Introduction to Organismal Biology (BIOL221) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe/

Neurons & Electrical Signaling

• Organisms have a need for mechanism to relay internal messages
• Messengers can be:  (1) chemical - hormones, organic molecules, slow, long-lasting or (2) electrical - action potentials, fast, transient
• Plants vs. animals - since plants lead a slow life, or in other words, are non-motile, they never had an evolutionary pressure to evolve a nervous system.  Thus, most signaling within plants occurs via chemical messengers.   Recall the plant way of life unit?  There are a few exceptions; for example the Venus fly trap and sensitive plants both have rapid movements that involve electrical signals.
  
  In contrast, the animal nervous system can be considered, at least in part, as a response to movement and the need to gather food.  Since animals lead a "fast" life, it required the evolution of rapid signaling system.  Animals rely on BOTH chemical and electrical signals.

II. Membrane Potentials - the basis of electrical signaling

A.  Definition/General - charge difference across the membrane
     At rest, neurons (and most other cells, too) exhibit an electrical charge difference across the membrane. This difference, or membrane potential, can be measured with tiny electrodes connected to a voltmeter or an oscilloscope. At rest, the membrane potential is -70 millivolts (mV), which means that the inside of the cell is negative in comparison to the outside.

B.  Charge distribution
    This is ultimately due to an unequal distribution of ions, especially sodium and potassium, between the outside and inside of the cell; check out Table 1.

C.  Why the unequal charge distribution? 
    The membrane is permeable to both potassium and sodium (recall that ions move through membranes slowly).  These ions diffuse through protein channels in the membrane.  There are separate "leakage channels" for both potassium and sodium and they are always "open", thereby permitting the diffusion of sodium and potassium down their concentration gradient.  Thus, potassium diffuses from inside to outside and sodium diffuses from outside to inside. 

    The membrane is much more permeable (50-75 times) to potassium so there is a net flux (leakage) of potassium, and hence positively-charged particles, out of the cell. This is the result of more potassium leakage channels and a steeper concentration gradient for potassium between inside and outside of the cell.  Since the cell is permeable to both sodium and potassium, over time, you would expect an equilibrium state would be reached at which point the concentration of sodium and potassium would be approximately equal inside and outside of the cell, and hence no membrane potential.

    So, what maintains the unequal charge distribution?  The sodium-potassium pump!  The pump is another integral protein complex that requires ATP and actively transports sodium out of the cell and potassium into the cell. For every three sodium that are moved out, two potassium are moved in.  Thus, the sodium-potassium pump maintains the ion gradient across the membrane.  It is an active (energy-requiring, ATP, process) because it is moving the ions against their concentration gradient.

D.  The concentration of ions and electrical charge are "balanced" at equilibrium. 
     The tendency for potassium to diffuse out of the cell following its concentration gradient is balanced by the electrical "pull" of the negatively charged interior.  And, the tendency for sodium to diffuse into the cell following its concentration gradient is balanced by the tendency of the electrical pull of negative charges outside the cell (which results from the positive sodium entering the cell leaving a greater proportion of anions and negatively charged ions).  

E. Nernst Equation
    Relates the ion distribution to the membrane potential when the cell is at rest.  The equation is:

E = 2.3 (RT/zF) (log [C_out]/[C_in])  which, under standard conditions for sodium and potassium, simplifies to

E = 61 log [C_out]/[C_in]

for an example:   E_K+ = 61 x log [5/150] = 61 x -1.477 = -90 mV

Thus, if the ion distribution is know, the membrane potential can be calculated and it reflects the measure of the electrical gradient that counter balances the concentration gradient.  Potassium is negative - to resist the tendency to loose potassium in response to the concentration gradient; sodium is positive - to counter the tendency of sodium to enter the cell following it concentration gradient.

III. Excitability

A.  General
    Some cells are excitable, that is, they can change their membrane potential. This occurs when the membrane depolarizes (switches polarity - the inside becomes positive relative to the outside) or hyperpolarizes (becomes more negative inside relative to outside).  A nerve impulse is simply a rapid, transient, change (reversal) in neuronal membrane permeability.

B.  Excitable cells in plants
    Few plant cells are excitable.  Those that are are generally non-specific. 

C. Excitable cells in animals

1.  Neurons - conduct nerve impulses

• dendrites – short, highly branched, receive signal and carry it toward cell body
• axons – long, carry signal away from cell body; terminal branches
• cell body – large section, contains the nucleus and other goodies
• myelin – lipid coating derived from membrane of Schwann cells; acts as a kind of insulation
• Nodes of Ranvier – gaps between Schwann cells (see below)
• synaptic terminal – end of the axon, junction with another neuron or effector
• neuron form/function related - see diagram/overhead of different neurons

2.  Supporting, or glial, cells (1-3 not on exam)
    The function of these cells is to structurally reinforce, protect, insulate or otherwise assist the neurons.  Some even serve as nerve housekeeper cells.  There are lots of glial cells; in fact, in the brain there are 10-50x more glial cells than neurons. Glial cells include:

1. Astrocytes (star-shaped; they have many functions including - line capillaries, part of the blood brain barrier which serves to keep toxins from the brain, phagocytosis)
2. Microglia - phagocytic cells, remove cellular debris
3. Oligodendrocytes (wrap and insulate nerves). 
4. Schwann cells – wrap around a neuron like a "jelly roll".  Have a myelin (fatty substance) sheath which is like insulation on a wire.  Vertebrates;  outside CNS in peripheral nervous system

3.  Construction
    Neurons are bundled together to form nerves. 
    Ganglia - clumps of cell bodies

IV. Action Potentials

A. General - membrane polarity rapidly changes

• Large change in polarity (~100 mV)
• inside becomes temporarily positive relative to outside
• propagates along cell; strength doesn't diminish
• like "the wave" at a sports stadium
• Thus, a nerve impulse is rapid (lasts 1-2 msec), transient, depolarization of the membrane. It sweeps down the neuron like a wave (at a rate of about 100 m sec^-1). 

B.  Anatomy of an action potenial - spike, depolarization, hyperpolarization, undershoot, repolarization - see diagram.

C.  Action potentials are caused by ion movement across the membrane which changes the membrane polarity.  The ions move across the membrane through voltage-gated channels (or voltage-gated ion channels or voltage-activated ion channels).  These are membrane proteins that respond to changes in membrane potential/voltage.  In response, these gates permit the passage of sodium or potassium in/out of the cell.  Let's examine the voltage-activated gates of sodium and potassium.

Sodium Channel – this channel permits the diffusion of sodium. It has two gates: 

• Gate 1 (Activation Gate) - closed at rest; depolarization causes it to open rapidly
• Gate 2 (Inactivation Gate) - open at rest, depolarization causes it to close slowly. 
• At rest, sodium cannot pass through these gates because gate 1 is closed. 

Potassium Channel - this channel permits the diffusion of potassium 

• closed at rest
• depolarization causes it to open slowly
• At rest, potassium cannot pass through these gates because it is closed

D. Opening the gates.  Consider what happens when...

1. the potassium gate opens:    potassium gate open → potassium diffuses out → inside becomes more negatively charged, outside more positive → hyperpolarization (membrane potential becomes more negative)
    
2. the sodium activation gate opens:  sodium gate open → sodium diffuses in → cell interior becomes positive, outside more negative → depolarization. If the depolarization reaches ca. -50 to -55 mV it will trigger an action potential, which is an all-or-nothing response.
    

VI.  Mechanics    

A.  Phases - an action potential can be dissected into the following phases:

1. Resting phase
           Membrane potential -70 mV; potassium gate closed; sodium gate 1 closed; sodium gate 2 open. No "excess" movement of sodium or potassium across membrane
   
2. Depolarization phase
       Local depolarization of membrane occurs → membrane potential reaches threshold potential (-55 mV) → acts on ion gates.  Sodium gate 1 opens quickly → rapid entry of sodium into cell → cell depolarizes. Sodium gate 2 begins to close; it is slow so has little immediate effect. The potassium gate begins to open, it is slow so has little immediate effect.
   
3. Repolarization phase
       Sodium gate 2 now closed, prevents entry of additional sodium. Potassium gate now fully open, and allows loss of potassium. The net result is a loss of positive charge from inside the cell which causes the repolarization.
   
4. Undershoot
       Excess potassium may leak out of the cell causing a hyperpolarization of the membrane.

B.  Refractory period
    Time period following an action potential during which the cell gates are returning to their original positions. The cell cannot respond to another nerve impulse during this period.

C.  Nerve impulse
    Action potential wave spreads along the neuron. It doesn’t turn back on itself because of the refractory period.

D.  Rate of conduction
    In unmyelinated nerves, speed is directly related to the diameter of the axon.  The larger the neuron, the faster the rate.  Rates vary from about 1 – 10 m s^-1.  Since vertebrate neurons are rather skinny, how do they conduct impulses rapidly enough?  Answer:  Saltatory conduction; this occurs when action potentials jump between nodes of Ranvier. These occur much faster.  Voltage channels occur just in node area.

VII. Communication between adjacent nerve cells
    A few neurons are hard-wired (i.e., are directly connected to one another, as in gap junctions). These are called electrical synapses and important for really fast responses. However, most neurons are not in contact with one another, rather there is a gap (ca. 25 nm), called the synapse or synaptic cleft, between the adjacent cells. So, how does the nerve impulse get across the gap in these chemical synapses? Chemical signals = neurotransmitters.

Action potential arrives at synaptic terminal → stimulates voltage-gated calcium channels to open → uptake of calcium → stimulates migration of vesicles containing neurotransmitter to the membrane surface → vesicles fuse with the membrane releasing their contents into the gap → diffuses across gap → binds to appropriate receptor → opens gates for sodium and potassium [or stimulates a secondary messenger system see below)

A. Various neurotransmitters have been identified. 
    Many known. Acetylcholine is a common one involved in muscle contractions and other responses. Norepinephrine, serotonin, and dopamine are involved in mood, mental state, ADD, schizophrenia. Gamma-amino butyric acid (GABA) is in the spinal cord and brain.  The opiate types, endorphins and enkephalins are involved in pain perception.

B. Too much neurotransmitter is a bad thing. 
    In other words, the neurotransmitters must be removed after they do their job. Many are broken down by enzymes. For example, acetylcholinesterase breaks down acetylcholine. Others like serotonin are reabsorbed by the axon. Some drugs work by the inhibition of the reuptake of the neurotransmitter.  For example, cocaine prevents the reuptake of serotonin.

C.  Secondary messenger system.
    As mentioned, the action of the neurotransmitter may be mediated by a secondary messenger system.  In this case:  neurotransmitter → binds to the receptor protein → activates G proteins  → adenyl cyclase → converts ATP to cyclic AMP (cAMP) → activates kinases → phosphorylates proteins → closes potassium channel → depolarization

VIII.  Excitatory & Inhibitory Impulses
    Some neurons cause depolarization of membrane by allowing sodium uptake (i.e, motor neuron and skeletal muscle).  These are called excitatory post-synaptic potentials (ESPS).  Other neurons cause hyperpolarization (inhibitory) by permitting potassium loss and are called inhibitory post-synaptic potentials (ISPS).  GABA and glycine act in this manner.  Ultimately, the final response of a nerve is a sum of the excitatory and inhibitory responses received - typically at the axon hillock (at base of axon, not insulated by glial cells, many voltage gated channels) 

IX. Chemicals and Nerves
   These can exert their effect in a variety of places. For example, DDT interferes with the sodium/potassium pump; the anesthetics cocaine, lidocaine and procaine block sodium ion channels.  Some poisons mimic the effect of the naturally-occurring neurotransmitter (i.e., hallucinogenic drugs, muscarine).  Check out the PowerPoint in the Public Folder for a look at how THC & opiates impact neuron function.

X. Videoss/animations

• Action potential  

• Synaptic transmission