PHYSIOLOGY EXAMINATION SYLLABUS
Theoretical exam
1. Cell membranes. Transport of substances through cell membranes.
cell membranes structure and function
cell membranes, also known as plasma membranes envelops the cell and is a think pliable elastic structure which is 7.5-109
nanometers thick it is composed of proteins and lipids.
The approximate concentration is 55% proteins, 25% phospholipids and 13%cholestrol and 4% other lipids and 3%
carbohydrates.
The cell is the basic unit of life, and all organisms are made up of one or many cells.
One of the things that all cells have in common is a cell membrane. It is a
barrier that separates a cell from its surrounding environment. This
outer boundary of the cell is also called the plasma membrane. It is
composed of four different types of molecules:
1 Phospholipids
2 Cholesterol
3 Proteins
4 Carbohydrates
The fluid mosaic model describes the structure of a cell
membrane. It indicates that the cell membrane is not solid. It is flexible and has a
similar consistency to vegetable oil, so all the individual molecules are just
floating in a fluid medium, and they are all capable of moving sideways within the cell
membrane. Mosaic refers to something that contains many different parts. The
plasma membrane is a mosaic of phospholipids, cholesterol molecules, proteins and
carbohydrates.
Phospholipids
Phospholipids make up the basic structure of a cell membrane. A single phospholipid molecule has two different ends: a
head and a tail. The head end contains a phosphate group and is hydrophilic. This means that it likes or is attracted to water
molecules.
The tail end is made up of two strings of hydrogen and carbon atoms called fatty acid chains. These chains are hydrophobic,
or do not like to mingle with water molecules. This is just like what happens when you pour vegetable oil in water. The
vegetable oil will not mix with the water.
This arrangement of phospholipid molecules makes up the lipid bilayer.
The phospholipids of a cell membrane are arranged in a double layer called the lipid bilayer. The hydrophilic phosphate
heads are always arranged so that they are near water. Watery fluids are found both inside a cell (intracellular fluid) and
outside a cell (extracellular fluid). The hydrophobic tails of membrane phospholipids are organized in a manner that keeps
them away from water.
Cholesterol, Proteins and Carbohydrates
When you hear the word cholesterol, the first thing you probably think of is that it is bad. However, cholesterol is actually a
very important component of cell membranes. Cholesterol molecules are made up of four rings of hydrogen and carbon
atoms. They are hydrophobic and are found among the hydrophobic tails in the lipid bilayer.
Cholesterol molecules are important for maintaining the consistency of the cell membrane. They strengthen the membrane
by preventing some small molecules from crossing it. Cholesterol molecules also keep the phospholipid tails from coming
into contact and solidifying. This ensures that the cell membrane stays fluid and flexible.
Some plasma membrane proteins are located in the lipid bilayer and are called integral proteins. Other proteins, called
peripheral proteins, are outside of the lipid bilayer. Peripheral proteins can be found on either side of the lipid bilayer:
inside the cell or outside the cell. Membrane proteins can function as enzymes to speed up chemical reactions, act as
receptors for specific molecules, or transport materials across the cell membrane.
Carbohydrates, or sugars, are sometimes found attached to proteins or lipids on the outside of a cell membrane. That is,
they are only found on the extracellular side of a cell membrane. Together, these carbohydrates form the glycocalyx.
The glycocalyx of a cell has many functions. It provides cushioning and protection for the plasma membrane, and it is also
important in cell recognition. Based on the structure and types of carbohydrates in the glycocalyx, your body can recognize
cells and determine if they should be there or not. The glycocalyx can also act as a glue to attach cells together.
,Functions:
its function is to protect the integrity of the interior of the cell by allowing certain substances into the cell, while keeping
other substances out. It also serves as a base of attachment for the cytoskeleton in some organisms and the cell wall in
others. Thus the cell membrane also serves to help support the cell and help maintain its shape. Another function of the
membrane is to regulate cell growth through the balance of endocytosis and exocytosis. In endocytosis, lipids and proteins
are removed from the cell membrane as substances are internalized. In exocytosis, vesicles containing lipids and proteins
fuse with the cell membrane increasing cell size. Animal cells, plant cells, prokaryotic cells, and fungal cells have plasma
membranes. Internal organelles are also encased by membranes.
transport of substances through cell membranes
There are different types of transport through cell membranes, they can be active requiring ATP or passive not requiring
ATP.
There are 3 models of carrier mediated membrane transport and these are facilitated diffusion. Primary active transport
and secondary active transport by exchangers (cotransport or antiports) and cotransporters (symports)
Passive transport is the movement of molecules across the cell membrane and it is passive so therefore does not require
energy it is dependent on the permeability of the cell membrane. There are three main types of passive transport and these
are diffusion, osmosis and facilitated diffusion. Examples of molecules, which move in this way, are sodium and glucose and
calcium.
Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration, the
diffusion rates can be calculated by ficks law of diffusion – J = P x A (c1-c2)
P is the permeability coefficient, a = surface area and c1-c2 is the concentration gradient driving diffusion.
Facilitated diffusion doe not require ATP, however the molecules don’t diffuse directly through the cell membrane, they
require cell membrane proteins which are called carrier proteins. These proteins carry the molecules across the cell
membrane from an area of high concentration to an area of low concentration. The transmembrane proteins that permit
facilitated diffusion can be open or closed, and they are said to be gated ion channels soe types of these ion channels
include ligand gates, mechanically gated, voltage gated or light gated. External ligands include ACH, GABA. Internal ligands
bind to a site on the channel protein exposed to the cytosol and examples include second messengers like cyclic AMP and
cyclic GMP. The protein channels that enable facilitated diffusion are selective top certain ions, and carry only that ion or
molecule through the protein.
Sometimes, ions can be co-transported through the membrane with another ion, for example glucose is transported by
facilitated diffusion with sodium in association with the glucose and this is known as co-transport.
Osmosis is the movement of water across a semi permeable membrane, from a lower water potential to a higher water
potential, it does not require any energy so therefore it is a form of passive transport.
Endocytosis/exocytosis. Large macromolecules (e.g., proteins, viruses, lipoprotein particles) require more complex
mechanisms to traverse membranes, and are transported into and out of cells selectively via endocytosis and exocytosis
(secretion). Interestingly, endocytosis and exocytosis are not only important for the import/export of large molecules.
Often, essential small molecules that are hydrophobic or toxic (e.g., iron) travel through the bloodstream bound to proteins,
which enter and exit cells via these mechanisms.
primary Active Transport: H+, Na+, Ca+2 all utilize ATP to force these things across the membrane. There is one major
distinction between active transport and facilitated diffusion other than one needing energy and the other not. Active
transport always causes a molecule to be forced to move from a lower concentration to higher, which it does not want to
naturally do. The most important of all transport pumps are the one that uses ATP to simultaneously force K+ (potassium
ions) into the cell and Na+ (sodium ions) out of the cell. This is famously called the Sodium-Potassium protein
pump. Active transport processes account for about 40% of the energy used in the body so this is a major thing in case you
thought this wasn’t important.
Secondary active transport: These is when one chemical is being forced in or out of the cell and another chemical just sort-
of follows through. The two terms used for this is SYMPORT and ANTI-PORT. Sym- means together and two chemicals are
going together in the same direction. Anti-port is when one chemical is forced in one direction and another is passively
,going in the other direction.
Active transport is the transport of substances from lower to a higher concentration against their concentration gradient,
and requires energy from ATP. Chemical energy in the form of ATP drives specialised protein carrier moelecules that
transport substances across the membrane in either direction. The carrier sites are specific and can be used by only one
substance, therefore the rate at which substances is transferred depends on the number of sites available.
The sodium potassium pump is present in all cells, and it is essential in maintaining the electrical potential of a cell, it is also
needed to make nerve impulses. It actively transpoirts three sodiums out of the cell and two potassiums into the cell.
2. Membrane potentials. Resting membrane potential of nerves.
If we insert one electrode into the interior of the cell and another to the exterior of the cell, a potential difference is
observed, this is the resting membrane potential of the cell. The imnside of the cell will be negative relative to the outside
of the cell.
In order for a potential difference to be present, there must be some conditions that are met, firstly there must be an
unequal distribution of ions of one or more species across the membrane and the membrane must be permeable to one or
more of these ion species, the permeability is dependent on the channels or pores in the membrane through which the ion
species can travel.
The resting membrane potential represents an equilibrium situation in which the driving force for the membrane permeate
ions down their concentration gradients across the membrane is equal and opposite to the driving force for these ions
down their electrical gradients. In neurons, the concentration of potassium is much higher inside the cell than outside the
cell.
For sodium ions, the opposite is true and the concentration of sodium ions is much higher outside the cell than inside. The
concentration difference is established by Na, K, ATPase the outward K+ concentration gradiebt results in passive
movement of k+ out the crll when K+ sseelective channels are open, similarly the inward Na+ concentration gradient
resulktds in pasasive movement of Na+ into the cell when Na+ selective ion channels are open.
In neurons, the resting membrane potential is usually around -70mV which is close to the equilibrium potential for K+.
because the cell membrane has more open K+ channels than Na+ channels, the membrane is more permeable to potassium
ions than to sodium ions. As a consequence of this, the intracellular and extracellular K+ concentrations are the prime
determinants of the resting membrane potential, the ion leaks occur, and to make sure the resting membrane potential is
not disrupted, huge changes in ion concentrartions are prevented by the sodium potassium pump which maintains the
RMP.
In cells of all types, there is an electrical potential difference between the inside of the cell and the surrounding
extracellular fluid. This is termed the membrane potential of the cell. While this phenomenon is present in all cells, it is
especially important in nerve and muscles cells, because changes in their membrane potentials are used to code and
transmit information.
First, what is an electrical potential difference? An electrical potential difference exists between two locations when there is
a net separation of charge between the two locations. This is illustrated in the figure on the right. Electrical potentials are
measured in units of volts. (A volt is defined in terms of energy per unit charge; that is, one volt is equal to one
joule/coloumb.)
When a nerve or muscle cell is at "rest", its membrane potential is called the resting membrane potential. In a typical
neuron, this is about –70 millivolts (mV). The minus sign indicates that the inside of the cell is negative with respect to the
surrounding extracellular fluid.
It is essential to realize that only a very small number of negative and positive ions need to be separated by the membrane
to create the resting membrane potential. For example, for each pair of negative and positive ions separated by the
membrane, there are roughly 1000 pairs of positive and negative ions within the cytosol of the neuron.
Thus, two energetic factors influence the movement of an ion across a membrane.
• The concentration gradient
• The electrical potential difference
The concentration gradient, of course, applies to uncharged molecules too. But with ions, we must always consider the
electrical potential difference as well. Thus, the total energy change for the movement of an ion across the membrane is the
the sum of the energy change due to the concentration gradient and the energy change due to electrical potential
, difference. These two factors may act in the same direction or in opposite directions.
If some event, such as the opening of a gated ion channel, causes the membrane potential to become less negative, this is
termed depolarization. Conversely, if some factor causes the membrane potential to become more negative, this is termed
hyperpolarization.
3. Nerve action potential. Propagation of the action potential. Rhythmicity.
An impulse is initiated by stimulation of sensory nerve endings or by the passage of an impulse from another nerve,
transmission of the impulse or action potential is due to movement of ions across the nerve cell membrane. In the resting
vell the membrane is polarised due to differences in the concentrations of ions across the plasma membrane. This means
there is a potential difference, and this is the resting membrane potential, in nerve cells this is -70mV.
In the resting state, there is a continual tendency for these ions to diffuse along their concentration gradients, K+ outwards
and Na+ inweards. When stimulated the permeability of the nerve cell membrane to these ions changes.
Initially NA+ floods into the neuron from the ECF which causes depolarisation creating an action potential. Depolarisation is
very rapid, and only occurs in one direction.
An action potential consists of a set of ionic movements, the ions move across the cell membrane when the correct
channels are opened.
An action potential consists of the following stages:
The membrane starts in its resting state – polarized with the inside of the cell being -60Mv compared to the outside
Sodium ion channels open and some sodium ions diffuse into the cell.
The membrane depolarizes – it become negative with respect to the outside and reaches a threshold value of -50Mv
Voltage gated sodium ion channels open and many sodium ions flood in. As more sodium ions enter, the cell becomes
positively charged inside compared with outside.
The potential difference across the plasma membrane reaches +40Mv. The inside of the cell is positive compared with
the outside.
The sodium ion channels close and the potassium channels open.
Potassium ions diffuse out of the cell bringing the potential difference back to negative inside compard with outside –
this process is known as repolarization
The potential difference overshoots slightly making the cell hyperpolarized.
The original potential difference is restored so that the cell returns to its resting state.
Can only travel in one direction.
Summary
5 the Na+ channels to open. If the opening is sufficient to drive the interior potential from -70 mV up to -55 mV, the process
continues.
6 Having reached the action threshold, more Na+ channels (sometimes called voltage-gated channels) open. The Na+ influx
drives the interior of the cell membrane up to about +30 mV. The process to this point is called depolarization.
7 The Na+ channels close and the K+ channels open. Since the K+ channels are much slower to open, the depolarization has
time to be completed. Having both Na+ and K+ channels open at the same time would drive the system toward
neutrality and prevent the creation of the action potential.
8 With the K+ channels open, the membrance begins to repolarize back toward its rest potential.
9 The repolarization typically overshoots the rest potential to about -90 mV. This is called hyperpolarization and would
seem to be counterproductive, but it is actually important in the transmission of information. Hyperpolarization
prevents the neuron from receiving another stimulus during this time, or at least raises the threshold for any new
stimulus. Part of the importance of hyperpolarization is in preventing any stimulus already sent up an axon from
triggering another action potential in the opposite direction. In other words, hyperpolarization assures that the
signal is proceeding in one direction.
After hyperpolarization, the Na+/K+ pump eventually brings the membrane back to its resting state of -70 mV .
4. Signal transmission in nerve fibers. Excitation - the process of eliciting the action potential. Threshold for
excitation, refractory period. Inhibition of excitability.
Theoretical exam
1. Cell membranes. Transport of substances through cell membranes.
cell membranes structure and function
cell membranes, also known as plasma membranes envelops the cell and is a think pliable elastic structure which is 7.5-109
nanometers thick it is composed of proteins and lipids.
The approximate concentration is 55% proteins, 25% phospholipids and 13%cholestrol and 4% other lipids and 3%
carbohydrates.
The cell is the basic unit of life, and all organisms are made up of one or many cells.
One of the things that all cells have in common is a cell membrane. It is a
barrier that separates a cell from its surrounding environment. This
outer boundary of the cell is also called the plasma membrane. It is
composed of four different types of molecules:
1 Phospholipids
2 Cholesterol
3 Proteins
4 Carbohydrates
The fluid mosaic model describes the structure of a cell
membrane. It indicates that the cell membrane is not solid. It is flexible and has a
similar consistency to vegetable oil, so all the individual molecules are just
floating in a fluid medium, and they are all capable of moving sideways within the cell
membrane. Mosaic refers to something that contains many different parts. The
plasma membrane is a mosaic of phospholipids, cholesterol molecules, proteins and
carbohydrates.
Phospholipids
Phospholipids make up the basic structure of a cell membrane. A single phospholipid molecule has two different ends: a
head and a tail. The head end contains a phosphate group and is hydrophilic. This means that it likes or is attracted to water
molecules.
The tail end is made up of two strings of hydrogen and carbon atoms called fatty acid chains. These chains are hydrophobic,
or do not like to mingle with water molecules. This is just like what happens when you pour vegetable oil in water. The
vegetable oil will not mix with the water.
This arrangement of phospholipid molecules makes up the lipid bilayer.
The phospholipids of a cell membrane are arranged in a double layer called the lipid bilayer. The hydrophilic phosphate
heads are always arranged so that they are near water. Watery fluids are found both inside a cell (intracellular fluid) and
outside a cell (extracellular fluid). The hydrophobic tails of membrane phospholipids are organized in a manner that keeps
them away from water.
Cholesterol, Proteins and Carbohydrates
When you hear the word cholesterol, the first thing you probably think of is that it is bad. However, cholesterol is actually a
very important component of cell membranes. Cholesterol molecules are made up of four rings of hydrogen and carbon
atoms. They are hydrophobic and are found among the hydrophobic tails in the lipid bilayer.
Cholesterol molecules are important for maintaining the consistency of the cell membrane. They strengthen the membrane
by preventing some small molecules from crossing it. Cholesterol molecules also keep the phospholipid tails from coming
into contact and solidifying. This ensures that the cell membrane stays fluid and flexible.
Some plasma membrane proteins are located in the lipid bilayer and are called integral proteins. Other proteins, called
peripheral proteins, are outside of the lipid bilayer. Peripheral proteins can be found on either side of the lipid bilayer:
inside the cell or outside the cell. Membrane proteins can function as enzymes to speed up chemical reactions, act as
receptors for specific molecules, or transport materials across the cell membrane.
Carbohydrates, or sugars, are sometimes found attached to proteins or lipids on the outside of a cell membrane. That is,
they are only found on the extracellular side of a cell membrane. Together, these carbohydrates form the glycocalyx.
The glycocalyx of a cell has many functions. It provides cushioning and protection for the plasma membrane, and it is also
important in cell recognition. Based on the structure and types of carbohydrates in the glycocalyx, your body can recognize
cells and determine if they should be there or not. The glycocalyx can also act as a glue to attach cells together.
,Functions:
its function is to protect the integrity of the interior of the cell by allowing certain substances into the cell, while keeping
other substances out. It also serves as a base of attachment for the cytoskeleton in some organisms and the cell wall in
others. Thus the cell membrane also serves to help support the cell and help maintain its shape. Another function of the
membrane is to regulate cell growth through the balance of endocytosis and exocytosis. In endocytosis, lipids and proteins
are removed from the cell membrane as substances are internalized. In exocytosis, vesicles containing lipids and proteins
fuse with the cell membrane increasing cell size. Animal cells, plant cells, prokaryotic cells, and fungal cells have plasma
membranes. Internal organelles are also encased by membranes.
transport of substances through cell membranes
There are different types of transport through cell membranes, they can be active requiring ATP or passive not requiring
ATP.
There are 3 models of carrier mediated membrane transport and these are facilitated diffusion. Primary active transport
and secondary active transport by exchangers (cotransport or antiports) and cotransporters (symports)
Passive transport is the movement of molecules across the cell membrane and it is passive so therefore does not require
energy it is dependent on the permeability of the cell membrane. There are three main types of passive transport and these
are diffusion, osmosis and facilitated diffusion. Examples of molecules, which move in this way, are sodium and glucose and
calcium.
Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration, the
diffusion rates can be calculated by ficks law of diffusion – J = P x A (c1-c2)
P is the permeability coefficient, a = surface area and c1-c2 is the concentration gradient driving diffusion.
Facilitated diffusion doe not require ATP, however the molecules don’t diffuse directly through the cell membrane, they
require cell membrane proteins which are called carrier proteins. These proteins carry the molecules across the cell
membrane from an area of high concentration to an area of low concentration. The transmembrane proteins that permit
facilitated diffusion can be open or closed, and they are said to be gated ion channels soe types of these ion channels
include ligand gates, mechanically gated, voltage gated or light gated. External ligands include ACH, GABA. Internal ligands
bind to a site on the channel protein exposed to the cytosol and examples include second messengers like cyclic AMP and
cyclic GMP. The protein channels that enable facilitated diffusion are selective top certain ions, and carry only that ion or
molecule through the protein.
Sometimes, ions can be co-transported through the membrane with another ion, for example glucose is transported by
facilitated diffusion with sodium in association with the glucose and this is known as co-transport.
Osmosis is the movement of water across a semi permeable membrane, from a lower water potential to a higher water
potential, it does not require any energy so therefore it is a form of passive transport.
Endocytosis/exocytosis. Large macromolecules (e.g., proteins, viruses, lipoprotein particles) require more complex
mechanisms to traverse membranes, and are transported into and out of cells selectively via endocytosis and exocytosis
(secretion). Interestingly, endocytosis and exocytosis are not only important for the import/export of large molecules.
Often, essential small molecules that are hydrophobic or toxic (e.g., iron) travel through the bloodstream bound to proteins,
which enter and exit cells via these mechanisms.
primary Active Transport: H+, Na+, Ca+2 all utilize ATP to force these things across the membrane. There is one major
distinction between active transport and facilitated diffusion other than one needing energy and the other not. Active
transport always causes a molecule to be forced to move from a lower concentration to higher, which it does not want to
naturally do. The most important of all transport pumps are the one that uses ATP to simultaneously force K+ (potassium
ions) into the cell and Na+ (sodium ions) out of the cell. This is famously called the Sodium-Potassium protein
pump. Active transport processes account for about 40% of the energy used in the body so this is a major thing in case you
thought this wasn’t important.
Secondary active transport: These is when one chemical is being forced in or out of the cell and another chemical just sort-
of follows through. The two terms used for this is SYMPORT and ANTI-PORT. Sym- means together and two chemicals are
going together in the same direction. Anti-port is when one chemical is forced in one direction and another is passively
,going in the other direction.
Active transport is the transport of substances from lower to a higher concentration against their concentration gradient,
and requires energy from ATP. Chemical energy in the form of ATP drives specialised protein carrier moelecules that
transport substances across the membrane in either direction. The carrier sites are specific and can be used by only one
substance, therefore the rate at which substances is transferred depends on the number of sites available.
The sodium potassium pump is present in all cells, and it is essential in maintaining the electrical potential of a cell, it is also
needed to make nerve impulses. It actively transpoirts three sodiums out of the cell and two potassiums into the cell.
2. Membrane potentials. Resting membrane potential of nerves.
If we insert one electrode into the interior of the cell and another to the exterior of the cell, a potential difference is
observed, this is the resting membrane potential of the cell. The imnside of the cell will be negative relative to the outside
of the cell.
In order for a potential difference to be present, there must be some conditions that are met, firstly there must be an
unequal distribution of ions of one or more species across the membrane and the membrane must be permeable to one or
more of these ion species, the permeability is dependent on the channels or pores in the membrane through which the ion
species can travel.
The resting membrane potential represents an equilibrium situation in which the driving force for the membrane permeate
ions down their concentration gradients across the membrane is equal and opposite to the driving force for these ions
down their electrical gradients. In neurons, the concentration of potassium is much higher inside the cell than outside the
cell.
For sodium ions, the opposite is true and the concentration of sodium ions is much higher outside the cell than inside. The
concentration difference is established by Na, K, ATPase the outward K+ concentration gradiebt results in passive
movement of k+ out the crll when K+ sseelective channels are open, similarly the inward Na+ concentration gradient
resulktds in pasasive movement of Na+ into the cell when Na+ selective ion channels are open.
In neurons, the resting membrane potential is usually around -70mV which is close to the equilibrium potential for K+.
because the cell membrane has more open K+ channels than Na+ channels, the membrane is more permeable to potassium
ions than to sodium ions. As a consequence of this, the intracellular and extracellular K+ concentrations are the prime
determinants of the resting membrane potential, the ion leaks occur, and to make sure the resting membrane potential is
not disrupted, huge changes in ion concentrartions are prevented by the sodium potassium pump which maintains the
RMP.
In cells of all types, there is an electrical potential difference between the inside of the cell and the surrounding
extracellular fluid. This is termed the membrane potential of the cell. While this phenomenon is present in all cells, it is
especially important in nerve and muscles cells, because changes in their membrane potentials are used to code and
transmit information.
First, what is an electrical potential difference? An electrical potential difference exists between two locations when there is
a net separation of charge between the two locations. This is illustrated in the figure on the right. Electrical potentials are
measured in units of volts. (A volt is defined in terms of energy per unit charge; that is, one volt is equal to one
joule/coloumb.)
When a nerve or muscle cell is at "rest", its membrane potential is called the resting membrane potential. In a typical
neuron, this is about –70 millivolts (mV). The minus sign indicates that the inside of the cell is negative with respect to the
surrounding extracellular fluid.
It is essential to realize that only a very small number of negative and positive ions need to be separated by the membrane
to create the resting membrane potential. For example, for each pair of negative and positive ions separated by the
membrane, there are roughly 1000 pairs of positive and negative ions within the cytosol of the neuron.
Thus, two energetic factors influence the movement of an ion across a membrane.
• The concentration gradient
• The electrical potential difference
The concentration gradient, of course, applies to uncharged molecules too. But with ions, we must always consider the
electrical potential difference as well. Thus, the total energy change for the movement of an ion across the membrane is the
the sum of the energy change due to the concentration gradient and the energy change due to electrical potential
, difference. These two factors may act in the same direction or in opposite directions.
If some event, such as the opening of a gated ion channel, causes the membrane potential to become less negative, this is
termed depolarization. Conversely, if some factor causes the membrane potential to become more negative, this is termed
hyperpolarization.
3. Nerve action potential. Propagation of the action potential. Rhythmicity.
An impulse is initiated by stimulation of sensory nerve endings or by the passage of an impulse from another nerve,
transmission of the impulse or action potential is due to movement of ions across the nerve cell membrane. In the resting
vell the membrane is polarised due to differences in the concentrations of ions across the plasma membrane. This means
there is a potential difference, and this is the resting membrane potential, in nerve cells this is -70mV.
In the resting state, there is a continual tendency for these ions to diffuse along their concentration gradients, K+ outwards
and Na+ inweards. When stimulated the permeability of the nerve cell membrane to these ions changes.
Initially NA+ floods into the neuron from the ECF which causes depolarisation creating an action potential. Depolarisation is
very rapid, and only occurs in one direction.
An action potential consists of a set of ionic movements, the ions move across the cell membrane when the correct
channels are opened.
An action potential consists of the following stages:
The membrane starts in its resting state – polarized with the inside of the cell being -60Mv compared to the outside
Sodium ion channels open and some sodium ions diffuse into the cell.
The membrane depolarizes – it become negative with respect to the outside and reaches a threshold value of -50Mv
Voltage gated sodium ion channels open and many sodium ions flood in. As more sodium ions enter, the cell becomes
positively charged inside compared with outside.
The potential difference across the plasma membrane reaches +40Mv. The inside of the cell is positive compared with
the outside.
The sodium ion channels close and the potassium channels open.
Potassium ions diffuse out of the cell bringing the potential difference back to negative inside compard with outside –
this process is known as repolarization
The potential difference overshoots slightly making the cell hyperpolarized.
The original potential difference is restored so that the cell returns to its resting state.
Can only travel in one direction.
Summary
5 the Na+ channels to open. If the opening is sufficient to drive the interior potential from -70 mV up to -55 mV, the process
continues.
6 Having reached the action threshold, more Na+ channels (sometimes called voltage-gated channels) open. The Na+ influx
drives the interior of the cell membrane up to about +30 mV. The process to this point is called depolarization.
7 The Na+ channels close and the K+ channels open. Since the K+ channels are much slower to open, the depolarization has
time to be completed. Having both Na+ and K+ channels open at the same time would drive the system toward
neutrality and prevent the creation of the action potential.
8 With the K+ channels open, the membrance begins to repolarize back toward its rest potential.
9 The repolarization typically overshoots the rest potential to about -90 mV. This is called hyperpolarization and would
seem to be counterproductive, but it is actually important in the transmission of information. Hyperpolarization
prevents the neuron from receiving another stimulus during this time, or at least raises the threshold for any new
stimulus. Part of the importance of hyperpolarization is in preventing any stimulus already sent up an axon from
triggering another action potential in the opposite direction. In other words, hyperpolarization assures that the
signal is proceeding in one direction.
After hyperpolarization, the Na+/K+ pump eventually brings the membrane back to its resting state of -70 mV .
4. Signal transmission in nerve fibers. Excitation - the process of eliciting the action potential. Threshold for
excitation, refractory period. Inhibition of excitability.
