Download Ch 27 Neurones and Neural Pathways

Neurones and Neural
Pathways
Chapter 27
The nervous system consists of
a complex network of nerve
cells called NEURONES. The
diagram shows the three types
of neurone. SENSORY,
ASSOCIATION and MOTOR
Structure of a Neurone
Neurones all share
the same basic
structure. They
consist of a cell
body, axon and
dendrite
DENDRITES
Information from
receptors is
transmitted along
several dendrites
which gather into one
fibre to carry the
information towards
the cell body
CELL BODY
The Cell body of a
neurone contains the
nucleus and most of
the cytoplasm. It is
the control centre of
the cell’s metabolism
and contains
ribosomes. These are
required to make
proteins including
enzymes for
synthesis of
neurotransmitters.
AXONS
An axon is a single nerve
fibre which carries nerve
impulses away from a cell
body and, in the case of
sensory and association
neurones, on to the next
neurone in the sequence.
AXONS
The axons of motor neurones
are extremely long. They can
be more than a metre long if
reaching body parts like the
toes.
Each axon from a motor
neurone carries a
message from the cell
body to an effector
AXON
CELL
BODY
The direction in which a nerve
impulse travels is always:
dendritescell bodyaxon
DENDRITE
Myelin Sheath
The presence of the
myelin sheath
greatly increases
the speed at which
impulses can be
transmitted along
the axon of a
neurone
This is a jacket of fatty material
around a nerve fibre. A nerve
fibre lacking myelin is described
as unmyelinated.
Post-Natal Development of
Myelination
• Myelination, the development of myelin
around axon fibres of individual neurones,
takes time and is not complete at birth but
continues during post–natal development.
Infants
Multiple Sclerosis
Chemical Transmission at a
Synapse
• A synapse is a tiny region of functional contact
between an axon ending of one neurone and the
membrane of the dendrite (or sometimes the cell
body) of the next neurone
• The nerve cell before the synapse is called the
presynaptic neurone; the one after is called the
postsynaptic neurone
• It is at the synapse that information is passed on
by means of a chemical called a
neurotransmitter
A synapse
Presynaptic
neurone
Postsynaptic
neurone
A region of
contact
between a
motor
neurone and
an effector is
called a
neuro-effector
junction
Neurotransmitters
• There are many neurotransmitters passed
on at the synapse. Two examples of the
many neurotransmitters are:
• ACETYLCHOLINE
• NORADRENALINE
The two neurones at a
synapse are separated by a
narrow space called the
synaptic cleft
A synaptic knob is full of
vesicles of one type of
neurotransmitter
When a nerve impulse
passes through the
presynaptic neurone and
reaches the synaptic knob,
several vesicles fuse with
the knob surface
membrane and release
neurotransmitter from the
vesicles into the synaptic
cleft
Since vesicles containing neurotransmitter
occur on one side only of a synapse, this
ensures that nerve impulses are transmitted in
one direction only
The neurotransmitter
molecules briefly combine
with receptor molecules at
sites on the postsynaptic
dendrite and a nerve
impulse is able to be passed
Threshold
Fate of neurotransmitter after
transmission of impulse
• As soon as an impulse has been transmitted the neurotransmitter is
rapidly removed.
• Acetylcholine is broken down into non-active products by an
enzyme present on the postsynaptic membrane, as in the following
equation:
acetylcholinesterase
• Acetylcholine
 Non-active products
The non-active products are then reabsorbed by the presynaptic
neurone and resynthesised into active neurotransmitter stored in
vesicles ready for reuse. Mitochondria present in the presynaptic
knob provide the energy.
Noradrenaline is rebsorbed by the presynaptic membrane and stored
in vesicles ready for reuse.
Frequency of impulses
• The nerve impulses transmitted are equal
in size, however the number of impulses
transmitted per second can vary
depending on the intensity of the original
stimulus
Loud Music Causes More Impulses
to be Sent
It is important to remove
neurotransmitter quickly
between impulses. If it
doesn’t happen then only
a limited number of
impulses can be passed.
We would therefore be
unable to distinguish
between loud and soft
sounds or similarly
between mild and severe
pain
Excitatory and inhibitory signals
In the CNS, one
postsynaptic neurone
normally forms
synapses with many
presynaptic axons from
several different
neurones
At some of these
synapses, the receptor
sites in the postsynaptic
membrane respond to
the arrival of
neurotransmitter( e.g.
acetylcholine) by having
an excitatory effect
which increases the
chances of reaching
threshold and
transmitting a nerve
impulse
Excitatory and Inhibitory Signals
At other synapses the
receptor sites respond to the
neurotransmitter (e.g.
acetylcholine) by having an
inhibitory effect which
reduces the chance of
reaching threshold and
transmitting a nerve impulse
When the sum of the
excitatory effects from the
postsynaptic membrane is
greater than the inhibitory
effects and threshold is
reached, a nerve impulse is
transmitted.
When the inhibitory effects
are in excess, no signal is
fired
Heart rate
decreases as
receptor sites
receive
acetylcholine and
cause inhibition
Peristalsis rate increases as
acetylcholine combines with
receptor sites that cause excitation
Complex Neural Pathways
• Neurones are found to be connected to
one another in many different ways in the
CNS. Two examples of neural pathways
are:
• DIVERGING NEURAL PATHWAY
• CONVERGING NEURAL PATHWAY
Diverging Neural Pathway
To diverge means to
branch out from a common
point. In a diverging neural
pathway, the route along
which an impulse is
travelling divides. This
allows information to be
transmitted to several
destinations
e.g.Temperature Control
Examples of a diverging
neural pathways would be
the hypothalamus having
a neural pathway that
diverges into branches
which lead to sweat
glands, skin arterioles and
skeletal muscles.
Diverging Neural Pathway
Fine Motor Control
Another example of a
diverging neural pathway
would be the cerebrum
transmitting impulses to
different muscles in the
hand. This allows the fine
motor control of the fingers
and thumbs and allows
them to work in unison
Converging Neural Pathway
To converge means
to come together
and meet at a
common point. In a
converging neural
pathway, impulses
from several sources
are channelled
towards one point
Rods and Cones
Rods and cones are visual
receptors present in the retina
of the eye. They contain
pigments which break down in
the presence of light. In each
case, this breakdown forms a
chemical which triggers off
nerve impulses along a pathway
of neurones
The pigment in cones is not very
sensitive to light and needs
bright light to break it down and
trigger nerve impulses
The pigment in rods is so
sensitive to light that dim
light triggers off its
breakdown and sends
impulses. It is inactive in
bright light
Convergence of Signals from Rods
• As the intensity of light entering the eye decreases,
cones cease to respond and rods take over. Unlike
cones several rods form synapses with the next neurone
in the pathway
Several rods form synapses
with the next neurone in the
pathway
The nerve impulse transmitted by
one rod in dim light is weak. It would
mean not enough neurotransmitter
would be released to carry on the
impulse. Several rods are needed to
allow enough neurotransmitter to be
released. A nerve impulse is then
passed through the optic nerve to the
brain
Plasticity of response of the
Nervous System
Investigating the brain’s capacity to
suppress the blinking reflex
Blinking the eye is an example
of a reflex action. A reflex
action is a rapid, automatic
response designed to protect
the body from danger
In this experiment, ten
attempts are made to make
the volunteer blink using
their right eye. A ten second
interval is allowed between
each attempt to allow the
volunteer the opportunity to
summon maximum
willpower. Some can
suppress the blinking but
others cannot resist blinking
no matter how hard they try
Investigating the ability of the brain
to suppress sensory impulses
If a person is given a
task to do that requires a
lot of concentration and
is subjected to auditory
and visual distractions,
some people are good at
suppressing the sensory
impulses from the
distractions and perform
well each time. Other
people find it hard to
block out the sensory
impulses
Plasticity of Response
• Sometimes the brain can be persuaded to
temporarily suppress a reflex action or
block out certain sensory impulses. This
demonstrates Plasticity of Response of
the nervous system.
Plasticity of Response
• Plasticity is thought to occur as you have two
conflicting messages- one saying to blink and
the other not to blink meeting in a convergent
pathway.
• If the overall effect at the synapse is
excitatory then the nerve impulse is fired and
blinking occurs.
• If the overall effect is inhibitory then no
impulse is fired and blinking fails to occur.
• This explains why some people can resist
blinking whilst others can’t help themselves