C2006/F2402 '07 -- Outline for Lecture 24

(c) 2007 Deborah Mowshowitz . Last updated 04/26/2007 03:42 PM.  

Handouts 24A to 24C.  (Not on web.) 24A = Muscle AP's; 24B = Heart Structure & Regulation of Blood Pressure; 24C = Cells involved in immune system; clonal selection

I. Wrap up of muscle contraction

1. Where does Ca⁺⁺ comes from?

2. What is affected by Ca⁺⁺, actin or myosin?

3. What protein binds Ca⁺⁺?

    B. How Ca⁺⁺ triggers contraction in Smooth Muscle

    C. How does Ca⁺⁺ trigger contraction in skeletal muscle? See Becker fig. 16-23 & 21 (23-20 & 21) or Purves 47.5 (47.9)

1. Overall: Ca⁺⁺ is released from SR (ER).

2. Presynaptic side: AP comes down motor neuron → releases transmitter (AcCh)

3. Post synaptic side -- events at membrane/motor endplate:

a. AcCh binds to nicotinic receptors on motor endplate

b. Depolarization of muscle membrane = EPP (end plate potential)

c. One AP in neuron → One EPP = sufficient depolarization to trigger AP in membrane of muscle fiber (One EPSP is not sufficient to trigger an AP in postsynaptic neuron.)

4. T tubules & SR

a. AP in muscle plasma membrane (sarcolemma) spreads to T tubules. For a picture, click here.
For another picture that shows another aspect, click here.

b. AP in T tubule → Ca⁺⁺ release from SR. Changes in membrane potential in T tubule → change in shape of protein in T tubule membrane → opening of channels in SR (SR = sarcoplasmic reticulum = ER of muscle cell)  → release of stored Ca⁺⁺

Try problems 9-2 & 9-4.

   D. Twitches and Contractions

1. What's a twitch = 1 contraction = response to one EPP; measured by force exerted by muscle fiber when it contracts.

2. Twitches are summed. See Purves 47.7 (47.11) and handout 24-A.

a. Twitch lasts longer than muscle membrane AP

b. Second AP can trigger twitch before first is over →  more contraction (shortening)

c. Tetanus: Multiple AP's can → fully contracted muscle that stays contracted = tetanus (requires continual splitting of ATP to maintain contraction).

3. Speed. Speed of twitch depends on multiple factors; for details (FYI) see notes of last time.

1. ATP must be split to run bridge cycle in all types of muscle. In smooth muscle, ATP is needed, in addition, to activate myosin using myosin kinase.

2. Myosin (not actin) has the ATPase activity. The catalytic site that splits ATP during the bridge cycle is in the myosin head (myosin is the 'motor' molecule).

3. Speed of cycle. The bridge cycle is similar in smooth and skeletal muscle, but speed of cycle is much slower in smooth muscle. In smooth muscle, cross bridges stay intact longer.

II. Heart Structure/function

    A. Two Types of cardiac muscle cells

1. Contractile cells

a. Bridge cycle etc. much like skeletal.

b. FYI: Similar to oxidative/slow twitch skeletal (see notes of last time) -- low fatigue rate but very oxygen dependent.

c. Cells are coupled electrically (gap junctions at intercalated disks)

d. Special features of AP in membrane (see handout 24-A)

(1). AP lasts much longer (as long as contraction) so tetany is impossible. Each contraction ends before next AP arrives. (see fig. 14-15 on handout & 47.7  & 49.8 (47.11) of Purves)

(2). Prolonged AP (long depolarized phase) is due to delay in opening slow voltage gated K⁺ gates and opening of Ca⁺⁺ channels. (see fig. 14-14 on handout & 49.8 in Purves.)

e. Role of Pacemakers.

(1). Trigger for contraction is signal from pacemaker cells of heart, not from AP of nerve.

(2). Contractile cells do not have pacemaker activity.

2. Pacemaker cells -- see handout 24A (fig. 14-16) or Purves 49.6 (49.7).

a. Have pacemaker activity -- Fire spontaneously

b. Mechanism of pacemaker activity: Depolarize slowly to threshold → pacemaker potential → AP when reaches threshold. (See notes of last time.)

c. Set pace of heart beat -- Autonomic neurons release transmitters that slow or speed up pace; discussed last time.

d. Special Features of AP -- AP (spike in potential) in pacemaker cells is largely due to inrush of Ca⁺⁺ not  Na⁺.  (see fig. 14-6, panel (c) on handout 24A). When cells depolarize to threshold, voltage gated Ca⁺⁺ channels, not voltage gated  Na⁺ channels, are opened.

3. What accounts for differences in function between the two types of cardiac cells? Have different channels. Look at handouts to see how differences in electrical properties correlate with differences in channels, ion flows, etc.

See Problems 11-1 & 11-2.

    B. Structure of heart --  Where are the contractile and pacemaker cells? See handout 24B, Purves Fig. 49.3 (49.4). See also 19A for 'subway diagram' version.

    C. Position, function of pacemaker cells (nodes), bundle of His, Purkinje fibers  -- see Purves fig. 49.7 (49.8) & handout 24B.

1. All these cells have pacemaker activity -- make up the conduction system -- carry the AP to all parts of the heart. Note these cells are muscle, not nerve.

2. SA node usually in charge. SA node has the fastest firing rhythm -- normally controls heart beat. Fires first.

3. Role of AP in SA node. Causes atria to contract, pushing blood into ventricles. Causes AV node to fire after a short delay

4. AP in AV node spreads to bundle of His and Purkinje fibers

5. Bundle of His etc. causes ventricles to contract, from bottom up, pushing blood out top of heart. 

    D. Overall view of circulation  -- see handout 19A and Purves p. 945 (870).

See problems 11-3 to 11-5.

III. Summary of regulation of blood pressure (see  handout 23B. & Purves figs. 49.19 & 49.20 [46.20 & 46.21] )

    A. Co-ordination of control

1. Major circuit that controls BP

a. IC = cardiovascular control center in medulla -- part of brain stem (Purves 49.20 [46.21]).

b. sensors = stretch receptors (= baroreceptors) in major arteries

c. effectors = heart, peripheral blood vessels (constrict/relax)

d. circuits -- uses PS and S.

See problems 11-10 & 11-16.

2. IC has other inputs and outputs

IV. Specific (= Acquired) Immune Response -- Major cells and Features

We have already discussed antibodies as chemical reagents. How do antibodies, and the entire immune system, really work  physiologically?

    A. Specific Immune system has 2 branches

1. In both branches:  Cells make a specific protein that binds to a foreign substance = antigen. Protein and antigen match up like ligand and receptor (or enzyme and substrate). Binding of specific protein to its target antigen is specific, and leads to destruction of target.

2. Humoral response -- Specific cell protein is an antibody. Why 'humoral?' Binding and destruction of antigen done by proteins in "humors" = antibodies in blood and secretions (for ex. milk, tears).

Example: B cells → release antibody → Ab (antibody) binds Ag (antigen -- usually on surface of microbe) → trigger destruction of microbes (microbes are engulfed by phagocytes or lysed) often with the help of a set of proteins called complement. (See Purves 18.11 (19.12) & table below.)  Allergies are a side effect of this system.

2. Cellular or cell-mediated response -- Specific cell protein is on surface of T cells, not released. Protein is called a TCR (T cell receptor). Binding and destruction of antigen done by whole cells bearing a TCR.

Example: T cells → TCR on surface; TCR's (of cytotoxic T cells) bind to Ag on surface of virus infected eukaryotic cell → destroy target cell by triggering apoptosis. (Apoptosis is triggered by juxtacrine signaling and/or perforin. See table below.) This is probably why grafts fail; foreign cells of graft look like infected (defective?) cells and are destroyed. 

    B. What Cells are involved?  What are B cells and T cells? See handout 24C. White blood cells (leukocytes) -- contain no hemoglobin. WBC divided into two main types

    C. Table: Summary of Major features of 2 branches of specific immune system. Any features not covered yet will be covered later; this is here for reference. See notes after the table.

Notes:
*T cell receptor is NOT the receptor for T cells -- it is the protein on the T cells that is the receptor for an antigen. It is the receptor of T cells, not the receptor for T cells. 

** Complement = a series of proteins found in blood. Activation of complement involves a cascade of activations similar to that involved in blood clotting. Complement binds to antibody-antigen complexes attached to microbes and triggers phagocytosis or lysis of the microbe bearing the complex.

^# Antigen must be attached to a euk. cell surface protein called MHC. (Details later.)

^##Cytotoxic T cells use proteins called perforins to make holes in their targets. Then other proteins enter the holes and trigger apoptosis.  Note complement is similar to perforins but works on prokaryotic invaders; perforins work on rogue eukaryotic cells. (See Purves 18.14 (19.15)) Many texts say perforin lyses cells -- it makes holes in membrane, and then water enters, causing cells to swell and burst. (This is the way complement kills bacteria.) Newer data indicates perforin works to trigger apoptosis.

    D. What are the Important Features of the Immune Response that need to be explained?

1. Specificity & Diversity -- each Ab or TCR is directed against one epitope or antigenic determinant (= piece of antigen -- see Purves 18.6 (19.6), and there are many, many different antigens. How can you make so many different Ab's or TCR's, each specific for a particular antigen or piece of it?

2. Memory -- secondary response is faster, larger, better than primary response. In secondary response, make more Ab, Ab is more effective (binds better to Ag because of slight changes in amino acid sequence of Ab), and Ab response lasts longer. (Purves 18.8 (19.8)) How is this done?

3. Tolerance -- can distinguish self/nonself or normal/abnormal -- make Ab only to foreign/abnormal stuff (except in disease states). TCR only directed against infected cells, not normal ones. How does this work?

4. Response is adaptable -- response depends on amount and type of antigen. How do you "know" which antibody (or TCR) to make in response to a particular antigen?

5. How do helper T cells fit in? How do helper T's and cytotoxic T's distinguish their targets?

3. Role of helper T cells -- needed for function of both B and cytotoxic T cells

V. Clonal Selection -- How do you account for the "important features" listed above?

    A. B cells (See Handout 24C bottom = Purves fig. 18.7 (19.7))

1. Each cell differentiates → produces a single type of Ab on surface ("virgin" or "naive" B). Each cell rearranges its DNA during differentiation, so each cell has a unique set of Ab coding genes and makes a unique antibody -- that is, with a unique set of "grabbers."

Note: As B cells mature and specialize, changes in the antibody they make may occur because of alternative splicing and/or additional rearrangements of the DNA. This is why Ab made in secondary response is better at binding Ag. (More on this next time.)

2. Ab on surface of cell acts as a "trap". Surface antibody (also called BCR or B cell antigen receptor) acts as trap/receptor for Ag.

3. Activation or destruction of B cell is triggered by binding of Ag to surface Ab (BCR)

a. Destruction. If Ag is perceived as "self" → cell destroyed or suppressed (→ tolerance).

b. Activation. If Ag is perceived as foreign → cell divides → clonal expansion, further differentiation into

(1). Effector cells -- short lived but secrete lots of Ab → destroy or inactivate targets; class of Ab determines fine points. (In earlier lecture we explained how alternative splicing can allow cell to switch from making surface bound Ab to secreted Ab.)

(2). Memory cells -- long lived and more specialized to make Ab; wait for next time (responsible for memory). 

c. Whether antigen is perceived as "self" or "foreign" depends on time of exposure to the antigen (embryonic vs adult) and additional factors. (This turns out to be very complicated, so we are ignoring the "additional factors.")

4. What's the point?

a. Clonal Selection: Each cell makes a little Ab before any Ag present. Each cell makes a different Ab. This antibody stays on the cell surface and acts as BCR = trap for antigen. Ag acts as a trigger -- binding of Ag to "trap" stimulates only those cells that happen to make Ab that binds to that particular trigger.

b. Clonal expansion: The cells triggered by binding of Ag grow and divide →  (more) effector cells  & memory cells . Both types of cells make only the antibody that binds to the trigger Ag.

c. Clonal suppression: The cells triggered by binding of Ag are destroyed or suppressed (prevented from multiplying &/or making Ab.)

d. What does this explain?

1. Clonal selection is the part that accounts for specificity, diversity, and adaptability.

2. Clonal expansion and suppression are the parts that account for memory & tolerance -- memory when Ag triggers expansion (as in b) , and tolerance when Ag triggers destruction or suppression (as in c) .

5. Why do you need helper T cells? For most antigens, helper T must bind to B cell-Ag complex in order to activate B cell. (Activated cell makes secreted Ab.)

Try Problem 13-4.

    B. T cells -- similar process as with B cells -- DNA rearrangement occurs so one type of protein with unique binding site made per cell -- but there are differences. More details next time.

    C. Clonal vs. Natural Selection. Note how clonal selection and natural selection compare. In both cases, need to have many variants (diff. antibodies or dif. organisms) to be able to respond to unpredictable environmental challenges. How is this done? In both cases, make many variants and conditions select (promote propagation of) cells making the few suitable Ab (or carrying out a rare, useful function); the rest are wasted. Random generation of variants seems wasteful, but is the biological solution to preparing for change without conscious planning ahead.