adrenal Gland Released epinephrine

No effect

NEURON ANATOMY Sympathetic nerves have a short preganglion

and a long postganglion Parasympathetic nerves have a long

preganglion and a short postganglion Sympathetic nerves originate from the

thoracic and lumbar vertebrae Parasympathetic nerves originate from the

cervical and caudal vertebrae

NATURAL AND ARTIFICIAL PAINKILLERS The body produces its own natural painkillers

in response to injury Endorphins and enkephalins are

manufactured in the brain Specialized cells called SG (substantia

gelanosa) cells produce a transmitter chemical that signals that damage or injury has occurred

The endorphins and enkephalins fit into receptor sites on the SG cells, reducing the amount of transmitter that is produced

Opitates such as heroin, morphine and its derivatives have a shape that is similar to the body’s nautral painkillers

Endorphin structure

Morphine structure

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As a result, opiates can also fit into the receptor sites that are usually used by endorphins

However, the use of opiates reduces the body’s production of the natural endorphins

Therefore, after the opiate breaks down, there is little or none of the natural painkiller being produced

This results in a return of pain, often perceived as being greater than the pain associated with the original injury

ACTIVATING YOUR NATURAL PAINKILLERS A number of different stimuli (not

necessarily all extremely painful) will release endorphins and other similar chemicals:

Acupuncture Consumption of capsaicin (the active

ingredient in chili peppers – this is probably why I have hot sauce on everything…)

Strenuous exercise (although the chemical released is actually anandamide – which is related to the THC found in marijuana)

OTHER DRUGS…

Depressants such as Valium and Librium will enhance the action of inhibitory synapses

This increases the production of the inhibitory neurotransmitter, GABA

Alcohol actually changes the neuron membrane, and does not act as a neurotransmitter – it increases the effect of GABA

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