C2006/F2402 '10 -- Outline for Lecture 18 --  (c) 2010 D. Mowshowitz  -- Lecture updated 04/04/10

Useful Web Sites & Animations

• Sliding filaments.  See diagram of sarcomere, with corresponding electron micrograph. (Go to pp.3 & 4)  By Michael Ferenczi.
• Actin-myosin crossbridge.  Watch the myosin head swivel, attaching and detaching from actin. From San Diego State University.
• Muscle structure and contraction

I. Signaling with Lipids (PI Derivatives) -- Wrap up of handout 17C

    A. Many inositol derivatives are involved in signaling -- this is a current hot subject of investigation.

1. What is PI? Phosphatidyl inositol (PI) is a membrane lipid; inositol part faces cytoplasm.

2. Role of Kinases

a. Kinases add additional phosphates to hydroxyls on PI.

b. Many different products are possible, as PI has 5 free hydroxyls. Examples:

• PIP2 or PI 4,5 bisphosphate-- has phosphate added to 4 & 5 positions.
• PIP3  -- has phosphates on 3, 4, & 5 positions.

c. Activation of kinases (to add phosphates) and phosphatases (to remove the added phosphates) is regulated by hormones, growth factors, etc.

3. What does phosphorylated PI do?

a. Generates second messengers: Phosphorylated PI (for example, PIP2) can be split to generate second messengers, such as IP3 and DAG. The second messengers bind to and activate enzymes.

b. Recruitment: Phosphorylated PI (for example, PIP3) can recruit some enzymes to the membrane; binding to the modified PI  activates the enzymes (no second messengers).

   B. Details for DAG/IP3/Ca⁺⁺ pathway. See handout 17C & Becker figs. 14-9 & 14-10 or Sadava 15.13 (15.11)

1. How IP3 and DAG are generated

a. Activated G protein binds to phospholipase C (PLC)

b. PLC cleaves PIP2 in membrane → two parts. For structures see handout 15B and Becker fig. 14-9.

(1). PIP3  -- soluble, in cytoplasm; also known as InsP3

(2). DAG = diacyl glycerol = glycerol with two fatty acids. DAG remains in membrane.

2.  Role of IP3/Ca⁺⁺

a. IP3 opens Ca⁺⁺ channels in the ER, raising Ca⁺⁺ in cytoplasm. (Becker fig. 14-11 for IP3 effect; fig 14-12 for overall Ca⁺⁺ regulation.)

b. Ca⁺⁺ acts alone or as complex with calmodulin (a protein) -- complex (calmodulin-Ca⁺⁺) or Ca⁺⁺ alone binds to and alters activity of many proteins. See Becker fig. 14-14 (14-13) for pictures of calmodulin.

c.  Ca⁺⁺ affects many processes -- sometimes called "3rd messenger." For example, changes in [Ca⁺⁺]

• can rigger exocytosis (& secretion)
• can trigger muscle contraction
• are involved in egg fertilization (see Becker fig.14-13 (14-14) or Sadava  15.14 (15.12) for some nice pictures). 

3. Role of DAG

a. DAG (in membrane) activates protein kinase C (= PKC, not to be confused with PLC).

b. "PKC" is a family of related enzymes involved in many different processes -- act by phosphorylating other proteins. If you are interested, see Becker or advanced texts for details. Some PKC's require Ca⁺⁺.  PKC and PKA have different target proteins.

Try problems 6-5, 6-10 & 6-16.
 

II. How Nervous System is Organized

    A. Simple circuits  -- see handout 18A, bottom or Sadava fig. 46.3

1. One synapse, 2 neurons -- monosynaptic circuit -- how sensory neuron signals an effector.

2. Circuit with multiple synapses -- how antagonistic muscles are controlled. For example, when stretch is detected in a muscle, that muscle contracts and the antagonistic muscle relaxes.

a. Signal to skeletal muscle is always +; a signal (+) means contract; no signal means relax.

b. Signal to neuron (than controls muscle) can be excitatory or inhibitory.

c. What makes the (antagonistic) skeletal muscle relax?

• Relaxation occurs because the antagonistic muscle gets no signal to contract.

• The muscle does not get an inhibitory signal -- it just doesn't get an excitatory signal.

d. Who gets an inhibitory signal? The motor neuron that innervates (goes in to) the antagonistic muscle gets an inhibitory signal, not the muscle itself. The motor neuron is inhibited and does NOT fire an AP, so the muscle is not triggered to contract.

3. Role of brain -- adds up/down (as vs. in/out) component

4. FYI: Where is all this located? see Sadava fig. 46.3; will not be discussed in class.

    B. How is NS organized overall? See handout 18A top, Becker 13-1 (6th ed. only) or Sadava fig. 46.1

1. CNS = brain + spinal cord

2. PNS -- Names of Divisons

a. Afferent vs Efferent.

(1) Afferent = carrying info into the CNS
(2) Efferent = carrying info away from the CNS

b. Efferent subdivided into: Somatic vs autonomic

(1) Somatic = controls skeletal muscle
(2). Autonomic = controls everything else

c. Autonomic subdivided into: Parasympathetic (PS) vs Sympathetic (S)

Try problem 8-8, part I.

    C. How do PS and S co-operate? (See Sadava fig. 46.10) What do they do?

1. What do they innervate?

a. Many organs innervated by both

b. Some organs innervated (stimulated) by only one

(1). sweat glands -- S only

(2). tears -- PS only

2. What results does stimulation produce?

a. Not always S excites; PS inhibits. Ex: salivation -- S inhibits; PS excites

b. Usually:

(1). S → response needed in a crisis

(2). PS → response needed in relaxed state. ("para makes you pause")

c. Examples:

(1). S → heart rate up; liver releases glucose; bladder relaxes (to hold more);

(2). PS → heart rate down, digestion, salivation up.

    D. General Set up of wiring of efferent PNS -- see handout 18B

1. First neuron -- same in Somatic and Autonomic

a. Location -- body in CNS

b. Neurotransmitter -- releases AcCh (Acetyl choline)

c. Receptor -- AcCh receptor (on effector/next neuron) is nicotinic (direct)

d. Effect -- effect of transmitter on next cell is always excitatory

2. Second neuron (post ganglionic) -- found in autonomic only

a. Location -- Body in ganglion (outside CNS).

b. Neurotransmitter

(1). Parasympathetic -- releases AcCh

(2). Sympathetic -- usually releases NE (norepinephrine)

c. Receptor (on effector) -- indirect (metabotropic)

(1). AcCh (cholinergic) receptor is muscarinic

(2). NE (adrenergic) receptor can be alpha or beta (see table in last lecture) -- 2nd messenger varies

(3). Effect of transmitter can be excitatory or inhibitory.

d. Adrenal medulla

        (1). Medulla ≡ ganglion -- but neurons have short axons. (Usual 'neuron #2' has long axon.)

        (2). Release neurotransmitter (mostly E = epinephrine) from end of short axons (within medulla).

        (3). E goes into blood, so E acts as neuroendocrine instead of neurotransmitter.

Try problem 8-8 part J.

    E. Major receptors in the PNS -- Reference & summary -- See table in last lecture.

III. Muscle Overview  -- See handout 18B

The major features of smooth and skeletal muscle are outlined separately below, but actual lecture may skip back and forth. Details of cardiac muscle, skeletal muscle contraction, & anything on smooth muscle we don't get to, will be covered next time.

1. Structure -- See handout 18B

a. Cells Fused?

    (1). No -- Smooth muscle -- individual, unfused cells.

    (a). Single Unit smooth muscle -- the cells are connected by gap junctions and contract as a unit -- are electrically coupled.

    (b). Multi unit smooth muscle -- the cells are not coupled electrically; cells are stimulated and contract as multiple individual units.

    (2) Fused at ends -- cardiac

    (3) Fused, multinucleate  -- skeletal

b. Striated vs. not. How are actin/myosin fibrils arranged? Bundles or sarcomeres?

c. Structure of nerve/muscle synapse. Details for nerve-smooth muscle synapse next time; see handout 16B, or text, for diagram of nerve-skeletal muscle synapse. 

2. What controls contraction?

a. Skeletal Muscle

(1). Innervated by somatic system

(2). Signal is always excitatory.

(3). Neurotransmitters at nerve/muscle synapse use nicotinic receptors (direct -- ionotropic).

(4). Stimulus generates an AP in the muscle membrane, which causes release of Ca⁺⁺ from ER. (ER called sarcoplasmic reticulum or SR in muscle.)

Note: Skeletal muscle does get hormonal stimulation (has receptors for hormones), but hormones do not affect contraction.

b. Smooth Muscle

(1). Innervated (not enervated) by autonomic neurons. See handouts. 

(2). Contraction influenced by hormones as well as autonomic neurotransmitters

(3). Stimulus can be excitatory or inhibitory.

Examples in previous lectures: epinephrine can cause smooth muscle contraction (through IP3) or relaxation (through cAMP). Response depends on receptors on smooth muscle.

(4). Stimulus doesn't always generate an AP in muscle membrane.

(5). Pacemaker activity, not external stimulus, controls contraction in some smooth muscles. See below.

(6). Ca⁺⁺ to trigger contraction comes from outside cell &/or ER.

(7). Has latch state; unlike striated muscle. Can remain contracted longer without input of ATP.

c. Cardiac Muscle

(1). Pacemaker controls contraction. (Remember the Loewi experiment -- isolated heart beats without any innervation.)

(2). What is pacemaker activity? Cell membrane gradually depolarizes without any external stimulus. (No stable RMP.)

(3). Depolarization caused by opening/closing of ion channels.

(4). It's a Cycle: When depolarization reaches threshold, cell fires an AP. Membrane then hyperpolarizes, and depolarization starts again.

(5). Autonomic NS can affect pacemaker. (As shown in the Loewi experiment.) Transmitters can effect opening/closing of channels and thereby alter time required to reach threshold. (More details another time.)

(6). All cardiac muscle and some smooth muscle has pacemaker activity. Some cells in each case, not every single cell, have pacemaker activity.

Details: Not all individual cardiac muscle cells or all individual smooth muscle cells (in single unit muscle) have pacemaker activity. Only a few specialized cells act as pacemakers. All cardiac muscle and some smooth muscle will contract without nerve input because these muscles contain pacemaker cells. The pacemaker cells fire APs simultaneously, and this stimulates the other cells, the contractile cells (that do not have pacemaker potentials), to contract.

3. Control of bridge cycle -- all use Ca⁺⁺, but details differ. Main differences between striated and smooth muscle that are summarized here for reference; will be discussed later:

a. What protein binds Ca⁺⁺? Calmodulin or troponin?

b. What protein is altered to allow contraction -- actin or myosin?

c. When actin or myosin is altered, what is nature of change? Change in conformation or in state of phosphorylation?

d. Where does the Ca⁺⁺ come from? Primarily extracellular or ER?

        e. Is ATP needed to maintain contraction?

At this point, it helps to start making a table that summarizes all the similarities and differences between the 3 types of muscle. Fill it in as you go -- add to it as you find out more about the structure and function of the different types.

IV. Smooth muscle

    A. What use is smooth muscle? What unique properties does it have?

1. Location -- much of it makes up walls of hollow organs & tubes. Maintains shape and pushes contents along.

2. Contraction speed & rate of fatigue -- relatively slow for both.

3.  Latch state possible -- can remain contracted for prolonged period without further input of ATP.  (One ATP split per bridge cycle, but cycle is much slower than for skeletal.)

4. Length over which it contracts/stretches  -- relatively long.

5. Contracts/relaxes in response to many different stimuli -- nerves, hormones, stretch, etc.

Overall: Can integrate multiple signals and maintain "tone" over wide range of length with economical use of ATP.

     B. Details of Structure Next Time