The nervous system

C E L L S I N T H E N E RVO U S S Y S T E M

THE NERVOUS SYSTEM

NERVOUS SYSTEM

The NERVOUS SYSTEM is the body's information gatherer, storage center and control system. Its overall functions are to collect information about the body's external/internal states and transfer this information to the brain, to analyze this information, and to send impulses out to initiate appropriate motor responses to meet the body's needs.

It consists of the CENTRAL NERVOUS SYSTEM (CNS) , essentially the processing area and the PERIPHERAL NERVOUS SYSTEM which detects and sends electrical impulses that are used in the nervous system.

CELLS IN THE NERVOUS SYSTEM

• Neurons and glial cells• Action potential• Synapse• Gallery

CELLS IN THE NERVOUS SYSTEM

• The human body is made up of trillions of cells. Cells of the nervous system are specialized to carry "messages" through an electrochemical process.

• There are two broad classes of cells in the nervous system: neurons, which process information, and gl ia, which provide the neurons with mechanical and metabolic support.

NEURONS

Neurons The basic building blocks of the nervous system. These specialized cells are the information-processing units of the brain responsible for receiving and transmitting information. Each part of the neuron plays a role in the communication of information throughout the body. They come in many shapes and sizes enabling them to conduct specialized functions such as storing memories or controlling our muscles.

NEURONSN E U R O N S V S . O T H E R C E L L S I N T H E B O DY

NEURONS ARE SIMILAR TO OTHER CELLS IN THE BODY BECAUSE:

NEURONS DIFFER FROM OTHER CELLS IN THE BODY BECAUSE:

• Neurons are surrounded by a cell membrane.• Neurons have a nucleus that contains genes.• Neurons contain cytoplasm, mitochondria and other

organelles.• Neurons carry out basic cellular processes such as

protein synthesis and energy production.

• Neurons have specialized extensions called dendrites and axons. Dendrites bring information to the cell body and axons take information away from the cell body.

• Neurons communicate with each other through an electrochemical process.

• Neurons contain some specialized structures (for example, synapses) and chemicals (for example, neurotransmitters).

NEURONST H R E E G E N E RA L C AT E G O R I E S O F N E U R O N S

Motor Neurons• Efferent Neuron • Moving toward a central organ or point• Relays messages from the brain or spinal cord to the

muscles and organs


Sensory Neurons• Afferent Neuron • Moving away from a central organ or point• Relays messages from receptors to the brain or spinal

cord


In terneurons• Relay Neuron • Relays message from sensory neuron to motor neuron• Make up the brain and spinal cord


SENSORY NEURON INTERNEURON MOTOR NEURON

LENGTH OF FIBERS

Long dendrites and short axon

Short dendrites and short or long

axon

Short dendrites and long axon

LOCATION

Cell body and dendrite are

outside of the spinal cord; the

cell body is located in a dorsal

root ganglion

Entirely within the spinal cord or CNS

Dendrites and the cell body are located in the

spinal cord; the axon is outside of

the spinal cord

FUNCTION Conduct impulse to the spinal cord

Interconnect the sensory neuron with appropriate

motor neuron

Conduct impulse to an effector

(muscle or gland)

NEURONST H E T H R E E D I S T I N C T PA RT S O F T H E N E U R O N

• The ce l l body or soma, contains the nuc leus of the cell and its associated intracellular structures.

• Dendr i tes are specialized extensions of the cell body. They function to obtain information from other cells and carry that information to the cell body, conduct incoming s igna ls .

• The axon or nerve fi ber , varying in length from a millimeter to a meter, which conduct outgo ing s igna ls emitted by the neuron. Axons are encased in a fat-like sheath, called myel in , which acts like an insulator and, along with the Nodes of Ranv ier , speeds impulse transmission. Dendrites and axons, both extensions of the cell body, are also referred to as processes .


A neuron has a cell body with extensions leading off it. Numerous dendrons and dendr i tes provide a large surface area for connecting with other neurons, and carry nerve impulses towards the cell body. A single long axon carries the nerve impulse away from the cell body. Most neurons have many companion cells called Schwann ce l ls , which wrap their cell membrane around the axon many times in a spiral to form a thick insulating lipid layer called the myel in sheath . Nerve impulse can be passed from the axon of one neuron to the dendron of another at a synapse. A nerve is a discrete bundle of several thousand neuron axons.



NEURONSC E L L B O DY O R S O M A

• Also called the soma , the cell body contains a spherical nucleus with a nucleolus and lots of cytoplasm.

• Like many cells, the neuron cell body of the neuron contains the usual cellular particles or o rgane l les , except cent r io les .


• Centrioles are the basis by which cells are able to

divide and form new cells. Because the neurons lack

centrioles , they are unable to divide and reproduce

themselves. Nevertheless, neurons do have

specialized hardworking endoplasmic ret icu lum

(ER) , which help transport proteins and molecules at

high speeds due to the fact that neurons work at

lightning speeds. Also, the neurofi br i ls , bundles of

micro filaments and micro tubules, which are

important in intracellular transport, are seen through

the body. A pigment called l ipofusc in , which is

yellow-brown, is one of the many pigments believed

to be in the neuron.


• The cell body integrates synapt ic input and determines the message to be transmitted to other cells by the axon

• The cell body also is responsible for a variety of complex b iochemica l processes

• The highly active proteins that serve as chemical messengers between cells are manufactured and packaged in the cell body

NEURONSD E N D R I T E S

• Dendrites are short, thick branched extensions which extend like the roots of a tree over other neurons or body cells. The dendrites all branch off dendr i t i c sp ines , which in turn branch of the cell body.

• Dendrites are the recept ive s i tes of the neurons. Here, the neurons receive electric messages from other neurons or body cells. The site where one dendrite meets another neuron's impulse is called the synapse. Usually, neurons have hundreds of dendrite extensions. These extensions are spread over a large area, giving the neuron better reception of signals. Some dendrites are specialized for the accumulation of information. These cells are finer than other dendrites and found near the brain.

NEURONSA XO N

• Axon Is the elongated fiber that extends from the cell body to the terminal endings and transmits the neural signal. The larger the axon, the faster it transmits information. Some axons are covered with a fatty substance called myelin that acts as an insulator. These myelinated axons transmit information much faster than other neurons.

• Axon H i l lockLocated at the end of the soma and controls the firing of the neuron. If the total strength of the signal exceeds the threshold limit of the axon hillock, the structure will fire a signal (known as an action potential) down the axon.

NEURONSA XO N

• Termina l Buttons Located at the end of the neuron and are responsible for sending the signal on to other neurons. At the end of the terminal button is a gap known as a synapse. Neurotransmitters are used to carry the signal across the synapse to other neurons.

Between the button and the dendrite of the next neuron is a gap called the synapse (or synaptic gap, or synaptic cleft),. For every neuron, there are between 1000 and 10,000 synapses.

NEURONSD E N D R I T E S V S . A XO N

AXONS DENDRITESTake information away

from the cell bodyBring information to the

cell body

Smooth Surface Rough Surface (dendritic spines)

Generally only 1 axon per cell

Usually many dendrites per cell

No ribosomes Have ribosomes

Can have myelin No myelin insulation

Branch further from the cell body

Branch near the cell body

GLIA AND OTHER SUPPORTING CELLS

• The focus of attention in studying the biological basis of behavior is on neurons and their activities, but neurons are not the only cells in the central nervous system. They are supported by g l ia ce l ls , which appear to perform a variety of housekeeping functions in the brain

• The term glia means "g lue," a reflection of the fact that glial cells really do hold the brain together, occupying the space between neurons. Glia are usually very small cells, but there are a great many of them. They outnumber neurons by a factor of between 10 and 50

GLIA AND OTHER SUPPORTING CELLST W O T Y P E S O F G L I A L C E L L S

• There are two types of glial cells in the nervous system: the large-bodied macrog l ia and the smaller microg l ia .

• There are two classes of macroglia in the central nervous system: astrocytes and o l igodendrocytes .

Astrocytes • A numerous type of glia named for their star-

shaped appearance when Golgi-stained. When examined at greater magnification, these small cells show a characteristics lack of organel les within their cell bodies.

• Astrocytes provide structural support for the neurons of the brains and aid in the repair of neurons following damage to the brain. They also regulated the flow of ions and larger molecules in the region of the synapses.

GLIA AND OTHER SUPPORTING CELLST W O C L A S S E S O F M A C R O G L I A

Ol igodendrocytes

• Their cell bodies contains a large number of organel les . They also contain many microtubules that are arranged in parallel arrays.

• The o l igodendrocytes produce myel in , which surrounds the axons of many neurons. This insulating coating is called a myel in sheath .

GLIA AND OTHER SUPPORTING CELLSS C H WA N N C E L L

Outside the central nervous system. along the peripheral nerves that connect the brain and spinal cord with the muscles, glands, and sensory organs of the body, there is another type of supporting cell that is similar in many ways to the oligodendrocytes, this is the Schwann ce l l .

In the developing nervous system, the Schwann cell first encircles an axon, then wraps itself around the neuron, building a myelin sheath. As it moves, the cytoplasm is pushed forward, leaving only the membrane of the Schwann cell wrapped around the once-naked axon. Myel inat ion greatly increases the speed with which action potentials are carried along an axon.

GLIA AND OTHER SUPPORTING CELLSM I C R O G L I A

In contrast, the microglia perform "housekeeping" functions within the central nervous system. Among their duties is the removal of dead cells within the brain.

ast rocy tes o l igodendrocy tes

GLIA AND OTHER SUPPORTING CELLSA C T I O N P O T E N T I A L

• Neurons send messages electrochemically. This means that chemicals cause an electrical signal. Chemicals in the body are "e lectr ica l ly -charged" -- when they have an electrical charge, they are called ions .

• The important ions in the nervous system are sod ium and potass ium (both have 1 positive charge, +), ca lc ium (has 2 positive charges, ++) and ch lor ide (has a negative charge, -).

• It is also important to remember that nerve cells are surrounded by a membrane that allows some ions to pass through and blocks the passage of other ions. This type of membrane is called semi-permeable .


• When a neuron is not sending a signal, it is "at rest ." When a neuron is at rest, the inside of the neuron is negat ive re lat ive to the outside.

• At rest, potassium ions (K+) can cross through the membrane easily. Also at rest, chloride ions (Cl-)and sodium ions (Na+) have a more difficult time crossing. The negatively charged protein molecules (A-) inside the neuron cannot cross the membrane.


• In addition to these selective ion channels, there is a pump that uses energy to move three sod ium ions out of the neuron for every two potass ium ions i t puts in . Finally, when all these forces balance out, and the difference in the voltage between the inside and outside of the neuron is measured, you have the rest ing potent ia l .

• The resting membrane potential of a neuron is about -70 mV (mV=mi l l ivo l t )

• At rest, there are relatively more sodium ions outside the neuron and more potassium ions inside that neuron.


• The rest ing potent ia l tells about what happens when a neuron is at rest. An act ion potent ia l occurs when a neuron sends information down an axon, away from the cell body.

• The action potential is an explosion of electrical activity that is created by a depolar iz ing current . This means that some event (a st imulus) causes the resting potential to move toward 0 mV. When the depolarization reaches about -55 mV a neuron will fire an action potential. This is the threshold .


• The rest ing potent ia l tells about what happens when a neuron is at rest. An act ion potent ia l occurs when a neuron sends information down an axon, away from the cell body.

• The action potential is an explosion of electrical activity that is created by a depolar iz ing current . This means that some event (a st imulus) causes the resting potential to move toward 0 mV. When the depolarization reaches about -55 mV a neuron will fire an action potential. This is the threshold .


• Action potentials are caused by an exchange of ions across the neuron membrane. A st imulus first causes sodium channels to open.

• Because there are many more sodium ions on the outside, and the inside of the neuron is negative relative to the outside, sodium ions rush into the neuron.

• When potassium channels open, potassium rushes out of the cell, reversing the depolar izat ion . Also at about this time, sodium channels start to close. This causes the action potential to go back toward -70 mV (a repolar izat ion).

• The action potential actually goes past -70 mV (a hyperpo lar izat ion)

• Gradually, the ion concentrations go back to resting levels and the cell returns to -70 mV.


SYNAPSE

Information from one neuron flows to another neuron across a synapse. The synapse contains a small gap separating neurons.

The synapse consists of:• a presynaptic ending that contains neurotransmitters, mitochondria and other cell organelles• a postsynaptic ending that contains receptor sites for neurotransmitters• a synaptic cleft or space between the presynaptic and postsynaptic endings.

SYNAPSE

Electr ica l Tr igger for Neurotransmiss ion

For communication between neurons to occur, an electrical impulse must travel down an axon to the synaptic terminal.

Neurotransmitter Mobi l i zat ion and ReleaseAt the synaptic terminal (the presynaptic ending), an electrical impulse will trigger the migration of vesicles containing neurotransmitters toward the presynaptic membrane. The vesicle membrane will fuse with the presynaptic membrane releasing the neurotransmitters into the synaptic cleft.

SYNAPSE

Diff usion of Neurotransmit ters Across the Synapt ic C lef t

The neurotransmitter molecules then diffuse across the synaptic cleft where they can bind with receptor sites on the postsynaptic ending to influence the electrical response in the postsynaptic neuron.

When a neurotransmitter binds to a receptor on the postsynaptic side of the synapse, it changes the postsynaptic cell's excitability: it makes the postsynaptic cell either more or less likely to fire an action potential. If the number of excitatory postsynaptic events is large enough, they will add to cause an action potential in the postsynaptic cell and a continuation of the "message.“

SYNAPSET Y P E S O F S Y N A P S E S

CREDITS

http://library.thinkquest.org/2935/Natures_Best/Nat_Best_Low_Level/Nervous_page.L.html

http://www.biology-online.org/8/1_nervous_system.htm

http://webschoolsolutions.com/patts/systems/nervous.htm

http://www.innerbody.com/image/nerv06.html

http://learn.genetics.utah.edu/content/addiction/reward/

http://faculty.washington.edu/chudler/ap.html

http://www.albany.edu/faculty/cafrye/apsy601/Ch.02cellsofthenervoussystem.html

http://www.biologymad.com/nervoussystem/nervoussystemintro.htm

http://psychology.about.com/od/biopsychology/ss/neuronanat.htm

http://webspace.ship.edu/cgboer/actionpot.html

http://webspace.ship.edu/cgboer/theneuron.html

http://www.mind.ilstu.edu/curriculum/neurons_intro/neurons_intro.php

























C E L L S I N T H E N E RVO U S S Y S T E M

NERVOUS SYSTEM

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The nervous system