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• Eventually, as an action potential passes down
the axon, it reaches the end of the axon and all of
its branches
• These branches may form junctions with the
dendrites of other neurons, muscle cells, or gland
cells; such intercellular junctions are called
• The neuron whose axon transmits the action
potential to the synapse is called the presynaptic
cell, while the receiving cell is called the
postsynaptic cell
• The synapse is the gap between the synaptic
terminals of an axon and its target cell
• There are two basic types of synapses:
– Electrical synapses – involve direct cytoplasmic
connections between the two cells formed by gap
junctions; the gap junctions allow ion currents to
continue; relatively rare in vertebrates
– Chemical synapses – electrical impulses must be
converted to a chemical signal that crosses the
synapse; more common in vertebrates
• Chemical synapses have a synaptic cleft – a
narrow space that separates the two cells
• The end of the
presynaptic cell
contains synaptic
vesicles which are
packed with
Chemical synapses
• When an action potential arrives at the end of an
axon, it stimulates the opening of voltage-gated
calcium (Ca+2) channels; causes a rapid influx of
Ca+ into the cell
• Rapid influx of Ca+2 causes the synaptic vesicles to
fuse with the plasma membrane, and release
neurotransmitters by exocytosis
• Release neurotransmitters diffuse to other side of
cleft and binds to receptor proteins in the
membrane of the postsynaptic cell (produces
graded potentials in postsynaptic membrane)
Chemical synapses/Release of
• Neurotransmitters are chemical signals in an
otherwise electrical system
• Must be rapidly removed from the synaptic
cleft to allow new signals to be transmitted;
accomplished by:
– Enzymatic digestion
– Re-uptake of neurotransmitter molecules by the
– Uptake by glial cells
• No one chemical property defines a
• Acetylcholine (Ach) is the neurotransmitter
that crosses the synapse between a motor
neuron and a muscle fiber
– This type of synapse is known as a neuromuscular
– Ach binds to its receptor proteins in the
postsynaptic membrane, causing ligand-gated ion
channels to open
Neuromuscular junction
Release of neurotransmitter
• Binding of Ach to receptor produces a
depolarization called an excitatory
postsynaptic potential (EPSP) which can open
the voltage-gated ion channels for Na+ and K+
that are responsible for action potentials
• Produces a muscle contraction (the
postsynaptic cell is a skeletal muscle fiber)
• For the muscle to relax, Ach must be
eliminated; an enzyme cleaves Ach into
inactive fragments, causes muscle relaxion
• Glycine and GABA are inhibitory
• These neurotransmitters cause the opening of
ligand-gated channels for Cl-, diffuses into
neuron, makes the inside of the membrane
more negative than it is at rest
– Hyperpolarization; called an inhibitory
postsynaptic potential (IPSP)
Excitatory (a) & Inhibatory (b)
More Neurotransmitters…
• Epinepherine (aka adrenaline) – responsible for
“fight or flight” response
– Faster and stronger heartbeat; diversion of blood to
muscles and heart; increased blood glucose
• Dopamine – used in some areas of the brain that
control body movement
– Important roles in behavior, motor activity, motivation
and reward, sleep, mood, attention and learning
• Seratonin – involved in the regulation of sleep
and emotional state
– Insufficient activity of neurons that release seratonin
may be one of the causes of clinical depression
More Neurotransmitters…
• Endorphins – block the perception of pain;
produced in the brain stem
– Opium, Morphine, and Heroin share chemical
similarities with endorphins, such that they bind
to receptors normally used by endorphins
Neurotransmitters play a role in drug
• Prolonged exposure to a stimulus that produces a
chemically-mediated signal may cause cells to
lose the ability to respond to it; habituation
• When receptor proteins are exposed to high
levels of neurotransmitter molecules for
prolonged periods, the postsynaptic cell often
responds by decreasing the number of receptor
proteins in its membrane; makes the cell more
efficient ( a natural function of the cell)
Neurotransmitters play a role in drug
• Drugs produce artificial neurotransmitter
effects, such that long-term use requires more
of the drug in order to obtain the same effect
• Cocaine affects neurons in the brain’s
“pleasure pathway” (limbic system)
– Binds dopamine transporters on presynaptic
membranes that normally remove dopamine from
the synaptic cleft, preventing the reuptake of
dopamine; dopamine ‘survives’ longer in the
synapse and fires pleasure pathways repeatedly
Cocaine addiction
• Prolonged exposure to cocaine reduces the number
of dopamine receptors; the cocaine user now needs
the drug to maintain normal levels of limbic activity
• The cocaine user is now addicted
• When a cocaine addict stops using cocaine, a
‘crushing’ depressive state ensues; feelings of
pleasure are essentially impossible; user wants more
drug to end depression
• Drugs are bad, mmmKay?
Effects of Cocaine
Transporter protein
Receptor protein
1. Reuptake of neurotransmitter by transporter
at a normal synapse.
2. Drug molecules block
transporter and cause
overstimulation of the
postsynaptic membrane.
3. Neuron adjusts to
overstimulation by
decreasing the number
of receptors.
4. Decreased number of
receptors make the
synapse less sensitive
when the drug is removed.
The Central Nervous System
All vertebrate brains have 3 basic divisions:
Hindbrain (rhombencephalon)
Midbrain (mesencephalon)
Forebrain (prosencephalon)
Through evolution, the relative sizes of
different brain regions have changed
– Forebrain became dominant feature
The Central Nervous System
Primitive fish brain
The Central Nervous System
• The forebrain is composed of two elements:
– Thalamus (integration and relay center) and
hypothalamus (basic drives and emotions; controls
pituitary gland)
– Cerebrum (aka “end brain”) – devoted to associative
• The increase in brain size in mammals reflects the
great enlargement of the cerebrum
• The cerebral cortex is the outer layer of the
cerebellum; contains ~10% of all brain neurons
The Cerebral Cortex
• Highly convoluted surface (increases surface
area of the brain)
• Divided into three regions, each with a specific
– Motor
– Sensory
– Associative
The Spinal Cord
• The spinal cord is a cable of neurons extending
from the brain down through the backbone
• The spinal cord is enclosed and protected by
the vertebral column and the meninges; layers
of membrane
• Serves as the body’s “information highway”;
relays messages between the body and brain
• Also functions in reflexes, the sudden,
involuntary movement of muscles
The Knee-Jerk Reflex
The Peripheral Nervous System
• The peripheral nervous system (PNS) collects
information and carries out responses
• The PNS consists of nerves and ganglia
• The PNS has somatic and autonomic systems
– Somatic associated with voluntary control of body
movements through the action of skeletal
muscles, and with reception of external stimuli
– Autonomic consists of motor neurons that control
smooth muscle, cardiac muscles and glands;
monitors visceral organs and blood vessels
The Autonomic System
• The Autonomic Nervous System (ANS) is
further divided into the sympathetic and
parasympathetic nervous systems
• Sympathetic nervous system prepares the
body for situations requiring alertness or
strength, or situations that arouse anger, fear,
excitement of embarassment
– Stimulates cardiac muscles to increase the heart
rate, causes dilation of the bronchioles of the
lungs , and cause dilation of blood vessels
The Autonomic System
• The parasympathetic system is active during
periods of digestion and rest; stimulates the
production of digestive enzymes and the
processes of digestion, urination, and
defecation; reduces blood pressure and heart
and respiratory rates, and conserves energy
through relaxation and rest
• The SNS and PNS act in opposition; one
system stimulates an organ, while the other
Stop secretion
Secrete saliva
Dilate bronchioles
Constrict bronchioles
Speed up heartbeat
Slow down heartbeat
Spinal cord
Adrenal gland
Secrete adrenaline
Decrease secretion
Increase secretion
Large intestine
Decrease motility
Increase motility
Small intestine
Retain colon contents
Delay emptying
Empty colon
Empty bladder