Types of Neurons Structure of Neurons The Synapse Nerve Impulses Resting Neuron
“All Or Nothing” Law Movement of the Impulse Neural Impulse
Terms The Brain Structure of the
Brain
The Cerebrum The Cerebellum The Medulla Oblongata The Thalamus The Hypothalamus
The Spinal Cord The Reflex Action Web Links
The nervous system allows
the animal to quickly detect, communicate and co-ordinate information about its
external and internal environment so it can make efficient 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 cranial nerves
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 the
nervous system. They carry information or impulses as electrical
signals from one place to another in the body. There are 3 types of
neurons:
Sensory Neurons- Sensory neurons carry electrical
signals (impulses) from receptors or sense organs to the CNS.
Sensory neurons are also called afferent neurons. The cell body of
sensory neurons is outside the CNS in ganglia.
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 associative neurons. They carry information between sensory and motor
neurons. They are found in the CNS.
A
Neuron consists of THREE MAIN PARTS:
A. CELL BODY -
The largest part, contains the nucleus and much of the cytoplasm (area between
the nucleus and the cell membrane), most of the metabolic activity of the cell,
including the generation of ATP (Adenine Triphosphate 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 the ENVIRONMENT or
from other NEURONS AND CARRY THEM TOWARD THE CELL BODY.
C. AXON - A Long
Fibre that CARRIES IMPULSES AWAY FROM THE CELL BODY. Each neuron
has only ONE AXON. The Axon Ends in a series of small swellings called
AXON TERMINALS.
Neurons
may have Dozens or even Hundreds of DENDRITES but usually ONLY ONE AXON.
Sensory Neuron or Afferent Neuron– Moving away from a central organ or point. Relays messages from
receptors to the brain or spinal cord.
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.
Interneurons- Relay message
from sensory neurone to motor neurone. 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 axons |
Short
dendrites and long axons |
|
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) |
The Axons of
most Neurons are covered with a Lipid Layer known as the MYELIN SHEATH. The Myelin Sheath both Insulates and Speeds
Up transmission of Action Potentials through the Axon. In the Peripheral
Nervous System, Myelin is produced by SCHWANN CELLS, which surround the
Axon. GAPS (NODES) in the Myelin Sheath along the length of the Axon are known
as the NODES OF RANVIER. These gaps allow the impulses to travel faster
than if they travelled along the entire length of the neuron.
The Axon ends
with many small swellings called AXON TERMINALS. At these Terminals the neuron may make
contact with the DENDRITES of another neuron, with a RECEPTOR, or with an
EFFECTOR. RECEPTORS are special SENSORY NEURONS in SENSE ORGANS that RECEIVE
Stimuli from the EXTERNAL ENVIRONMENT. EFFECTORS are MUSCLES or GLANDS
that bring about a COORDINATE RESPONSE.
The points of
contact at which impulses are passed from one cell to another are known as THE
SYNAPTIC CLEFT OR SYNAPSE. Neurons that transmit impulses to other neurons
DO NOT actually touch one another. The Small Gap or Space between the
axon of one neuron and the dendrites or cell body on the next neuron is called
the Synapse. One importance of the presence of Synapses is that they
ensure one-way transmission of impulses in a living person. A nerve
impulse CANNOT go backward across a Synapse.
The Axon
Terminals at a Synapse contain tiny vesicles, or sacs called neurotransmitter
swellings. These tiny swellings are filled with CHEMICALS known as NEUROTRANSMITTERS.
Acetylcholine (Ach) and noradrenialin,
also called norepinephrine, are 2 of the main neurotransmitters.
A
NEUROTRANSMITTER is a chemical substance that is used by one neuron to signal
another. Some are made in the cell body while others are made in the
neurotransmitter swellings. The impulse is changed from and Electrical Impulse
to a Chemical Impulse (Electrochemical Impulses). The molecules of the
neurotransmitter diffuse across the gap and attach themselves to SPECIAL
RECEPTORS on the membrane of the neuron receiving the impulse. This now causes
the electrical impulse to be regenerated. After the neurotransmitter relays it
message it is rapidly REMOVED or DESTROYED, thus halting its effect. ENZYMES,
taken up again by the axon terminal and recycled, may break down the molecules
of the neurotransmitter or they may simply diffuse away.
Synapses are
the slowest part of the nervous system. The advantage to having many
neurons, with gaps between them, is that we can control and receive information
from different parts of the body at different times. They also ensure one-way
transmission of impulses in a living person. The number of synapses associated
with each neuron varies from 1000 for a cell body of the spinal cord to up to
10,000 for cell bodies in the brain.
To
Review: The main functions of the synapse are:
1.
To transmit impulses
from one neuron to another neuron or to an effector.
2.
To control the
direction of the impulse. Impulses can only go one way. The neurotransmitter
swellings are only found on the presynaptic side of the synapse. Thus, the
impulse can only travel from the presynaptic side to the postsynaptic side.
3.
To prevent over
stimulation of effectors. Constant stimulation causes neurotransmitter
production to cease. In this way we get used to stimuli such as pain or noise.
4.
Certain chemicals can
block the impulse. This is why doctors prescribe certain drugs for pain relief.
Nerve
impulses are electrical as they run along the nerve. They then become chemical
as the travel over the synaptic cleft.
When
a neuron receives a stimulus of sufficient strength the electrical current
moves along the dendrite and axon to the neurotransmitter swellings. The
movement of ions causes these electrical impulses.
When
a neuron is not carrying an impulse the inside of the axon has a negative
charge and the outside has a positive charge.
The
threshold is the minimum stimulus needed to cause an impulse to be
carried. It must be of sufficient strength. Not all stimuli cause an impulse. A
stimulus below the threshold has no effect on the neuron. Some people have
higher thresholds for pain, heat or other stimuli. This means they can tolerate
a stronger stimulus before their nervous system reacts with an impulse.
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
doesn’t matter how strong the stimulus. The same impulse is sent regardless of
strength. The sensitivity to mild or severe pain depends on the number of
neurons stimulated as well as the frequency of their stimulation.
When
the threshold is reached the axon or dendrite changes. The inside, at the point
of the stimulation, becomes positive and the outside becomes negative. This
creates unlike charges along the length of the neuron and the impulse travels
along the neuron. This is called 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 from left to
right. The AP has not yet reached point 4; the membrane there is still at rest.
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
the peak of the AP; the sodium channels are open and ions are flowing into the
axon. The AP has passed by point 1; the sodium channels are inactivated, and
the membrane is hyperpolarized.
Refractory Period
While the ions are moving in and out of
each region of the neuron, there is a
brief period during which the neuron is unable to have another action potential.
This delay is called the refractory period.
In
Summary:
The
resting potential tells about what happens when a neuron is at rest. An action potential occurs when 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 action potential. The
action potential is an explosion of electrical activity that is created by a depolarising current. This means that some event (a stimulus) causes
the resting 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 the neuron does not reach this critical
threshold level, then no action potential will fire. Also, when the threshold
level is reached, an action potential of a fixed sized will always fire...for
any given neuron, the size of the action potential is always the same. There
are no big or small action potentials in one nerve cell - all action potentials
are the same size. Therefore, the neuron either does not reach the threshold or
a full action potential is fired - this is the "ALL OR NONE"
principle.
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
and carrying it through the axon and on to the next neuron. Let's take a look
at what'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+) and Potassium (K+).
2) selectively permeable membrane - the outer membrane of the neuron is not
impermeable, but instead selectively allows some ions to pass back and forth.
The way it selects is easy - it has pores that are only so big. So, only very
small ions can fit through. Any large ions simply can't pass through the small
pores.
3) charge of the neuron - inside the neuron, the ions are mostly
negatively charged. Outside the neuron, the ions are mostly positively charged.
In this state (with mostly negative charge inside and positive charge on the
outside) the neuron is said to be Polarized.
4) resting potential - while the neuron is Polarized, it is in a
stable, negatively charged, inactive state The charge is approx. -70
millivolts, and it means that the neuron is ready to fire (receive and send
information).
5) stimulus - eventually, some stimulation occurs (ex. hand to
close to a flame), and the information is brought into the body by a sensory
receptor and brought to the dendrites of a neuron.
6) action potential - once the stimulation (the heat) reaches a
certain threshold (come to later) the neural membrane opens at one area and
allows the positively charged ions to rush in and the negative ions to rush
out. The charge inside 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 pumping out the positively charged ions and bringing back the negative ones.
This can occur fast enough to allow up to 1,000 action potentials per second.
8) absolute refractory period - after the action potential occurs, there is a
brief 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. The speed
depends on whether a myelin sheath is present or not. If there is no myelin
sheath then the impulse travels all along the axon or dendrite. This acts to
slow down the impulse. If there is a myelin sheath then the impulse charges can
only move in and out at the nodes of Ranvier. These impulses move more rapidly
than the non-myelinated neurons. Also, the larger the diameter of the axon 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
neural impulse will begin and will go through the complete cycle.
11)
Threshold - a dividing line that
determines if a stimulus is strong enough to warrant action. If the threshold
is reached, an action potential will occur.
The Central Nervous System
After sensory neurons carry impulses most eventually reach the brain.
The brain acts to interpret, sort, and process the incoming impulses and then
decide on a response.
The brain’s grey matter is
composed of cell bodies and synapses. The white matter is made of nerve
fibres (axons and dendrites). There are about 12,000 million neurons that form
the brain.
3 membranes called the meninges protect
the brain and the spinal cord. The space between the 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.
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
h. judgement
i.
personality
The right hemisphere controls the left side of the body while the left
hemisphere controls 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 is
divided 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 more efficiency.
The
inner part of the cerebrum is white matter. It is made of millions of nerve
fibres. These nerve fibres connect different areas of the cerebral cortex as
well as the 2 sides of the brain.
1.
Second largest part
of the brain
2.
Heavily folded
3.
Controls muscular
coordination
4.
Allows for smooth,
refined muscular action
5.
Responses involuntary
once they are learned
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
d.
coughing
e.
salivation
f.
sneezing
g.
vomiting
1.
Located below the
cerebrum
2.
Acts as a sorting
centre for the brain. It relays incoming impulses to the relevant part of the
brain.
1.
Lies below the
thalamus
2.
Regulates the
internal environment (homeostasis) of the body by monitoring:
a.
blood temperature
b.
appetite
c.
thirst
d.
osmoregulation
e.
blood pressure
3.
Regulates the production of many hormones of the
pituitary gland.
The spinal cord is a long, fragile
tubelike structure that begins at the end of the brain stem and continues down
almost to the bottom of the spine (spinal column). The spinal cord consists of
nerves that carry both incoming and outgoing messages between the brain and the
rest of the body. It is also the centre for reflexes, such as the knee jerk
reflex. Like the brain, the spinal cord
is covered 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, the spine is composed of 26 vertebrae, which are
the individual bones of the back. Just as the skull protects the brain, vertebrae
protect the spinal cord. The vertebrae are separated by disks made of
cartilage, which act as cushions, reducing the forces generated by movements
such as walking and jumping.
Like the
brain, the spinal cord consists of grey and white matter. The butterfly-shaped
centre of the cord consists of grey matter. The grey matter contains
dendrites and cell bodies. The front or ventral root contain
motor nerves, which transmit information from the brain or spinal cord to
muscles, stimulating movement. The back or dorsal root contain
sensory nerves, which transmit sensory information from other parts of the body
through the spinal cord to the brain. The surrounding white matter
contains columns of axons that carry sensory information to the brain from the
rest of the body (ascending tracts) 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.
A reflex is the
simplest, quickest form of activity in the nervous system. It is an automatic,
involuntary, unthinking response to a stimulus. The reflex arc are the neurons
that form the pathway of the impulses of a reflex. Examples of reflex actions
are breathing, eye blinking, iris size, and many protective actions such as
moving away from a burning flame. (see below)
When
we move our finger away from a flame we are performing a withdrawl reflex.
These satge 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 dorsal root.
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 the impulse to the brain.
4.
The motor neurons
take the impulse to the effector (muscle) and the finger is pulled away. At the
same time, the impulse reaches the brain and we are aware of the pain.
Another reflex
action is The Knee Jerk Reflex:
The knee jerk reflex is one that you may have had tested
at a check up at the doctor'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 legs crossed so that his leg can swing freely. Hit
his leg just below the knee with the side 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 reflex because there is only one synapse in the
circuit needed to complete the reflex. It only 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 horn of the spinal cord, the information
is sent back out to the muscle...and there you have the reflex.
See a movie about what
happens at the synapse
Take
an Online Quiz on the Neuron
See
Various Animations Of The Synapse
See
another animation of a synapse
See
an animation of The Nervous System
See An
Animation of the Formation of the Nervous System
See An
Animation of the Parts of the Brain
View
an Animation of a Reflex Arc