Pp 173 - 184
Nervous System
Nervous system
consists of two parts:
 Central Nervous System (CNS);
 brain and spinal cord
 relays messages, processes and analyzes
information
 Peripheral Nervous System
(PNS)
 peripheral nerves: cranial and spinal
(outside CNS)
 connects sense organs to CNS
 connects CNS to muscles and glands
Central Nervous System
• The central nervous
system (CNS) consists of
the brain and spinal cord.
• Both structures receive
sensory information from
receptors all over the body
and they interpret the
information, process it and
decide if a response is
required.
• A response by the brain or
spinal cord is known as a
motor response.
Peripheral Nervous System
• The peripheral nervous system
includes all other nerves that
connect to the CNS.
• Peripheral nerves are composed of
sensory neurons and motor
neurons. The peripheral system
has two categories of peripheral
nerves.
• There are 31 pairs of spinal
nerves which emerge directly
from the spinal cord and are a mix
of sensory neurons and motor
neurons. It also has 12 pairs of
cranial nerves which emerge
from the brain stem.
Neurons
• The nervous system is
made up of special nerve
cells called neurons.
• These cells are very
different in shape from
other eukaryotic cells and
they transmit messages in
the form of electrical
impulses at incredible
speed.
• Many neurons grouped
together form a nerve.
3 main types of nerve cells
sensory
neurone
relay
neurone
motor
neurone
Sensory neurons
 Conducts impulses from receptors e.g. pain receptors
in skin to the CNS( brain or spinal cord)
Relay neuron
 Conducts impulses within the CNS, from
sensory neurons to motor neurons.
Motor neuron
 Conducts impulses from CNS to effectors e.g.
muscle to bring about movement or gland to
bring about secretion of hormone
Draw a labelled diagram of a motor
neurone
Parts of a Motor Neuron
• Cell body: Contains the nucleus, rough ER and other
•
•
•
•
•
organelles.
Axon: a very long and thin extension of cytoplasm from the
cell body. Carries impulses away from the cell body.
Dendrites: much smaller cytoplasm extensions which
carry impulses to the cell body.
Myelin Sheath: a type of insulation that is wrapped
around the axons of neurons; composed of Schwann cells.
Helps reduce signal loss.
Schwann Cells: special type of glial cells that produce the
myelin sheath
Nodes of Ranvier: regularly occurring gaps between
sections of myelin sheath where the axon is “naked”.
Reflex
How the impulse is transmitted
 Impulse begins when a neuron
is stimulated by another
neuron or by the environment
 Electrical impulse moves in
one direction:
Dendrites → Cell Body → Axon
 Synapse: gap between 2
neurons
 Neurotransmitters send the
signal to the following neuron
 No myelin = 5-25m/s
 With myelin = 10-120m/s
Resting potential and Action potential
(depolarization and repolarization).
• Resting potential can be defined as the electrical
potential across the plasma membrane of a cell that is
not sending an impulse. The resting potential of all
neurons depends on the ionic gradients that exist
across the plasma membrane of the cell.
• Action potential can be defined as the reversal and
restoration of the electrical potential across the plasma
membrane of a cell as an electrical impulse passes
along it. It is also measured in mV.
Nerve Impulse – Animated Tutorial
Action Potential – Animated Tutorial
Resting Potential
 All cells have an electrical potential difference
(voltage) across their plasma membrane that is
measured in mV and the difference is known as the
membrane potential.
 Neurons typically have a membrane potential between
-60 and -80 mV (millivolts) when the cell is not
transmitting a signal and this is called the resting
potential
• Neurons use active transport to maintain a balance of ions
across their membranes. Sodium ions are pumped out and
potassium ions are pumped in.
• There are chloride ions and other negatively charged ions
inside the neuron that are fairly large and have a tendency to
stay inside which creates a net negative charge inside the
neuron as compared with the more positive charge outside
the neuron.
• This creates the resting potential and the membrane is said
to be polarized.
 NOTE: Sodium ions are highly concentrated outside of the
cell and they have a tendency to diffuse inside the cell.
Potassium ions diffuse out of the cell as sodium diffuses in
but the membrane is about 50 times more permeable to
potassium than sodium so the movement is unequal. The
sodium-potassium pumps use active transport to control
the movement of these ions.
 When an impulse is passing along a neuron the potassium
and sodium ions are allowed to diffuse (passive transport)
across the membrane through proteins known as voltagegated ion channels.
 This reverses the electrical potential of the neuron but it is
quickly restored and this is the action potential. The
neuron is depolarized and then quickly repolarized.
When there is a stimulus...
 Na+ gates open = Na + enter the cell
 Electrical potential of the cell changes
 depolarization (normal charge is reversed) = +30mV
 Action potential is recorded
 Na + channels close
 K + channels open
 repolarization occurs (charges back to normal)
 K + channels stay open longer
 hyperpolarization = -85mV (refractory period = prevents one
impulse to catch up with another)
 Stimulus = self-propagating
http://outreach.mcb.harvard.edu/animations/actionpotential.swf
Depolarization and Repolarization
Depolarization is the diffusion of
sodium ions into the nerve cell
resulting in a charge reversal.
Repolarization is the process of
restoring the original polarity of
the nerve membrane.
How a nerve impulse passes along a non-myelinated
neuron.
• Understanding of how an action potential works is the
key to understanding how a nerve impulse passes
along the axon of a neuron.
• An action potential in one part of a neuron will cause
the development of an action potential in the next
section of the neuron.
• This can occur because sodium ions flow from a region
with an action potential to a region with a resting
potential.
• As the ions move the resting potential is reduced
which results in the opening of the voltage-gated
channels.
• When a neuron is excited the membrane becomes more
permeable to sodium than potassium.
• Scientists believe this occurs because sodium gates open
while potassium gates close. The membrane potential is
reduced and more sodium channels open.
• As sodium ions flow into the neuron via diffusion and charge
attraction the inside of the membrane becomes positive
(charge reversal) and depolarization occurs. The membrane
potential has been reversed.
• Once depolarization occurs the sodium gates are
closed and the potassium channels open. Potassium
diffuses out of the neuron in the direction of the
concentration gradient.
• The loss of the positive potassium ions causes the
internal environment of the neuron to become
negative once again and the potential across the
membrane is restored.
• This is known as repolarization; the return to the
original polarity of the nerve membrane.
• Sodium-potassium pumps are used to restore the
concentration gradients of the ions via active transport.
Sodium is pumped back out of the neuron while potassium is
pumped back in.
• The resting potential of the neuron is now restored and can
now conduct another impulse.
• The action potential moves along the membrane of the
neuron creating a wave of depolarization and repolarization.
As the impulse moves along the axon of the neuron it moves
from a depolarized region and initiates depolarization in the
next region.
A synapse is a junction between two neurons
• At the end of the axons of neurons there are swollen
•
•


membranous areas called terminal buttons.
Inside the terminal buttons are small vesicles filled
with chemicals known as neurotransmitters.
These chemicals are used for synaptic transmission
and they always pass in the same direction; from the
presynaptic neuron to the postsynaptic neuron.
A synapse is a junction between two neurons and the
plasma membranes of those neurons are separated by
a narrow fluid-filled gap known as the synaptic cleft.
Synaptic transmission is the transmission of an action
potential across the synapse from the pre-synaptic
neuron to the postsynaptic neuron.
Steps in Synaptic Transmission
1.
The nerve impulse (action potential) arrives at the
end of the presynaptic neuron.
2. Depolarization of the membrane occurs and the
voltage gated calcium channels open allowing calcium
ions diffuse into the terminal button.
3. The movement of the calcium ions in causes the
vesicles containing the neurotransmitter such as
acetylcholine to fuse with the plasma membrane of the
neuron and release the neurotransmitter into the
synaptic cleft.
4. The neurotransmitter diffuses across the cleft from the
presynaptic neuron and binds to receptors (transmittergated ion channels) on the postsynaptic neuron.
5. The binding of the neurotransmitter results in the
opening of the Na+ ion channels and sodium ions along
with other positively charged ions diffuse into the
postsynaptic neuron.
6. The movement of the ions causes the depolarization
of the postsynaptic membrane and this initiates the
movement of the action potential down the neuron.
7. The neurotransmitter in the cleft is quickly broken
down by enzymes to prevent continuous synaptic
transmission and the calcium ions are pumped out of
the presynaptic neuron and into the synaptic cleft. The
ions channels close to sodium ions.
8. The fragments of the broken down neurotransmitters
diffuse back across the synaptic cleft and into the
vesicles where they can be reassembled.
9. Ca2+ ions are pumped back into the synaptic cleft from
presynaptic neuron
Endocrine System
The endocrine system
 The endocrine system consist of glands in various
locations around the body.
 Each gland secretes one or more hormone into the
bloodstream to be transported to target organ(s) i.e. a
specific organ of the body where hormones initiates a
specific response.
 Hormones are chemical messengers made of proteins
and secreted from the cells of ductless glands.
 Hormones help to control and coordinate body
activities then they are broken down in the liver.
Homeostasis
 maintaining the
internal environment
constant between
narrow limits
 Example of conditions
that need to be kept
constant: blood sugar,
blood pH, oxygen &
carbon dioxide
concentrations, body
temperature and
water balance
 Homeostasis involves
negative feedback
Homeostasis:- monitoring levels of variables and
correcting changes in levels by negative
feedback mechanisms
 change in internal
environment is detected
 response to bring the system
back to set point / within
narrow limits occurs
 when the set point is
reached, the response is
stopped to prevent over
reaction
 internal environment
fluctuates around the set
point (norm)
Negative feedback loop
Receptor
Coordination
Centre
Stimuli
Effector
Response
- ve feedback
Control of body temperature; - (thermoregulation)
 Humans are endothermic (i.e. they are able to
regulate their body temperature) they maintain a
constant body temperature at approximately 37 ˚C.
 Body temperature is regulated by a negative
feedback mechanism
 Thermoreceptors in thermoregulatory centre
(hypothalamus) in brain detect temperature
change
 The message is carried by neurons to various parts
of the body which responds by either increasing or
decreasing the output of heat depending on the
situation.
warming the body actions:
cooling the body actions:
 Shivering/ increased
 vasodilation of skin arterioles
metabolism to produce heat
through respiration
 no release of sweat/ sweating
 behaviours such as increased
motion, huddling, reduction
of exposed body surfaces
 vasoconstriction of skin
arterioles thus less blood flow
near the skin surface to
reduce heat loss leading to
retention of heat
thus more blood flow near
the skin surface increasing
loss of heat through radiation
 Increased sweating/
perspiration accompanied by
evaporative cooling
 reduction of metabolic
activity, relaxation of body
muscles resulting in lower
rate of respiration thus less
heat is produced
 behavioural responses such as
panting, licking of fur
resulting in cooling
Control of blood glucose concentration
 homeostasis maintains the
internal blood glucose
levels between narrow
limits i.e. 70-110 mg
glucose 100cm −3 of blood
 homeostasis involves both
nervous and endocrine
systems
 blood glucose level is
maintained constant by
negative feedback
mechanism
 islets cells in the pancreas
monitor blood glucose
levels
High blood glucose level
 after a carbohydrate meal




blood glucose increases
high blood glucose level
stimulates release of insulin
by β -islet cells in pancreas
insulin causes muscles,
adipose tissue and liver cells
to take up more glucose as
cells become more permeable
to glucose
glucose is converted to
glycogen for storage in muscle
& liver cells
storage of glucose in form of
glycogen lowers blood glucose
levels
low blood glucose level
 if blood glucose levels
drops after starvation,
glucagon is produced by
α -islet cells in pancreas
 glucagon causes liver to
break down glycogen to
glucose
 tissue cells become less
permeable to glucose
 glycogen breakdown into
glucose causes blood
glucose level increase
Diabetes
 In both types of diabetes, there
is:
 a build up of glucose in the



 There are two types of
diabetes:
 type I diabetes
 type II diabetes

blood stream and it will then
subsequently appear in urine
high concentrations of blood
glucose (hyperglycaemia)
results in the movement of
water from body cells by
osmosis into blood
this extra fluid in the blood
results in production of larger
quantities of urine
lack of glucose in cells means
that fats then proteins have to
be metabolised in respiration
breakdown of protein for
energy creates organ damage.
By use of a
table,
distinguish
between
type I and
type II
diabetes.
Type I diabetes
 autoimmune disease i.e.
the immune system attacks
and destroys its own beta (β)
- islet cells so that little or no
insulin is produced
 usually caused by body
producing antibodies against
insulin and (or) β cells in islet
of Langerhans
 it is treated by using insulin
injections to control blood
sugar levels
 usually the age of onset is
around the age of 14, majority
of sufferers are diagnosed
before their twentieth
birthday
Type II diabetes
 insulin is produced but the
body cells (target cells) do
not respond to insulin
(insulin resistance)
 associated with genetic
history, obesity, lack of
exercise, advanced age, and
certain ethnic groups
 it can be controlled by
adopting a low
carbohydrate diet
 usually the age of onset is 40+
years with the majority being
diagnosed between the ages
of 50 and 60
Side Effects of Diabetes
 Regardless of the type of diabetes, if it is not
controlled it can lead to many serious side
effects such as:
 Retina damage leading to blindness
 Kidney failure
 Nerve damage
 Increased risk of cardiovascular disease
 Poor healing of wounds which can result
in gangrene and amputation.