ELECTRICALSIGNALS IN ANIMALS (lec 21)
How is electrical signals in the heart and other parts of animal body affected in extreme cases, such as
• Animal movements are triggered by electrical signals conducted by nerve cells, or neurons.
• Complex processes such as moving, seeing, and thinking are based on seemingly simple events: flows of
ions across plasma membranes.
• Neurons transmit electrical signals; muscles can respond to electrical signals by contracting.
• There are two basic types of nervous systems:
• The diffuse arrangement of cells called a nerve net, found in cnidarians (jellyfish, hydra,
anemones) and ctenophores (comb jellies).
• Acentral nervous system (CNS) that includes large numbers of neurons aggregated into
clusters called ganglia.
Types of Neurons in the Nervous System
• Sensory receptors in the skin, eyes, ears, and nose transmit streams of data about the external
environment. Sensory cells inside the body monitor conditions that are important in homeostasis, such as
blood pH and oxygen levels.
• Asensory receptor transmits the information it receives from the environment by means of a nerve cell
called a sensory neuron.
• In vertebrates, the sensory neuron sends the information to neurons in the spinal cord or brain via nerves
—long, tough strands of nervous tissue containing thousands of neurons.
• The central nervous system (CNS), made up of the brain and spinal cord, integrates information from
many sensory neurons.
• Cells in the CNS called interneurons make connections between sensory neurons and motor neurons,
which are nerve cells that send signals to effector (response) cells in glands or muscles. • All of the components of the nervous system outside the CNS are part of the peripheral nervous
• In summary, sensory information from receptors in the PNS is sent to the CNS, where it is processed.
Then a response is transmitted back to appropriate parts of the body.
TheAnatomy of a Neuron
• Most neurons have the same three parts:
1. Adendrite receives electrical signals from the axons of adjacent cells.
2. The cell body, or soma, which includes the nucleus, integrates the incoming signals and
generates an outgoing signal.
3. The axon then sends the signal to the dendrites of other neurons.
• Each neuron makes many connections with other neurons.
Which structure generate electrical potential in cell membranes and how?
Potassium and sodium leakchannels (more potassium than sodium channels)Na+ ‐K+‐ATPase pump resting
potential: high intracellular concentration of K+ and proteins and low intracellular concentration of Na+ and
An Introduction to Membrane Potentials • Adifference of electrical charge between any two points creates a difference in electrical potential, or a
• If the positive and negative charges on ions that exist on the two sides of a plasma membrane do not
balance each other, the membrane will have an electrical potential.
• When an electrical potential exists on either side of a plasma membrane, the separation of charges is
called a membrane potential.
• Membrane potentials are a form of electrical potential and are measured in millivolts (mV).
• In neurons, membrane potentials are typically about 70–80 mV.
• Membrane potentials are always expressed as inside-relative-to-outside.
Electrical Potential, Currents, and Gradients
• When a membrane potential exists, the ions on both sides of the membrane have potential energy.
• If the membrane were removed, ions would spontaneously move from the area of like charge to the area
of unlike charge—causing a flow of charge, called an electric current.
• Ions move across membranes in response to concentration gradients as well as charge gradients.
– The combination of an electric gradient and a concentration gradient is an electrochemical
How Is the Resting Potential Maintained?
• When a neuron is at rest in extracellular fluid, its membrane has a voltage called the resting potential.
• This potential exists in part because neurons have a high intracellular concentration of K and low +
intracellular concentrations of Na and Cl . –
• The plasma membrane is selectively permeable; only certain substances can cross it.
• Ions such as these can only cross plasma membranes efficiently in one of three ways: • Along their electrochemical gradient through an ion channel, a pore in the membrane that allows
only specific ions to pass through.
• Carried via a membrane cotransporter protein or antiporter protein.
• Pumped against an electrochemical gradient by a membrane protein that hydrolyzes ATP.
The K Leak Channel
• When a neuron is not transmitting an electrical signal, the type of membrane ion channel most likely to
be open are those that admit K ions.
• Resting neurons are most permeable to K ions, which cross the membrane easily along their
• The potassium channels involved are sometimes called leak channels, because they allow K to leak out +
of the cell.
• As K moves out of the cell via K channels, the inside of the cell becomes more negatively charged
relative to the outside.
• Eventually, the membrane reaches a voltage at which there is equilibrium between the concentration
gradient that moves K out and the electrical gradient that moves K in. This is called the equilibrium
potential for K .
• Even though Cl and Na cross the plasma membrane much less readily than does K , each type of ion +
has its own equilibrium potential.
The Role of the Na /K -ATPase
• Na /K -ATPase imports K ions and exports Na ions, resulting in the concentration of K ions being +
higher inside the cell and Na being higher outside the cell.
• This results in the inside of the neuron being negatively charged with respect to the extracellular
environment. Thus, the neuron has a negative resting potential.
• The resting potential represents energy stored as concentration gradients in a series of ions.
What Is an Action Potential?
• An action potential is a rapid, temporary change in a membrane potential.
• Although Hodgkin and Huxley initially studied it in the squid giant axon, subsequent work has shown
that the action potential has the same general characteristics in all species and in all types of neurons.
• It has three phases: depolarization, repolarization, and hyperpolarization.
• How are electrical signals generated and propagated in animal neurons?
• Depolarization- threshold of about -55mv; voltage-gated Na+ channels open • Repolarization- at about 30mv, voltage-gated K+ channels open;voltage gated Na+ channels close
• Hyperpolarization-briefly more negative than about -70mv
1. The initial event is a rapid depolarization of the membrane.
– For an action potential to begin, the membrane potential must shift from its resting potential
-70mv to about –55 mV.
– If this threshold potential is reached, certain channels in the axon membrane open and ions rush
into the axon, following their electrochemical gradients.
– This current flow causes further depolarization.
2. The second phase is a rapid repolarization.
– When the membrane potential reaches about +40 mV, an abrupt change is triggered by the
closing of certain ion channels and the opening of other ion channels in the membrane.
– During this phase, ions flow out of the axon, changing the membrane potential from positive
back to negative.
3. The third phase is hyperpolarization.
– The repolarization event results in the membrane briefly becoming more negative than the
resting potential, a state called hyperpolarization.
How are electrical signals
generated and propagated
in animal neurons? How Is theAction Potential Propagated?
1. When Na enters a cell at the onset of an action potential, positive charges in the cell are repulsed and
negative charges are attracted. This results in the charge spreading away from the sodium channels.
2. As positive charges are pushed farther from the initial sodium channels, they depolarize adjacent
portions of the membrane.
3. Nearby voltage-gated Na channels pop open in response to depolarization. Positive feedback occurs,
and a full-fledged action potential results.
In this way, an action potential is continuously regenerated as it moves down the axon. The signal does not
diminish as it moves, because the response is all or none.
Action potentials do not propagate back up the axon because Na channels are refractory—once they have
opened and closed, they are less likely to open again for a short period of time.
Sodium channels “downstream” of the site are not in the refractory state, resulting in the one-way propagation
of the action potential. Excitatory postsynaptic
Potentials (EPSPs): membrane depolarizes Inhibitory postsynaptic potentials
(IPSPs): membrane hyperpolarizes
Both are not all--‐or--‐none (as isAP) but graded events. Neurotransmitters: release at the synapse determines
the strength of EPSP and IPSP
Summa5on: several EPSPs occur close together and their sum and causes the neuron to likely fire anAP.
An “All-or-None” Signal That Propagates
• In addition to being fast and having three distinct phases, the action potential is an all-or-none event:
– There is no such thing as a partial action potential.
– All action potentials for a given neuron are identical in magnitude and duration.
– Action potentials are propagated down the length of the axon.
• Neurons have excitable membranes, because neurons are capable of generating action potentials that
propagate rapidly along the length of their axons.
• The action potential suggests a mechanism for electrical signaling.
• In the nervous system, information is coded in the form of action potentials that travel along axons. The
frequency of action potentials—rather than their size—is the meaningful signal.
How do electrical signal lead to muscle contraction and body movement
Events @ Neuromuscular junction
1)AP arrives at the synapse (end of axon)
2) Ca++ enters the synapse (presynaptic
3) Synaptic vesicles release acetylcholine
(neurotransmitter) into the synaptic cleft
(gap bw presynaptic and muscle cell)
4)Ach binds to recepto on postsynaptic
muscle cell membrane, initiating anAP if
threshold is reached Events @ Neuromuscular junction and beyond
5.APs propagate along the membrane of the muscle cell and spread via invaginations called T tubules
6. T tubules are in close contact with sarcoplasmic reticulum (SR, smooth endoplasmic reticulum). When anAP
spreas via T tubule, a Protein in the T tubule changes shape opens Ca++ channel in SR
7.Ach broken down and taken back by the presynaptic cell to stop the cycle
Muscle fibers: made up skeletal and cardiac
muscle tissue in vertebrate
Myofibrils:bundles within each muscle fibre
Sarcomere: functional unit within
myofibrils,composed of thin (actin) and thick
(myosin) filaments; shorten when muscle
How is muscle contracted
Events in Muscle cell
When Ca++ is released from SR,
it binds to troponin-tropomyosin
complex shape in thin filaments
This change in shape exposes
active binding sites forATP on
OnceATP site on thin (actin)
filament is exposed the myosin
head of the thick filament binds
to it and the above cycle begins. End result:
actin of thin
the centre or
one of the ends
ANIMAL SENSORY SYSTEMSAND MOVEMENT (lec 22)
How do moths and vertebrates hear?
• Sensing changes in the environment and moving in response to this information is fundamental to how
• The ability to sense a change in the environment depends on four processes:
1. Transduction, the conversion of an external stimulus to an internal signal in the form of an
2. Amplification of the signal.
3. Transmission of the signal to the central nervous system (CNS).
4. Integration pr processing with other incoming signals Moths fly through the night sensory neurons will relay info about conditions in/out an animal to the to the CNS.
After integrating the info from many sensory neurons the CNS sends signal to the muscle.
Integrate sensory input-info from sensory neurons- and respond with motor output via electrical signals, to
specific muscle groups (effectors)
• Each type of sensory information is detected by a sensory neuron or by a specialized receptor cell that
makes a synapse with a sensory neuron.
• Transduction requires a sensory receptor cell to convert light, sound,