C2006/F2402 '09 Outline for Lecture 15 --  (c) 2009 D. Mowshowitz  -- Lecture updated 03/24/09.

Handouts: 15A & B. Not on line. Have diagrams for summation, sensors, (15A)  and IP3/Ca⁺⁺ as second messengers (15B).  Extra copies of class handouts are in boxes outside Dr. M's office; 7th floor Mudd.

Note on refractory periods: There is some disagreement between authorities on the timing of the refractory periods. According to handout 14A, and to Becker, the absolute refractory period comes right after the spike of the action potential. According to some,  the absolute refractory period coincides more or less with the spike; the relative refractory period follows after the spike. All agree about the underlying mechanism. The absolute refractory period corresponds to the time when the Na⁺ channels are inactivated (so depolarization is not possible). The relative refractory period corresponds to the time when the Na⁺ channels can be activated, but the voltage gated K⁺ channels are still open (so depolarization to threshold requires a larger stimulus).

A few Interesting links:
1991 Nobel Prize in Medicine for Patch Clamping (Neher & Sakmann)
1936 Nobel Prize in Medicine for Chemical Nature of Transmission at Synapses (Dale & Loewi)
1994 Nobel Prize in Medicine for discovery of G proteins (Gilman & Rodbell)
Two more animations by Steve Berg:  Action Potential and Voltage gated cation channel.

I. Nerve-Nerve Synapses, cont. -- What determines if a nerve impulse will be passed on to the next neuron? For nice overall pictures see Sadava 44.13 or Becker fig. 13-19. Some of this is review, but is included for clarity.

    A. Presynaptic Side -- Transmitters  -- See Sadava Table 44.1 & Becker fig. 13-18 (13-20).

1. One major type of transmitter per synapse (released from pre-synaptic side)

a. In CNS, many diff. transmitters. Usually amino acids or their derivatives. Major ones are glutamate (excitatory) & GABA (inhibitory).

b. In PNS usually norepinephrine (NE) or acetyl choline (AcCh).

2. One transmitter per Neuron. Usually only one major transmitter released by any neuron. Therefore one major transmitter released -- the same one -- at all synapses made by that neuron.

3. Release of Transmitter:

a. Location of Transmitter: Neurotransmitters are in vesicles (except for gaseous NTs)

b. Trigger for release: Action potential (AP) stimulates Ca⁺⁺ channel opening in plasma membrane, raising intracellular Ca⁺⁺

c. Exocytosis: High intracellular Ca⁺⁺ promotes exocytosis in general and release of transmitter.

4. Effects of transmitter (on target) can vary. Some transmitters are always excitatory or inhibitory; other transmitters vary in effect (depends on if you have one or more type of receptor for that transmitter -- see below.)

5. Getting rid of transmitters

a. Transmitter doesn't remain in synaptic cleft for long.

b. Different methods for getting rid of dif. transmitters

(1). Diffusion away from cleft

(2). Destruction by enzymes in cleft. For ex, acetyl choline esterase (AcChE) in cleft breaks up AcCh (Choline is reused). See Sadava fig.44.14

(3). Reuptake by transporters (secondary active transport) -- NE, serotonin removed from cleft by reuptake. (Endocytosis recovers membrane; transporters recover transmitters.)

    B. Post Synaptic Side -- PSP's = post synaptic potentials = small change in potential due to release of transmitter

1. Can be inhibitory (NT generates an IPSP) or excitatory (NT generates an EPSP)

a. Inhibitory -- causes hyperpolarization or stabilizes existing negative polarization due to opening of K⁺ or Cl^- channels. Either K⁺ goes out or Cl^- comes in.

b. Excitatory -- causes depolarization due to opening of cation channels; Na⁺  in>> K⁺ out.

c. Terminology -- a single IPSP or EPSP usually refers to the small change in potential due to release of transmitter caused by a single AP. The total PSP depends on the algebraic sum of multiple IPSP's and EPSP's as explained below.

2. Any given synapse is excitatory or inhibitory -- What determines it?

a. Receptor is crucial

(1). One type of receptor for neurotransmitter at each synapse.

(2). Receptor determines which kind of synapse it is -- excitatory or inhibitory. 

(3). Same neurotransmitter can be excitatory or inhibitory at different synapses.

(4). Receptor can be

(a) ionotropic (direct) -- receptor is itself an ion channel.

• Faster response.

• Ligand binding always opens channel.

• Not always excitatory -- depends on type of channel opened.

• Example -- the nicotinic AcCh receptor (see table below)

• For pictures see Sadava 44.17 or Becker 13-21 (13-23). Also see many pictures in both books of neuromuscular junction, or handout 14.

(b) metabotropic (indirect) -- receptor uses GPCR & 2nd messenger. 

• Response is slower.

• Effects can be more varied & more extensive.

• Can be used to open or close ion channels.

• Example: the muscarinic AcCh receptor & all adrenergic receptors (see table)

• For picture see Sadava 44.16.

(5) Agonists and antagonists are used as common tools to study receptors for both NTs and hormones. Some receptors named by their common ligand and most common agonist or antagonist. (For example, the nicotinic acetyl choline receptor. Ac Ch = NT; nicotine = agonist.) For more examples, see table at end of notes.

b. Overall: One major receptor/neurotransmitter pair per synapse.

        3. Major Types of Receptors in the PNS -- Reference & summary

*Both alpha and beta have subtypes that differ in location, mechanism & effect; same receptors used for epinephrine acting as a hormone or neurotransmitter.

** This is the receptor at the neuromuscular junction. See Becker fig. 13-21 (13-23). This is the AcCh receptor people mean when they talk about "THE" acetyl choline receptor.

Look at problem 8-16, part C. Assume you are looking at a standard synapse between two nerves that uses AcCh as a transmitter. What are the answers to parts 1-5? What effects will you expect on transmission at a standard synapse? (Note -- this problem as written does not refer to a standard synapse between two nerves. However the answers in the back of the book are still okay except for part 4. What is the right answer to part 4 for a standard synapse?)

4. Features of Total PSP's (To compare to AP's)

a. Total PSP's are graded  -- size is proportional to stimulus (as with receptor potentials, see below). Size is not all or none. (Unlike action potentials.)

b. PSP's are local -- die out if don't reach threshold. (Not regenerated like AP's.)

c. PSP's are caused by opening/closing of ligand gated channels. (What kind of channel is needed for AP's?)

To review IPSP's and EPSP's try problem 8-10.

    C. Post Synaptic Side -- Summation  -- See Handout 15A , bottom, or Sadava fig. 44.15 or Becker 13-22 (13-24).

a. Spatial summation: Multiple EPSP's delivered at different spots can add up → AP

b. Temporal summation: Multiple EPSP's delivered close enough together in time can add up → AP

c. Why you need summation:

    (1). A single EPSP is not enough to → AP

    (2). IPSP's & EPSP's are summed: net effect depends on both inhibitory and excitatory input. Remember there are about 1000 synapses (inputs) on body & dendrites of average neuron. See Becker fig. 13-22 (13-24). (See circuits next time for how you use this.)

Review Problem 8-8, parts A to H, and recitation problem 8-2. 

II. Sensors  See handout 15A, top. Sadava has a whole chapter on Sensory Systems. (Chapter # depends on which edition of the text you have.) Only a few general principles discussed here. See Sadava for examples & details.

    B. Receptor Proteins

    C.  Receptor Potentials

1. Response to stimulus is graded. Stimulus → local graded response. The more stimulus, the more channels open (or close), and the bigger the graded potential (bigger depolarization or bigger hyperpolarization) in the sensory/receptor cell.

2. Terminology -- graded response in receptor/sensory cell is called a generator potential or receptor potential.

    D. Receptor or Sensory Cells. Special cells with receptor proteins for detecting stimuli

1. Modified neuron  -- sensory cell is a modified neuron capable of generating AP itself. (See Sadava 45.2 & Handout 15A.) Example: sensory cells for detecting stretch or smell.

2. Cell that cannot generate an AP itself .

a. How get an AP? Sensory cell releases transmitter and triggers AP in next cell (a neuron). See Sadava 45.5 & handout. Example: sensory cells for detecting light, taste, sound and balance.

b. Type of cell. This type of sensory cell can be a modified neuron or epithelial cell.

Question to ask yourself: What type of channels does a cell need in order to generate an AP? If you are curious about the details of how the AP is generated, see notes of '08.

   E. All stimuli (whatever the modality) give same message to CNS (= AP's). If AP is all or nothing, how do you know which stimulus it was? And how much?

1. Number, frequency of AP's indicate length (duration) and strength (intensity) of stimulus.

2. Wiring (what part of brain is stimulated) = labeled lines = indicates location of stimulus and type (modality) of stimulus  -- taste, stretch, etc. If you get a punch in the eye, you set off light receptors. For a less violent example, take a very sharp pencil and tap your upper lip. What sensors did you trip off? (For contrast, tap your arm.)

III. How can one signal molecule (hormone, transmitter, etc.) produce different effects on different tissues?

      A . Two Basic Methods

1.  Using the Same Receptor & same 2nd messenger, but different Target Proteins

a. An example:

(1). In skeletal muscle: epinephrine causes glycogen breakdown.

(2). In smooth muscle of lung: epinephrine causes muscle relaxation.

b. Why does this make sense?

(1). Epinephrine (also called adrenaline) is produced in response to stress.

(2). In response to stress,  need to "mobilize" glucose -- release it from storage so it can be broken down to provide energy. Therefore need to increase glycogen breakdown (and decrease glycogen synthesis) in muscle (& liver).

(3). In response to stress, need to breathe more deeply. Therefore need smooth muscle around tubes that carry air (bronchioles) to relax.

c. How is this possible?  Same receptors, same 2nd messenger (cAMP) are used.

d. The solution: PKA is activated in both skeletal and smooth muscle. However the target proteins available to be phosphorylated are different in the two tissues. Therefore different proteins are phosphorylated and activated (or inactivated) in the two different tissue types.

(1). In skeletal muscle, PKA phosphorylates phosphorylase kinase,  glycogen phosphorylase etc, as on handout for review questions for exam #2.

(2).In smooth muscle surrounding the bronchioles, PKA phosphorylates a protein (MLCK) needed for contraction, inactivating it. Therefore contraction cannot occur.

  2. Using different receptors & second messengers in different cell types 

(See Becker fig. 14-24 (14-23). An example -- effects of epinephrine (adrenaline) on smooth muscle. Some smooth muscles relax, and some contract in response to epinephrine. In this case, different receptors & 2nd messengers are involved. How does this work? See below.

Try problem 6-11.

  B. Example of Using Different Second Messengers (& Different Receptors).

1. The phenomenon:

a. Epinephrine (secreted in response to stress) has different effects on different smooth muscles:

           (1).  On some smooth muscles, epi → contraction
 
           (2). On other smooth muscles, epi →  relaxation (as above)

b. How does this make sense?

(1). In peripheral circulation, smooth muscles around blood vessels (arterioles) contract, diverting blood from peripheral circulation to essential internal organs

(2). In lungs, smooth muscles around tubes carrying air (bronchioles) relax, so lungs can expand more and you can breathe more deeply.

2. How Ca⁺⁺ fits in:

a. Ca⁺⁺ stimulates muscle contraction.

b. Epinephrine binds to receptors on some smooth muscles  (ex: around arterioles) → Ca⁺⁺ released from ER → intracellular Ca⁺⁺ up → stimulates contraction. 

c. Epinephrine binds to receptors on some smooth muscles (ex: around bronchioles)  → phosphorylates protein needed for response to Ca⁺⁺,  preventing response.

3. Role of receptors

a. Two basic types of epinephrine  receptors -- called alpha and beta adrenergic receptors (adrenergic = for adrenaline). The two types are distinguished (primarily) by their relative affinities for epinephrine (adrenaline) and norepinephrine (noradrenaline).

b. Some types of smooth muscle have mostly one type of receptors; some the other. (See table below and table above for details of receptor properties.)

c. Two types of receptors activate different G proteins and generate different second messengers. (See Handout 15B. )

(1). Beta receptors → G protein of one type (G_s)→ activates enzyme (Adenyl cyclase)  → second messenger (cAMP) → PKA

(2). Alpha₁  receptors → different G protein (G_p) → activate different enzyme (phospholipase C or PLC) → different second messenger (IP3) → binds to receptors on ER membrane → opens Ca⁺⁺channels in ER → Ca⁺⁺ release from ER → contraction

4. How does this all work to allow appropriate response  to stress (epinephrine)?

a. Beta type receptors. Beta receptors are found in lung tissue in smooth muscle surrounding bronchioles.
Stress (pop quiz, lion in street, etc.)→ epinephrine → muscles relax → bronchioles dilate →  deeper breathing → more oxygen → energy to cope with stress. 

b. Alpha type receptors. Alpha receptors are found in smooth muscle surrounding blood vessels of peripheral circulation.
Stress → epinephrine → muscles contract → constrict peripheral circulation → direct blood to essential organs for responding to stress (heart, lungs, skeletal muscle).

To review effects of different receptors, try problems 6-21 & 6-22. Note that you do not need to know the details of how IP3 is generated. If you are curious, see Becker fig. 14-10 or class handout.

5. Medical Uses of all this.

Epinephrine can be used during an asthmatic attack to relax bronchi and ease breathing. Overuse of this type of broncho-dilator eases breathing temporarily but masks underlying problem (inflammation of lung tissue) and can have additional serious long term effects (from overstimulation of heart which also has beta receptors). Heart and lungs have slightly different types of beta receptors, so drugs (agonists) have been developed that stimulate one and not the other (unlike epinephrine).  Many drugs either imitate or block the effects of signaling molecules such as hormones, transmitters, etc.

• Agonist = mimic of hormone or ligand; binds to receptor and has same effect as ligand.
• Antagonist = blocker of effect of hormone or ligand; binds to receptor (and prevents binding of normal ligand) but does not activate the receptor.

Try Problem 6-8 & 6-9 if not yet. (To review agonists & antagonists.)

6. Summary of epinephrine effects on smooth muscle (in lung vs peripheral circulation)

Note: There are more than two types of epinephrine receptors on smooth muscle cells, so epinephrine may affect smooth muscle in other tissues in other ways.  (There are subtypes of alpha and subtypes of beta.)

* Details of how PLC generates IP3 are in texts and on class handout 15B. We will go over this in class if time.