Chapter 24
NERVES AND HORMONES
Nerves and Hormones
Two systems are involved in internal
communication: nervous system and endocrine
system. The endocrine system consists of glands
that release hormones. The nervous system
consists of nerve cells (neurons).
Afferent nerves take sensory information to the
CNS
Efferent nerves conduct signals from the CNS to
muscles and endocrine system.
Divisions of the Nervous
System
 Central Nervous system (CNS): includes brain
and spinal cord
 Peripheral Nervous system (PNS): every other
nerve.
Roles of the Nervous System
 Collect information from the senses
 Coordinate responses to external stimuli
 Coordinate muscle activity
 Control the secretion of hormones
 Monitor the functioning of other organ
systems
 Regulate the body’s response to internal
stimuli
Motor Neuron Structure
Nerve Impulse
 change in concentrations of sodium and
potassium ions across the membranes of the
neuron.
 Involves:
 Resting potential
 Action potential
 Depolarization
 Repolarization
 Restoration of the resting potential
Resting potential
 Is the potential of a cell that is not
transmitting an impulse. The
resting potential of the inside of a
neuron is about -70 mV, more
negative than the fluid outside
the neuron. This gradient exists
because of:
 Sodium potassium pump actively
transports 3 sodium ions per
every 1 potassium ion pumped in.
 The membrane is more
permeable to K+ than to Na+ ions
 A higher concentration of anions
(-) are found on the inside of the
membrane
Action Potential
depolarization
 Change of the membrane
potential from negative to
positive
 Occurs in the presence of a
stimulus causing the
membrane potential to rise
and become less negative.
 Sodium channels open
when the threshold
potential rises to -50 mV,
allowing sodium ions to
enter.
 Potassium channels remain
closed. Sodium diffuses
raising the membrane
potential to about +30 mV.
Action Potential
repolarization
 Change of the membrane




potential back from positive
to negative.
Occurs immediately after
depolarization.
Potassium channels are also
voltage gated. At +30 mV
potassium channels open
and sodium channels close.
Potassium ions diffuse to the
outside of the membrane,
making the inside negative
again.
Potassium channels remain
open until the inside is back
in resting potential at -70V
Restoration of the Resting
Potential
Right after
repolarization, the
sodium potassium
pump is reactivated to
rebuild concentration
gradients.
This involves the use
of energy in the form
of ATP.
Movement of the action
potential
 An actionpotential in
one part of the neuron
triggers and action
potential in a
neighboring part of the
neuron due to the
diffusion of sodium ions
inside and outside the
neuron.
 When a threshold
potential is reached,
sodium channels open
and depolarization
occurs.
 As the impulse is
passed down a neuron,
repolarization returns.
Synapses
 Junctions between two neurons
 The nerve impulse reaches the axon terminus. As a result Ca2+ ions
diffuse through channels in the membrane.
 The Calcium causes vesicles containing neurotransmitters to fuse
with the pre-synaptic membrane (exocytosis)
 Neurotransmitters are released into the synaptic cleft
Synapses
 Neurotransmitters bind to receptors in the post-synaptic
membrane
 As a result ion channels open and ions diffuse down a
concentration gradient
 An impulse is generated in the post-synaptic neuron
 Neurotransmitters are recycled, degraded or diffused out of
the synapse.
The Endocrine System
 The human body maintains many variables
within defined limits including body
temperature, blood, pH, carbon dioxide
concentration and blood glucose
concentration.
 The nervous system and endocrine system
play a role in homeostasis by maintaining
negative feedback mechanisms (end product
inhibition).
Thermorregulation
 The hypothalamus
detects changes in body
temperature and
produces a response
 Responses to heat:

blood vessels dilate in
the skin
 sweat glands secrete
sweat, keeping the skin
wet,
 shivering does not occur
to prevent overheating
 Responses to cooling:

arterioles in the skin
constrict,
 sweat glands stop
secreting sweat,
 shivering is involuntary
contraction of skeletal
muscles, it generates
heat.
 Blood glucose levels are
Control of glucose
monitored by cells in the
pancreas.
 Low glucose levels:

Alpha cells of islets of
Langerhans detect fall in
glucose concentration and
secrete glucagon into blood;
 the liver breaks down
glycogen to glucose and
converts aminoacids and
glycerol to glucose.
 High glucose levels:

Beta cells of islets of
Langerhans detect rise in
blood glucose and secrete
insulin into the blood;
 body cells take up more
glucose;
 liver cells use more glucose
for respiration and convert it
to glycogen and fat.
Diabetes:
condition where an individual
has consistently elevated levels of sugar in
the blood.
 Type I diabetes:
 early onset diabetes
 inability to produce sufficient
quantities of insulin.
 Type II diabetes:
 adult onset diabetes
 inability to process or
respond to insulin because of
a deficiency of insulin
receptors on target cells.