The Body Regulates pH in Several Ways

• Buffers are weak acid mixtures (such as bicarbonate/CO₂) which minimize pH change
  • Buffer is always a mixture of 2 compounds
    • One compound takes up H ions if there are too many (H acceptor)
    • The second compound releases H ions if there are not enough (H donor)
  • The strength of a buffer is given by the buffer capacity
    • Buffer capacity is proportional to the buffer concentration and to a parameter known as the pK
  • Mouth bacteria produce acids which attack teeth, producing caries (cavities). People with low buffer capacities in their saliva have more caries than those with high buffer capacities.
• CO₂ gas (a potential acid) is eliminated by the lungs
• Other acids and bases are eliminated by the kidneys

Events in Muscle Contraction - the sequence of events in crossbridge formation:

1) In response to Ca²⁺ release into the sarcoplasm, the troponin-tropomyosin complex removes its block from actin, and the myosin heads immediately bind to active sites.

2) The myosin heads then swivel, the Working Stroke, pulling the Z-lines closer together and shortening the sarcomeres. As this occurs the products of ATP hydrolysis, ADP and Pi, are released.

3) ATP is taken up by the myosin heads as the crossbridges detach. If ATP is unavailable at this point the crossbridges cannot detach and release. Such a condition occurs in rigor mortis, the tensing seen in muscles after death, and in extreme forms of contracture in which muscle metabolism can no longer provide ATP.

4) ATP is hydrolyzed and the energy transferred to the myosin heads as they cock and reset for the next stimulus.

Excitation-Contraction Coupling: the Neuromuscular Junction  

Each muscle cell is stimulated by a motor neuron axon. The point where the axon terminus contacts the sarcolemma is at a synapse called the neuromuscular junction. The terminus of the axon at the sarcolemma is called the motor end plate. The sarcolemma is polarized, in part due to the unequal distribution of ions due to the Sodium/Potassium Pump.

1) Impulse arrives at the motor end plate (axon terminus) causing  Ca²⁺ to enter the axon.

2) Ca²⁺ binds to ACh vesicles causing them to release the ACh (acetylcholine) into the synapse by exocytosis. 

3) ACH diffuses across the synapse to bind to receptors on the sarcolemma. Binding of ACH to the receptors opens chemically-gated ion channels causing Na⁺ to enter the cell producing depolarization.

4) When threshold depolarization occurs, a new impulse (action potential) is produced that will move along the sarcolemma. (This occurs because voltage-gated ion channels open as a result of the depolarization -

5) The sarcolemma repolarizes:

a) K⁺ leaves cell (potassium channels open as sodium channels close) returning positive ions to the outside of the sarcolemma. (More K⁺ actually leaves than necessary and the membrane is hyperpolarized briefly. This causes the relative refractory period) (b) Na⁺/K⁺ pump eventually restores resting ion distribution.  The  Na⁺/K⁺ pump is very slow compared to the movement of ions through the ion gates. But a muscle can be stimulated thousands of times before the ion distribution is substantially affected.

6) ACH broken down by ACH-E (a.k.a. ACHase, cholinesterase). This permits the receptors to respond to another stimulus. 

Excitation-Contraction Coupling:

1) The impulse (action potential) travels along the sarcolemma. At each point the voltaged-gated Na+ channels open to cause depolarization, and then the K+ channels open to produce repolarization.

2) The impulse enters the cell through the T-tublules, located at each Z-disk, and reach the sarcoplasmic reticulum (SR), stimulating it.

3) The SR releases Ca²⁺ into the sarcoplasm, triggering the muscle contraction as previously discussed. 

4) Ca²⁺ is pumped out of the sarcoplasm by the SR and another stimulus will be required to continue the muscle contraction.

Functional Divisions of the Nervous System:

1) The Voluntary Nervous System - (ie. somatic division) control of willful control of effectors (skeletal muscles) and conscious perception. Mediates voluntary reflexes.

2) The Autonomic Nervous System - control of autonomic effectors - smooth muscles, cardiac muscle, glands. Responsible for "visceral" reflexes

Phases of cardiac cycle :

1. Early diastole ( also called the atrial diastole , or complete heart diastole) : During this phase :

- Atria are  relaxed
- Ventricles are relaxed
- Semilunar valves are closed
- Atrioventricular valves are open
During this phase the blood moves passively from the venous system into the ventricles ( about 80 % of blood fills the ventricles during this phase.

2. Atrial systole : During this phase :

- Atria are contracting
- Ventricles are relaxed
- AV valves are open
- Semilunar valves are closed
- Atrial pressure increases.the a wave of atrial pressure appears here.
- P wave of ECG starts here
- intraventricular pressure increases due to the rush of blood then decrease due to continuous relaxation of ventricles.

The remaining 20% of blood is moved to fill the ventricles during this phase , due to atrial contraction.

3. Isovolumetric contraction : During this phase :

- Atria are relaxed
- Ventricles are contracting
- AV valves are closed
- Semilunar valves are closed
- First heart sound
- QRS complex.
The ventricular fibers start to contract during this phase , and the intraventricular pressure increases. This result in closing the AV valves , but the pressure is not yet enough to open the semilunar valves , so the blood volume remain unchanged , and the muscle fibers length also remain unchanged , so we call this phase as isovolumetric contraction ( iso : the same , volu= volume , metric= length).

4. Ejection phase : Blood is ejected from the ventricles into the aorta and pulmonary artery .

During this phase :

- Ventricles are contracting
- Atria are relaxed
- AV valves are closed
- Semilunar valves are open
- First heart sound
- Intraventricular pressure is increased , due to continuous contraction
- increased aortic pressure .
- T wave starts.

5. Isovolumetric relaxation:  This phase due to backflow of blood in aorta and pulmonary system after the ventricular contraction is up and the ventricles relax . This backflow closes the semilunar valves .

During this phase :

- Ventricles are relaxed
- Atrial are relaxed
- Semilunar valves are closed .
- AV valves are closed.
- Ventricular pressure fails rapidly
- Atrial pressure increases due to to continuous venous return. the v wave appears here. 
- Aortic pressure : initial sharp decrease due to sudden closure of the semilunar valve ( diacrotic notch) , followed by secondary rise in pressure , due to elastic recoil of the aorta ( diacrotic wave)  .
- T wave ends in this phase

Proteins:

• about 50 - 60% of the dry mass of a typical cell
• subunit is the amino acid & amino acids are linked by peptide bonds
• 2 functional categories = structural (proteins part of the structure of a cell like those in the cell membrane) & enzymes

Enzymes are catalysts. Enzymes bind temporarily to one or more of the reactants of the reaction they catalyze. In doing so, they lower the amount of activation energy needed and thus speed up the reaction

 Pain, Temperature, and Crude Touch and Pressure

General somatic nociceptors, thermoreceptors, and mechanoreceptors sensitive to crude touch and pressure from the face conduct signals to the brainstem over GSA fibers of cranial nerves V, VII, IX, and X.

The afferent fibers involved are processes of monopolar neurons with cell bodies in the semilunar, geniculate, petrosal, and nodose ganglia, respectively.

The central processes of these neurons enter the spinal tract of V, where they descend through the brainstem for a short distance before terminating in the spinal nucleus of V.

Second-order neurons then cross over the opposite side of the brainstem at various levels to enter the ventral trigeminothalamic tract, where they ascend to the VPM of the thalamus.

Finally, third-order neurons project to the "face" area of the cerebral cortex in areas 3, 1, and 2 .

Discriminating Touch and Pressure

Signals are conducted from general somatic mechanoreceptors over GSA fibers of the trigeminal nerve into the principal sensory nucleus of V, located in the middle pons.

Second-order neurons then conduct the signals to the opposite side of the brainstem, where they ascend in the medial lemniscus to the VPM of the thalamus.

 Thalamic neurons then project to the "face" region of areas 3, I, and 2 of the cerebral cortex.

 Kinesthesia and Subconscious Proprioception

Proprioceptive input from the face is primarily conducted over GSA fibers of the trigeminal nerve.

The peripheral endings of these neurons are the general somatic mechanoreceptors sensitive to both conscious (kinesthetic) and subconscious proprioceptive input.

Their central processes extend from the mesencephalic nucleus to the principal sensory nucleus of V in the pons

The subconscious component is conducted to the cerebellum, while the conscious component travels to the cerebral cortex.

Certain second-order neurons from the principal sensory nucleus relay proprioceptive information concerning subconscious evaluation and integration into the ipsilateral cerebellum.

Other second-order neurons project to the opposite side of the pons and ascend to the VPM of the thalamus as the dorsal trigeminothalamic tract.

Thalamic projections terminate in the face area of the cerebral cortex.