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Topic 6:
Human Health and Physiology
6.5 Nerves, Hormones and
Homeostasis
The Nervous System Bit
The Hormones and Homeostasis Bit
6.5.1 – State that the nervous system consists of the central
nervous system (CNS) and peripheral nerves, and is composed
of cells called neurons that carry rapid electrical impulses.
Nervous system is divided into
1. Central nervous system
• Brain and spinal cord
2. Peripheral nervous system
• The other nerves
• Peripheral nerves are called
neurons
• They transport messages in
the form of electrical impulses
to specific sites
• This is done very quickly by
local depolarization of the cell
membrane of the neuron.
6.5.2 – Draw and label a diagram of the structure of a motor
neuron.
A motor neuron - nerve cell which transmits impulses from the brain to a muscle or gland.
6.5.3 – State that nerve impulses are conducted from receptors
to the CNS by sensory neurons, within the CNS by relay
neurons, and from the CNS to effectors by motor neurons.
Reflex arc – a neural pathway that allows a quick reaction.
Ex–a person puts his or
her finger on a sharp
object (1)
The impulse travels from
the CNS to the muscle by
the motor neuron. (6)
Pain receptor sends out an
impulse (2)
The impulse is carried to the
CNS by a sensory neuron (3)
Inside the spinal cord
the impulse is passed
to a relay neuron (4)
6.5.4 – Define resting potential and action potential
(depolarization and repolarization)
Resting potential
• the electrical potential (measured in
millivolts, mV) across a cell
membrane when not sending an
impulse.
• The resting potential is maintained
by the active transport of Na+
outside the neuron.
Action potential
• the localized reversal
(depolarization) and then
restoration (repolarization) of the
electrical potential (mV)
• measured across the membrane of a
neuron as the impulse is passes
along it.
6.5.5 – Explain how a nerve impulse passes along a nonmyelinated neuron.
6.5.5 – Explain how a nerve impulse passes along a nonmyelinated neuron.
Resting potential
•
•
At rest, there is a potential difference between the outside of an axon membrane
and the inside
-70 mV, this means that the outside of the cell is positive compared to the inside.
• Concentrations of Na+ is higher
outside when K+ concentrations are
higher inside the axon.
• Both have a charge of +1 so this
doesn’t affect the potential
difference
• The Resting Potential is maintained
by:
1. It is the distribution of Cl-.
2. The situation is also maintained
by a selectively permeable
membrane. Na+/K+ pumps (3
Na+ out for every 2 K+ in)
6.5.5 – Explain how a nerve impulse passes along a nonmyelinated neuron.
The Action Potential
• Information travels down a neuron as an action potential
• An action potential is generated by a stimulus of a receptor and from an action potential
of another neuron
6.5.5 – Explain how a nerve impulse passes along a nonmyelinated neuron.
Step 1: Depolarization
• First, the Na+ pores suddenly open
• Because of the high concentration outside the cell, Na+ diffuses into
the cell
• Also the electrical forces will cause sodium to go from positively
charged environment to negatively charged environment
• Influx of positive ions reduces
the potential difference – called
depolarization
• When the potential difference is
above zero, The Na+ is only
driven by diffusion forces (high
to low conc. grad.)
• The inside of the axon is now
more positive than the outside
and the Na+ move into a more
positively charged area
6.5.5 – Explain how a nerve impulse passes along a nonmyelinated neuron.
Step 2: Repolarization
• When the value reaches +40 mV, the Na+ pores close and the K+
pores open.
• K+ moves through the potassium channels out of the axon. The forces
are both diffusion and electrical
• As a result the potential difference will begin to decrease
(repolarization)
• When it falls bellow zero, K+ is
only driven out by diffusion
forces
• K+ channels will shut when the
potential reaches approximately
-70 mV
• The potential is restored but the
Na+ and K+ are in the wrong
place.
• Active transport will restore
them to original positions.
6.5.5 – Explain how a nerve impulse passes along a nonmyelinated neuron.
Step 3: Repolarization
• Repolarization is called the refractory period
• Divided into two states
– Absolute refractory state (1 msec)
– Relative refractory state (up to 10 msec)
• During the absolute refractory
state, no new impulses are
possible
• During the relative refractory
state, the potential is below the
resting potential (-70 mV) and a
stronger stimulus is necessary to
generate an action potential
• The refractory period is followed
by the Na/K pump which returns
the ions to their original sides of
the membrane
6.5.5 – Explain how a nerve impulse passes along a nonmyelinated neuron.
The Threshold Potential
• An action potential is not generated by every impulse
• A threshold needs to be reached
• Depolarization must reach -40 to 50 mV.
• If not, the impulse fades out
• This is called the “all or nothing
response”
6.5.6 Explain the principles of synaptic transmissions
Synapse – the small space between the pre-synaptic motor end plate and
post synaptic membrane. There are electrical and chemical synapses.
Electrical Synapses
• Membranes of two neurons may be
very close together
• The have very small pores called “gap
junctions”
• An impulse can travel from one
membrane to another causing an
action potential (electrically)
• Electrical synapses are faster than
chemical synapses
6.5.6 Explain the principles of synaptic transmissions
Chemical Synapses
At non-electrical synapses, the impulse cannot “jump” from one neuron to the next.
It needs chemicals messengers (called neurotransmitters) to carry the impulse
6.5.6 Explain the principles of synaptic transmissions
Chemical Synapses
How it works…
1. At the synapse, when an action
potential arrives it causes a change
in the membrane permeability for
Ca2+ (1)
2. The result…Ca2+ flows into the
synaptic knob
3. This causes vesicles containing
neurotransmitters to move to the
plasma membrane (2)
4. The vesicles fuse with the
membrane and release the
neurotransmitters (exocytosis) into
the synaptic cleft (3)
6.5.6 Explain the principles of synaptic transmissions
Chemical Synapses
How it works…(continued)
5. The transmitters diffuse across the synaptic
cleft and attach to receptors on the postsynaptic membrane (4)
6. The receptor site changes its configuration
and opens Na+ channels
7. The influx of Na+ initiates an action potential
8. Enzymes in the cleft breakdown the
neurotransmitters and the Na+ channels
close (5)
Synaptic transmissions can also be inhibitory:
• The receptor site changes it configuration
and opens K+ and Cl- channels
• K+ moves out and Cl- moves in, which
increases the distance from the threshold
value (inside becomes more negative)
The Hormones and Homeostasis Bit
6.5.7 State that the endocrine system consists of glands that
release hormones that are transported by the blood
The endocrine system - made up of
endocrine glands that secrete hormones
into the blood
Hormones pass a cell and causes a
reaction/response only if it has a receptor
that the hormone recognizes – these cells
are called “target cells”
6.5.8 State that homeostasis involves maintaining the internal
environment between limits, including blood, pH, CO2
concentration, blood glucose concentration, body temperature
and water balance
Homeostasis – is the maintenance of the internal
environment (within reasonable limits) despite
fluctuations in the external environment
Internal environment – blood and tissue
Non-biological example – regulating the
temperature in your home
Biological examples
• pH – narrow limits (about 7.4) Blood plasma
has buffers to help avoid large fluctuations
• O2 and CO2 concentrations are maintained by
chemoreceptors on the walls of certain blood
vessels
6.5.9 Explain that homeostasis involves monitoring levels of
variables and correcting changes in levels by negative feedback
mechanisms
Negative feedback – the control process by maintaining homeostasis in such a way
that an increase (or decrease) in the “situation” is always reversed.
When body temperature becomes too high, the body will do everything to decrease
your body temperature.
Negative feedback requires sensors to measure the current situation and the sensors
pass the information to a “coordinator” which knows the desired (normal) value
6.5.9 Explain that homeostasis involves monitoring levels of
variables and correcting changes in levels by negative feedback
mechanisms
6.5.9 Explain that homeostasis involves monitoring levels of
variables and correcting changes in levels by negative feedback
mechanisms
6.5.10 Explain the control of body temperature, including the
transfer of heat in blood, and the roles of the hypothalamus,
sweat glands, skin arterioles and shivering.
Remember, one of the things that the blood carries is heat.
The body of mammals and birds have thermoreceptors in the skin and in the heat
center of the hypothalamus of the brain.
This allows the organism to monitor external and internal environmental conditions.
If an organism is too hot, it can
reduce its temperature by:
1. vasodilation
2. sweating
3. decreased metabolism
4. behavior
If an organism is too cold, it can
increase its temperature by:
1. vasoconstriction
2. shivering
3. increased metabolism
4. fluffing of hair
6.5.10 Explain the control of body temperature, including the
transfer of heat in blood, and the roles of the hypothalamus,
sweat glands, skin arterioles and shivering.
I’m Too Hot!
1. Vasodilation
Blood vessels (called arterioles) relax and
become wider(dilate) in areas near the skin.
This is called vasodilation.
Blood flows to the skin and the skin
increases in temperature.
The heat is radiated to the environment
and the body is cooled.
6.5.10 Explain the control of body temperature, including the
transfer of heat in blood, and the roles of the hypothalamus,
sweat glands, skin arterioles and shivering.
2.
•
•
•
Sweating
When water changes phase from liquid to gas it requires energy
This energy is taken from the body and as a result the body is cooled.
Panting has the same effect.
3. Decreased metabolism
• many biological reactions produce heat as a by- product
• By decreasing metabolism you can decrease body temperature
4.
•
•
•
Behavioral adaptations
bathing,
hiding from the sun (returning to burrow)
remove top layer of earth and lie on cool soil
6.5.10 Explain the control of body temperature, including the
transfer of heat in blood, and the roles of the hypothalamus,
sweat glands, skin arterioles and shivering.
I’m Too Cold!
1. Vasoconstriction
Blood vessels (called arterioles) constrict
and become narrower in areas near the
skin. This is called vasoconstriction.
Blood flow does not reach the skin surface
The heat is conserved in the body.
6.5.10 Explain the control of body temperature, including the
transfer of heat in blood, and the roles of the hypothalamus,
sweat glands, skin arterioles and shivering.
2. Shivering
• repeated muscle contractions will produce heat
3. Increased metabolism
• As mentioned before, biological reactions produce heat as a by-product
• The hypothalamus senses a decrease in body temperature and stimulates the
pituitary gland to produce TSH (thyroid stimulating hormone)
• TSH travels in the blood stream to the
thyroid gland and stimulates the
thyroid gland to make TH (thyroid
hormone).
• Nearly every cell in the body is a
target cell for TH. The thyroid
hormone binds to the receptor
causing the cell to increase its
metabolic rate.
• As a result, heat is produced.
6.5.10 Explain the control of body temperature, including the
transfer of heat in blood, and the roles of the hypothalamus,
sweat glands, skin arterioles and shivering.
4. Fluffing of hair or feathers
• increases the thickness of the insulating
layer of air (B)
• Also, the contraction of the hair muscles
will generate heat (A)
5. Behavior
• Reptiles lay in the sun to increase body
temperature
6.5.11 Explain the control of blood glucose concentration,
including the roles of glucagon, insulin and α and β cells in the
pancreatic islets.
Blood glucose levels are monitored by cells
in the pancreas by chemoreceptors
The pancreas is both an exocrine and
endocrine gland
The endocrine cells are clustered together in
groups called the islets of Langerhans.
The islets of Langerhans is where the α and
β cells are located.
Islets of Langerhans
6.5.11 Explain the control of blood glucose concentration,
including the roles of glucagon, insulin and α and β cells in the
pancreatic islets.
The digestion and absorption part
•
Carbohydrates are broken down into
glucose in the month and later the small
intestines by amylase
•
Glucose (and other monosaccharides) are
absorbed are absorbed in the small
intestines and used for cellular respiration.
•
If not completely used, It can be converted
to glycogen and stored in the liver and
muscle cells
6.5.11 Explain the control of blood glucose concentration,
including the roles of glucagon, insulin and α and β cells in the
pancreatic islets.
If blood glucose levels are too high (1.)
• the β cells in the islets of Langerhans
will secrete insulin.
• It is secreted into the blood and
carried to all parts of the body.
• The presence of insulin causes the
muscles to absorb more glucose and
the muscle cells and hepatocytes
convert glucose into glycogen.
• Also, all the cells in your body have
receptors for insulin. When insulin
binds to these cells, it sends a signal to
tell the cell to uptake glucose from the
blood.
• The result…glucose levels decrease.
6.5.11 Explain the control of blood glucose concentration,
including the roles of glucagon, insulin and α and β cells in the
pancreatic islets.
If blood glucose levels are to low (2.)
• the α cells secrete glucagon.
• The target cells for glucagon are the
liver cells (hepatocytes) and muscle
cells.
• Glucagon binds to receptors on the
muscle cells and the hepatocytes.
• The cells respond to the glucagon by
converting the stored glycogen in
the cell to glucose and releasing it
into the blood.
6.5.12 Distinguish between type I and type II diabetes
Type I (Early Onset)
Type II (Late Onset)
Epidemiology
combination of insufficient
amounts of insulin are
produced and the cells are
less sensitive to insulin
because the insulin
receptors are deficient in
target cells
no insulin or insufficient
levels are produced by the β
cells
Cause
an antibody is produced that
attacks the β cells and/or
insulin (Type I diabetes is and
autoimmune disease)
inject insulin daily or
pancreas transplantation
obesity, increase in age and
family history
Treatment
reduce the amount of
carbohydrates ingested,
weight loss, insulin injections
and medication