A heart rate that is persistently greater than 100bpm is termed tachycardia. A heart rate that is persistantly lower than 60 pulse per min  is termed bradycardia. Let's examine some factors that could cause a change in heart rate:

• Increased heart rate can be caused by:
  • Increased output of the cardioacceleratory center. In other words, greater activity of sympathetic nerves running to the heart and a greater release of norepinephrine on the heart.
  • Decreased output of the cardioinhibitory center. In other words, less vagus nerve activity and a decrease in the release of acetylcholine on the heart.
  • Increased release of the hormone epinephrine by the adrenal glands.
  • Nicotine.
  • Caffeine.
  • Hyperthyroidism - i.e., an overactive thyroid gland. This would lead to an increased amount of the hormone thyroxine in the blood.
• Decreased heart rate can be caused by:
  • Decreased activity of the cardioacceleratory center.
  • Increased activity of the cardioinhibitory center.
  • Many others.

• The Autonomic Nervous System (ANS) Controls the Body's Internal Environment in a Coordinated Manner

• The ANS helps control the heart rate, blood pressure, digestion, respiration, blood pH and other bodily functions through a series of complex reflex actions
• These controls are done automatically, below the conscious level
• To exert this control the activities of many different organs must be coordinated so they work to accomplish the same goal
• In the ANS there are 2 nerves between the central nervous system (CNS) and the organ. The nerve cell bodies for the second nerve are organized into ganglia:
  • CNS -> Preganglionic nerve -> Ganglion -> Postganglionic nerve -> Organ
• At each junction neurotransmitters are released and carry the signal to the next nerve or organ.

• The ANS has 2 Divisions, Sympathetic and Parasympathetic

   

• Comparison of the 2 systems:

• 	Anatomical Location	Preganglionic Fibers	Postganglionic Fibers	Transmitter (Ganglia)	Transmitter (Organs)
  Sympathetic	Thoracic/ Lumbar	Short	Long	ACh	NE
  Parasympathetic	Cranial/ Sacral	Long	Short	ACh	ACh

   

  The Sympathetic is the "Fight or Flight" Branch of the ANS

• Emergency situations, where the body needs a sudden burst of energy, are handled by the sympathetic system
• The sympathetic system increases cardiac output and pulmonary ventilation, routes blood to the muscles, raises blood glucose and slows down digestion, kidney filtration and other functions not needed during emergencies
• Whole sympathetic system tends to "go off" together
• In a controlled environment the sympathetic system is not required for life, but it is essential for any stressful situation

• The Parasympathetic is the Rest and Digest Branch of the ANS

• The parasympathetic system promotes normal maintenance of the body- acquiring building blocks and energy from food and getting rid of the wastes
• It promotes secretions and mobility of different parts of the digestive tract.
• Also involved in urination, defecation.
• Does not "go off" together; activities initiated when appropriate
• The vagus nerve (cranial number 10) is the chief parasympathetic nerve
• Other cranial parasympathetic nerves are: III (oculomotor), VII (facial) and IX (glossopharyngeal)

• The Hypothalamus Has Central Control of the ANS

• The hypothalamus is involved in the coordination of ANS responses,
• One section of the hypothalamus seems to control many of the "fight or flight" responses; another section favors "rest and digest" activities

• The Adrenal Medulla is an Extension of the Sympathetic Nervous System

• The adrenal medulla behaves like a combined autonomic ganglion and postsynaptic sympathetic nerve (see diagram above)
• Releases both norepinephrine and epinephrine in emergency situations
  • Releases a mixture of epinephrine (E = 80%) and norepinephrine (NE = 20%)
  • Epinephrine = adrenaline
• This action is under control of the hypothalamus

• Sympathetic & Parasympathetic Systems

• Usually (but not always) both sympathetic and parasympathetic nerves go to an organ and have opposite effects
• You can predict about 90% of the sympathetic and parasympathetic responses using the 2 phrases: "Fight or Flight" and "Rest and Digest".
• Special cases:
  • Occasionally the 2 systems work together: in sexual intercourse the parasympathetic promotes erection and the sympathetic produces ejaculation
  • Eye: the sympathetic response is dilation and relaxation of the ciliary muscle for far vision (parasympathetic does the opposite)
  • Urination: the parasympathetic system relaxes the sphincter muscle and promotes contraction of muscles of the bladder wall -> urination (sympathetic blocks urination)
  • Defecation: the parasympathetic system causes relaxation of the anal sphincter and stimulates colon and rectum to contract -> defecation (sympathetic blocks defecation)

• Organ	Parasympathetic Response "Rest and Digest"	Sympathetic Response "Fight or Flight"
  Heart (baroreceptor reflex)	Decreased heart rate Cardiac output decreases	Increased rate and strength of contraction Cardiac output increases
  Lung Bronchioles	Constriction	Dilation
  Liver Glycogen	No effect	Glycogen breakdown Blood glucose increases
  Fat Tissue	No effect	Breakdown of fat Blood fatty acids increase
  Basal Metabolism	No effect	Increases ~ 2X
  Stomach	Increased secretion of HCl & digestive enzymes Increased motility	Decreased secretion Decreased motility
  Intestine	Increased secretion of HCl & digestive enzymes Increased motility	Decreased secretion Decreased motility
  Urinary bladder	Relaxes sphincter Detrusor muscle contracts Urination promoted	Constricts sphincter Relaxes detrusor Urination inhibited
  Rectum	Relaxes sphincter Contracts wall muscles Defecation promoted	Constricts sphincter Relaxes wall muscles Defecation inhibited
  Eye	Iris constricts Adjusts for near vision	Iris dilates Adjusts for far vision
  Male Sex Organs	Promotes erection	Promotes ejaculation

   

Abnormalities of Salt, Water or pH

• Examples:
  • Hyperkalemia: caused by kidney disease & medical malpractice
    • High K+ in blood- can stop the heart in contraction (systole)
  • Dehydration: walking in desert- can lose 1-2 liters/hour through sweat
    • Blood becomes too viscous to circulate well -> loss of temperature regulation -> hyperthermia, death
  • Acidosis: many causes including diabetes mellitus and respiratory problems; can cause coma, death

Contractility : Means ability of cardiac muscle to convert electrical energy of action potential into mechanical energy ( work).
The excitation- contraction coupling of cardiac muscle is similar to that of skeletal muscle , except the lack of motor nerve stimulation. 

Cardiac muscle is a self-excited muscle , but the principles of contraction are the same . There are many rules that control the contractility of the cardiac muscles, which are:

1. All or none rule: due to the syncytial nature of the cardiac muscle.There are atrial syncytium and ventricular syncytium . This rule makes the heart an efficient pump.

2. Staircase phenomenon : means gradual increase in muscle contraction following rapidly repeated stimulation..

3. Starling`s law of the heart: The greater the initial length of cardiac muscle fiber , the greater the force of contraction. The initial length is determined by the degree of diastolic filling .The pericardium prevents overstretching of heart , and allows optimal increase in diastolic volume.

Thankful to this law , the heart is able to pump any amount of blood that it receives. But overstretching of cardiac muscle fibers may cause heart failure.

Factors affecting  contractility ( inotropism)

I. Positive inotropic factors:

1. sympathetic stimulation: by increasing the permeability of sarcolemma to calcium.
2. moderate increase in temperature . This due to increase metabolism to increase ATP , decrease viscosity of myocardial structures, and increasing calcium influx.
3. Catecholamines , thyroid hormone, and glucagon hormones.
4. mild alkalosis
5. digitalis
6. Xanthines ( caffeine and theophylline )

II. Negative inotropic factors:

1. Parasympathetic stimulation : ( limited to atrial contraction)
2. Acidosis
3. Severe alkalosis
4. excessive warming and cooling .
5. Drugs ;like : Quinidine , Procainamide , and barbiturates .
6. Diphtheria and typhoid toxins.

The Sliding Filament mechanism of muscle contraction.

When a muscle contracts the light I bands disappear and the dark A bands move closer together. This is due to the sliding of the actin and myosin myofilaments against one another. The Z-lines pull together and the sarcomere shortens

The thick myosin bands are not single myosin proteins but are made of multiple myosin molecules. Each myosin molecule is composed of two parts: the globular "head" and the elongated "tail". They are arranged to form the thick bands.

It is the myosin heads which form crossbridges that attach to binding sites on the actin molecules and then swivel to bring the Z-lines together

Likewise the thin bands are not single actin molecules. Actin is composed of globular proteins (G actin units) arranged to form a double coil (double alpha helix) which produces the thin filament. Each thin myofilament is wrapped by a tropomyosin protein, which in turn is connected to the troponin complex. 

The tropomyosin-troponin combination blocks the active sites on the actin molecules preventing crossbridge formation. The troponin complex consists of three components: TnT, the part which attaches to tropomyosin, TnI, an inhibitory portion which attaches to actin, and TnC which binds calcium ions. When excess calcium ions are released they bind to the TnC causing the troponin-tropomyosin complex to move, releasing the blockage on the active sites. As soon as this happens the myosin heads bind to these active sites.

Serum Proteins

Proteins make up 6–8% of the blood. They are about equally divided between serum albumin and a great variety of serum globulins.

After blood is withdrawn from a vein and allowed to clot, the clot slowly shrinks. As it does so, a clear fluid called serum is squeezed out. Thus:

Serum is blood plasma without fibrinogen and other clotting factors.

The serum proteins can be separated by electrophoresis.

• The most prominent of these and the one that moves closest to the positive electrode is serum albumin.
• Serum albumin
  • is made in the liver
  • binds many small molecules for transport through the blood
  • helps maintain the osmotic pressure of the blood
• The other proteins are the various serum globulins.
  • alpha globulins (e.g., the proteins that transport thyroxine and retinol [vitamin A])
  • beta globulins (e.g., the iron-transporting protein transferrin)
  • gamma globulins.
    • Gamma globulins are the least negatively-charged serum proteins. (They are so weakly charged, in fact, that some are swept in the flow of buffer back toward the negative electrode.)
    • Most antibodies are gamma globulins.
    • Therefore gamma globulins become more abundant following infections or immunizations.