Neuron System

8/8/2019 Neuron System

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The nervous system allows the animal to quickly detect, communicate and co-ordinate information about its external and internal environment so it can makeefficient appropriate responses for survival and/or reproduction.

The two major parts of our nervous system are the central nervous system(CNS) and peripheral nervous system (PNS).

The CNS is made of the brain and spinal cord.

The cranial nerves, spinal nerves and ganglia make up the PNS. The cranialnerves connect to the brain. The cranial and spinal nerves contain the axons(fibres) of sensory and motor nerve cells.

Nerve cells areas are also known as neurons. Neurons are the basic unit of thenervous system. They carry information or impulses as electrical signals fromone place to another in the body. There are 3 types of neurons:

Sensory Neurons - Sensory neurons carry electrical signals (impulses) fromreceptors or sense organs to the CNS. Sensory neurons are also calledafferent neurons. The cell body of sensory neurons is outside the CNS inganglia .

Motor Neurons - Motor neurons carry

impulses from the CNS to effector organs

Motor neurons are also called efferent neurons.

The cell bodies of motor neurons are inside the CNS.

Interneurons - These are also called intermediate , relay , or associativeneurons. They carry information between sensory and motor neurons. They arefound in the CNS.


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The Structure of Neurons

A Neuron consists of THREE MAIN PARTS:

A. CELL BODY - The largest part, contains the nucleus and much of thecytoplasm (area between the nucleus and the cell membrane), most of themetabolic activity of the cell, including the generation of ATP (AdenineTriphosphate Compound that Stores Energy) and synthesis of protein.

B. DENDRITES - Short branch extensions spreading out from the cell body.Dendrites Receive STIMULUS (Action Potentials) and carry IMPULSES from theENVIRONMENT or from other NEURONS AND CARRY THEM TOWARD THECELL BODY.

C. AXON - A Long Fibre that CARRIES IMPULSES AWAY FROM THE CELLBODY . Each neuron has only ONE AXON. The Axon Ends in a series of smallswellings called AXON TERMINALS.

Neurons may have Dozens or even Hundreds of DENDRITES but usually ONLYONE AXON.
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Sensory Neuron or Afferent Neuron Moving away from a central

organ or point. Relays messages from receptors to the brain or spinalcord.

Motor Neuron or Efferent Neuron Moving toward a central organ or point.Relays messages from the brain or spinal cord to the muscles and organs.


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Interneurons - Relay message from sensory neurone to motor neurone. Makeup the brain and spinal cord.


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Sensory neuron Interneuron Motor Neuron

Length of Fibers Long dendrites and shortaxon

Short dendrites and short or long axons

Short dendrites and longaxons

Location Cell body and dendrite areoutside of the spinal cord; thecell body is located in adorsal root ganglion

Entirely within the spinal cordor CNS Dendrites and the cell bodyare located in the spinal cord;the axon is outside of thespinal cord

Function Conduct impulse to the spinalcord

Interconnect the sensoryneuron with appropriate motor neuron

Conduct impulse to an effector (muscle or gland)

The Axons of most Neurons are covered with a Lipid Layer known as theMYELIN SHEATH. The Myelin Sheath both Insulates and Speeds Up transmission of Action Potentials through the Axon. In the Peripheral Nervous System, Myelin is produced bySCHWANN CELLS , which surround the Axon. GAPS (NODES) in the Myelin Sheath alongthe length of the Axon are known as the NODES OF RANVIER . These gaps allow theimpulses to travel faster than if they travelled along the entire length of the neuron.


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The Axon ends with many small swellings called AXON TERMINALS . At theseTerminals the neuron may make contact with the DENDRITES of another neuron, with aRECEPTOR, or with an EFFECTOR. RECEPTORS are special SENSORY NEURONS inSENSE ORGANS that RECEIVE Stimuli from the EXTERNAL ENVIRONMENT.

EFFECTORS are MUSCLES or GLANDS that bring about a COORDINATE RESPONSE.

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The Synapse

The points of contact at which impulses are passed from one cell to another areknown as THE SYNAPTIC CLEFT OR SYNAPSE. Neurons that transmitimpulses to other neurons DO NOT actually touch one another. The Small Gap orSpace between the axon of one neuron and the dendrites or cell body on thenext neuron is called the Synapse. One importance of the presence of Synapsesis that they ensure one-way transmission of impulses in a living person. A nerveimpulse CANNOT go backward across a Synapse.

The Axon Terminals at a Synapse contain tiny vesicles, or sacs calledneurotransmitter swellings . These tiny swellings are filled with CHEMICALSknown as NEUROTRANSMITTERS . Acetylcholine (Ach) and noradrenialin, alsocalled norepinephrine, are 2 of the main neurotransmitters.


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A NEUROTRANSMITTER is a chemical substance that is used by one neuron tosignal another. Some are made in the cell body while others are made in theneurotransmitter swellings. The impulse is changed from and Electrical Impulseto a Chemical Impulse (Electrochemical Impulses). The molecules of theneurotransmitter diffuse across the gap and attach themselves to SPECIALRECEPTORS on the membrane of the neuron receiving the impulse. This nowcauses the electrical impulse to be regenerated. After the neurotransmitterrelays it message it is rapidly REMOVED or DESTROYED, thus halting its effect.ENZYMES, taken up again by the axon terminal and recycled, may break downthe molecules of the neurotransmitter or they may simply diffuse away.

Synapses are the slowest part of the nervous system. The advantage to havingmany neurons, with gaps between them, is that we can control and receiveinformation from different parts of the body at different times. They also ensureone-way transmission of impulses in a living person. The number of synapsesassociated with each neuron varies from 1000 for a cell body of the spinal cordto up to 10,000 for cell bodies in the brain.


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Resting NeuronWhen a neuron is not carrying an impulse the inside of the axon has a negative charge andthe outside has a positive charge.

The threshold is the minimum stimulus needed to cause an impulse to be carried. It must beof sufficient strength. Not all stimuli cause an impulse. A stimulus below the threshold has noeffect on the neuron. Some people have higher thresholds for pain, heat or other stimuli. Thismeans they can tolerate a stronger stimulus before their nervous system reacts with animpulse.

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All Or Nothing Law

The All or Nothing Law states that if the threshold is reached an impulse is carried, but if the threshold is not reached then there will be no impulse. It doesnt matter how strong thestimulus. The same impulse is sent regardless of strength. The sensitivity to mild or severepain depends on the number of neurons stimulated as well as the frequency of their stimulation.

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Movement of the Impulse

When the threshold is reached the axon or dendrite changes. The inside, at the point of thestimulation, becomes positive and the outside becomes negative. This creates unlikecharges along the length of the neuron and the impulse travels along the neuron. This iscalled the action potential. Once the impulse moves along, the area behind the impulse is

changed back to its normal negative (resting) state.

Below is a cross-section of an axon, with an action potential (AP) moving fromleft to right. The AP has not yet reached point 4; the membrane there is still atrest. At point 3, positive sodium ions are moving in from the adjacent region,depolarising the region; the sodium channels are about to open. Point 2 is at thepeak of the AP; the sodium channels are open and ions are flowing into the
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axon. The AP has passed by point 1; the sodium channels are inactivated, andthe membrane is hyperpolarized.

Refractory Period

While the ions are moving in and out of each region of the neuron, there is a brief periodduring which the neuron is unable to have another action potential. This delay iscalled the refractory period .

In Summary:

The resting potential

tells about what happens when a neuron is at rest. An action potential occurswhen a neuron sends information down an axon, away from the cell body.Neuroscientists use other words, such as a "spike" or an "impulse" for the actionpotential. The action potential is an explosion of electrical activity that is createdby a depolarising current . This means that some event (a stimulus) causes theresting potential to move toward 0 mV. When the depolarisation reaches about

-55 mV a neuron will fire an action potential. This is the threshold . If theneuron does not reach this critical threshold level, then no action potential willfire. Also, when the threshold level is reached, an action potential of a fixed sizedwill always fire...for any given neuron, the size of the action potential is alwaysthe same. There are no big or small action potentials in one nerve cell - all actionpotentials are the same size. Therefore, the neuron either does not reach thethreshold or a full action potential is fired - this is the "ALL OR NONE" principle.


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Neural Impulse Terms

A. Neural impulse - takes the same path all the time - it is a process of conducting information from a stimulus by the dendrite of one neuron andcarrying it through the axon and on to the next neuron. Let's take a look atwhat's involved in the neural impulse:

1) ions - we have positively (+) and negatively (-) charged particles called ions.For the neural impulse, however, we are only concerned with Sodium (Na+) andPotassium (K+).

2) selectively permeable membrane - the outer membrane of the neuron isnot impermeable, but instead selectively allows some ions to pass back andforth. The way it selects is easy - it has pores that are only so big. So, only verysmall ions can fit through. Any large ions simply can't pass through the smallpores.

3) charge of the neuron - inside the neuron, the ions are mostly negativelycharged. Outside the neuron, the ions are mostly positively charged. In this state(with mostly negative charge inside and positive charge on the outside) theneuron is said to be Polarized.

4) resting potential - while the neuron is Polarized, it is in a stable, negativelycharged, inactive state The charge is approx. -70 millivolts, and it means thatthe neuron is ready to fire (receive and send information).

5) stimulus - eventually, some stimulation occurs (ex. hand to close to aflame), and the information is brought into the body by a sensory receptor andbrought to the dendrites of a neuron.

6) action potential - once the stimulation (the heat) reaches a certainthreshold (come to later) the neural membrane opens at one area and allows thepositively charged ions to rush in and the negative ions to rush out. The chargeinside the neuron then rises to approx. +40 mv. This only occurs for a brief moment, but it is enough to create a domino effect.

7) repolarization - the neuron tries to quickly restore its charge by pumpingout the positively charged ions and bringing back the negative ones. This can


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occur fast enough to allow up to 1,000 action potentials per second.

8) absolute refractory period - after the action potential occurs, there is abrief period during which the neuron is unable to have another action potential.Then the charge inside the neuron drops to about -90 mv (refractory period)

before restoring itself to normal.9) speed of an action potential - can travel from 10-120 meters/sec. Thespeed depends on whether a myelin sheath is present or not. If there is nomyelin sheath then the impulse travels all along the axon or dendrite. This actsto slow down the impulse. If there is a myelin sheath then the impulse chargescan only move in and out at the nodes of Ranvier. These impulses move morerapidly than the non-myelinated neurons. Also, the larger the diameter of theaxon or dendrite the faster the impulse.

10) all-or-none law - a neural impulse will either occur or not. There is no in

between. Once the threshold is reached, there is no going back, the neuralimpulse will begin and will go through the complete cycle.

11) Threshold - a dividing line that determines if a stimulus is strong enough towarrant action. If the threshold is reached, an action potential will occur.

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The Central Nervous System

The Brain

After sensory neurons carry impulses most eventually reach the brain. The brainacts to interpret, sort, and process the incoming impulses and then decide on aresponse.

The brains grey matter is composed of cell bodies and synapses. The white matter ismade of nerve fibres (axons and dendrites). There are about 12,000 million neurons thatform the brain.

3 membranes called the meninges protect the brain and the spinal cord. The space betweenthe inner 2 membranes is filled with a liquid called cerebrospinal fluid. There is a total of about 100 mL of this liquid in the CNS. It protects the CNS by acting as a shock absorber.

Inflammation of the meninges causes a sometimes-serious condition called meningitis.Refer to your text for a description of viral and bacterial meningitis.


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Structure of the Brain

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The Cerebrum

1. Largest part of the brain

2. Contains about 75% of the total neurons of the brain

3. Divided into 2 halves: The right and left cerebral hemispheres

4. Control:

a. voluntary movements

b. receiving and interpreting impulses from sense organs

c. thinking

d. intelligence

e. memory

f. language

g. emotions
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h. judgement

i. personality

The right hemisphere controls the left side of the body while the left hemispherecontrols the right side of the body.

Each hemisphere is specialised for different functions.

Generally

The left side is dominant for: The right side is dominant for:

1. hand use 1. art

2. language 2. music

3. mathematics 3. shape recognition

4. analysis 4. emotional responses

5. logic

The outer part of the cerebrum is grey and called the cerebral cortex. It isdivided into 4 lobes. Each lobe controls specific functions:

Notice that there are many infolds of the cerebral cortex. This gives it a larger surface area.This allows for more interconnections between different parts of the brain and for moreefficiency.

The inner part of the cerebrum is white matter. It is made of millions of nerve fibres. Thesenerve fibres connect different areas of the cerebral cortex as well as the 2 sides of the brain.


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The Cerebellum

1. Second largest part of the brain

2. Heavily folded

3.

Controls muscular coordination4. Allows for smooth, refined muscular action

5. Responses involuntary once they are learned

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The Medulla Oblongata

1. Connects the brain with the spinal cord

2. Contains clusters of nerve cells that control involuntary actions such as:

a. breathing

b. blood pressure

c. swallowing


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d. coughing

e. salivation

f. sneezing

g. vomiting

The Thalamus

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1. Located below the cerebrum

2. Acts as a sorting centre for the brain. It relays incoming impulses to therelevant part of the brain.

The Hypothalamus

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1. Lies below the thalamus

2. Regulates the internal environment ( homeostasis ) of the body bymonitoring:


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a. blood temperature

b. appetite

c. thirst

d. osmoregulation

e. blood pressure

3. Regulates the production of many hormones of the pituitary gland.

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The Spinal Cord


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The spinal cord is a long, fragile tubelike structure that begins at the end of thebrain stem and continues down almost to the bottom of the spine (spinalcolumn). The spinal cord consists of nerves that carry both incoming andoutgoing messages between the brain and the rest of the body. It is also thecentre for reflexes, such as the knee jerk reflex. Like the brain, the spinal cord iscovered by three layers of tissue called meninges . The spinal cord and meninges are

contained in the spinal canal, which runs through the centre of the spine. In most adults, thespine is composed of 26 vertebrae, which are the individual bones of the back. Just as theskull protects the brain, vertebrae protect the spinal cord. The vertebrae are separated bydisks made of cartilage, which act as cushions, reducing the forces generated by movementssuch as walking and jumping.

Like the brain, the spinal cord consists of grey and white matter. Thebutterfly-shaped centre of the cord consists of grey matter . The greymatter contains dendrites and cell bodies. The front or ventral rootcontain motor nerves, which transmit information from the brain orspinal cord to muscles, stimulating movement. The back or dorsalroot contain sensory nerves, which transmit sensory information fromother parts of the body through the spinal cord to the brain. Thesurrounding white matter contains columns of axons that carrysensory information to the brain from the rest of the body (ascendingtracts) and columns that carry impulses from the brain to the muscles(descending tracts). There are a total of 31 pairs of spinal nerves.These carry impulses to and from the spinal cord.


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Reflex Action

A reflex is the simplest, quickest form of activity in the nervous system. It is anautomatic, involuntary, unthinking response to a stimulus. The reflex arc are theneurons that form the pathway of the impulses of a reflex. Examples of reflexactions are breathing, eye blinking, iris size, and many protective actions such asmoving away from a burning flame. (see below)

When we move our finger away from a flame we are performing a withdrawl reflex . Thesesatge of this reflex are as follows:

1. The finger is the receptor . It contains sensory neurons .

2. Sensory neurons carry the impulse to the sensory nerves in the dorsalroot .

3. An interneuron carries the impulse across the spinal cord to the motor

neurons in the ventral root . At the same time, another neuron takes theimpulse to the brain.

4. The motor neurons take the impulse to the effector (muscle) and the finger ispulled away. At the same time, the impulse reaches the brain and we are awareof the pain.

Another reflex action is The Knee Jerk Reflex:


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The knee jerk reflex is one that you may have had tested at a check up at thedoctor's office. In this test, the doctor hits your knee at a spot just below your

kneecap and your leg kicks out. Try it! Have a partner sit with his or her legscrossed so that his leg can swing freely. Hit his leg just below the knee with theside of your hand. DO NOT USE A HAMMER!!!! The leg will kick out immediately(if you hit the right place). The knee jerk reflex is called a monosynaptic reflexbecause there is only one synapse in the circuit needed to complete the reflex. Itonly takes about 50 milliseconds between the tap and the start of the leg kick.That is fast! The tap below the knee causes the thigh muscle to stretch.Information is then sent to the spinal cord. After one synapse in the ventral hornof the spinal cord, the information is sent back out to the muscle...and there youhave the reflex.

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