The Nerve Impulse

When a nerve is stimulated the resting potential changes. Examples of such stimuli are pressure, electricity, chemicals, etc. Different neurons are sensitive to different stimuli(although most can register pain). The stimulus causes sodium ion channels to open. The rapid change in polarity that moves along the nerve fiber is called the "action potential." In order for an action potential to occur, it must reach threshold. If threshold does not occur, then no action potential can occur. This moving change in polarity has several stages:

Depolarization

The upswing is caused when positively charged sodium ions (Na+) suddenly rush through open sodium gates into a nerve cell. The membrane potential of the stimulated cell undergoes a localized change from -55 millivolts to 0 in a limited area. As additional sodium rushes in, the membrane potential actually reverses its polarity so that the outside of the membrane is negative relative to the inside. During this change of polarity the membrane actually develops a positive value for a moment(+30 millivolts). The change in voltage stimulates the opening of additional sodium channels (called a voltage-gated ion channel). This is an example of a positive feedback loop.

Repolarization

The downswing is caused by the closing of sodium ion channels and the opening of potassium ion channels. Release of positively charged potassium ions (K+) from the nerve cell when potassium gates open. Again, these are opened in response to the positive voltage--they are voltage gated. This expulsion acts to restore the localized negative membrane potential of the cell (about -65 or -70 mV is typical for nerves).

Hyperpolarization

When the potassium ions are below resting potential (-90 mV). Since the cell is hyper polarized, it goes to a refractory phrase.

Refractory phase

The refractory period is a short period of time after the depolarization stage. Shortly after the sodium gates open, they close and go into an inactive conformation. The sodium gates cannot be opened again until the membrane is repolarized to its normal resting potential. The sodium-potassium pump returns sodium ions to the outside and potassium ions to the inside. During the refractory phase this particular area of the nerve cell membrane cannot be depolarized. This refractory area explains why action potentials can only move forward from the point of stimulation.

Factors that affect sensitivity and speed

Sensitivity

Increased permeability of the sodium channel occurs when there is a deficit of calcium ions. When there is a deficit of calcium ions (Ca+2) in the interstitial fluid, the sodium channels are activated (opened) by very little increase of the membrane potential above the normal resting level. The nerve fiber can therefore fire off action potentials spontaneously, resulting in tetany. This could be caused by the lack of hormone from parathyroid glands. It could also be caused by hyperventilation, which leads to a higher pH, which causes calcium to bind and become unavailable.

Speed of Conduction

This area of depolarization/repolarization/recovery moves along a nerve fiber like a very fast wave. In myelinated fibers, conduction is hundreds of times faster because the action potential only occurs at the nodes of Ranvier (pictured below in 'types of neurons') by jumping from node to node. This is called "saltatory" conduction. Damage to the myelin sheath by the disease can cause severe impairment of nerve cell function. Some poisons and drugs interfere with nerve impulses by blocking sodium channels in nerves. See discussion on drug at the end of this outline.

SPECIAL VISCERAL AFFERENT (SVA) PATHWAYS

Taste

Special visceral afferent (SVA) fibers of cranial nerves VII, IX, and X conduct signals into the solitary tract of the brainstem, ultimately terminating in the nucleus of the solitary tract on the ipsilateral side.

Second-order neurons cross over and ascend through the brainstem in the medial lemniscus to the VPM of the thalamus.

Thalamic projections to area 43 (the primary taste area) of the postcentral gyrus complete the relay.

SVA VII fibers conduct from the chemoreceptors of taste buds on the anterior twothirds of the tongue, while SVA IX fibers conduct taste information from buds on the posterior one-third of the tongue.

SVA X fibers conduct taste signals from those taste cells located throughout the fauces.

Smell

The smell-sensitive cells (olfactory cells) of the olfactory epithelium project their central processes through the cribiform plate of the ethmoid bone, where they synapse with mitral cells. The central processes of the mitral cells pass from the olfactory bulb through the olfactory tract, which divides into a medial and lateral portion The lateral olfactory tract terminates in the prepyriform cortex and parts of the amygdala of the temporal lobe.

These areas represent the primary olfactory cortex. Fibers then project from here to area 28, the secondary olfactory area, for sensory evaluation. The medial olfactory tract projects to the anterior perforated sub­stance, the septum pellucidum, the subcallosal area, and even the contralateral olfactory tract.

Both the medial and lateral olfactory tracts contribute to the visceral reflex pathways, causing the viscerosomatic and viscerovisceral responses.

The cell membrane is called the sarcolemma. This membrane is structured to receive and conduct stimuli. The sarcoplasm of the cell is filled with contractile myofibrils and this results in the nuclei and other organelles being relegated to the edge of the cell.

Myofibrils are contractile units within the cell which consist of a regular array of protein myofilaments. Each myofilament runs longitudinally with respect to the muscle fiber. There are two types: the thick bands and the thin bands. Thick bands are made of multiple molecules of a protein called myosin. The thin bands are made of multiple molecules of a protein called actin. The thin actin bands are attached to a Z-line or Z-disk of an elastic protein called titin. The titin protein also extends into the myofibril anchoring the other bands in position. From each Z-line to the next is a unit called the 

    The sarcomere is the smallest contractile unit in the myofibril. Sarcomeres contract because the Z-lines move closer together. As the sarcomeres contract the myofibrils contract. As the myofibrils contract the muscle cell contracts. And as the cells contract the entire muscle contracts.

The arrangement of the thick myosin filaments across the myofibrils and the cell causes them to refract light and produce a dark band known as the A Band. In between the A bands is a light area where there are no thick myofilaments, only thin actin filaments. These are called the I Bands. The dark bands are the striations seen with the light microscope.

Control of processes in the stomach:

The stomach, like the rest of the GI tract, receives input from the autonomic nervous system. Positive stimuli come from the parasympathetic division through the vagus nerve. This stimulates normal secretion and motility of the stomach. Control occurs in several phases:

Cephalic phase stimulates secretion in anticipation of eating to prepare the stomach for reception of food. The secretions from cephalic stimulation are watery and contain little enzyme or acid.

Gastric phase of control begins with a direct response to the contact of food in the stomach and is due to stimulation of pressoreceptors in the stomach lining which result in ACh and histamine release triggered by the vagus nerve. The secretion and motility which result begin to churn and liquefy the chyme and build up pressure in the stomach. Chyme surges forward as a result of muscle contraction but is blocked from entering the duodenum by the pyloric sphincter. A phenomenon call retropulsion occurs in which the chyme surges backward only to be pushed forward once again into the pylorus. The presence of this acid chyme in the pylorus causes the release of a hormone called gastrin into the bloodstream. Gastrin has a positive feedback effect on the motility and acid secretion of the stomach. This causes more churning, more pressure, and eventually some chyme enters the duodenum.

Intestinal phase of stomach control occurs. At first this involves more gastrin secretion from duodenal cells which acts as a "go" signal to enhance the stomach action already occurring. But as more acid chyme enters the duodenum the decreasing pH inhibits gastrin secretion and causes the release of negative or "stop" signals from the duodenum.

These take the form of chemicals called enterogastrones which include GIP (gastric inhibitory peptide). GIP inhibits stomach secretion and motility and allows time for the digestive process to proceed in the duodenum before it receives more chyme. The enterogastric reflex also reduces motility and forcefully closes the pyloric sphincter. Eventually as the chyme is removed, the pH increases and gastrin and the "go" signal resumes and the process occurs all over again. This series of "go" and "stop" signals continues until stomach emptying is complete.

White Blood Cells (leukocytes)

White blood cells

• are much less numerous than red (the ratio between the two is around 1:700),
• have nuclei,
• participate in protecting the body from infection,
• consist of lymphocytes and monocytes with relatively clear cytoplasm, and three types of granulocytes, whose cytoplasm is filled with granules.

Lymphocytes: There are several kinds of lymphocytes, each with different functions to perform , 25% of wbc The most common types of lymphocytes are

• B lymphocytes ("B cells"). These are responsible for making antibodies.
• T lymphocytes ("T cells"). There are several subsets of these:
  • inflammatory T cells that recruit macrophages and neutrophils to the site of infection or other tissue damage
  • cytotoxic T lymphocytes (CTLs) that kill virus-infected and, perhaps, tumor cells
  • helper T cells that enhance the production of antibodies by B cells

Although bone marrow is the ultimate source of lymphocytes, the lymphocytes that will become T cells migrate from the bone marrow to the thymus where they mature. Both B cells and T cells also take up residence in lymph nodes, the spleen and other tissues where they

• encounter antigens;
• continue to divide by mitosis;
• mature into fully functional cells.

Monocytes : also originate in marrow, spend up to 20 days in the circulation, then travel to the tissues where they become macrophages. Macrophages are the most important phagocyte outside the circulation. Monocytes are about 9% of normal wbc count

Macrophages are large, phagocytic cells that engulf

• foreign material (antigens) that enter the body
• dead and dying cells of the body.

Neutrophils

The most abundant of the WBCs. about 65% of normal white count  These cells spend 8 to 10 days in the circulation making their way to sites of infection etc  Neutrophils squeeze through the capillary walls and into infected tissue where they kill the invaders (e.g., bacteria) and then engulf the remnants by phagocytosis. They have two types of granules: the most numerous are specific granules which contain bactericidal agents such as lysozyme; the azurophilic granules are lysosomes containing peroxidase and other enzymes

Eosinophils : The number of eosinophils in the blood is normally quite low (0–450/µl). However, their numbers increase sharply in certain diseases, especially infections by parasitic worms. Eosinophils are cytotoxic, releasing the contents of their granules on the invader.

Basophils : rare except during infections where these cells mediate inflammation by secreting histamine and heparan sulfate (related to the anticoagulant heparin). Histamine makes blood vessels permeable and heparin inhibits blood clotting. Basophils are functionally related to mast cells.  . The mediators released by basophils also play an important part in some allergic responses such as hay fever and an anaphylactic response to insect stings.

Thrombocytes (platelets):

Thrombocytes are cellular derivatives from megakaryocytes which contain factors responsible for the intrinsic clotting mechanism. They represent fragmented cells  which contain residual organelles including rough endoplasmic reticulum and Golgi apparati. They are only 2-microns in diameter, are seen in peripheral blood either singly or, often, in clusters, and have a lifespan of 10 days.

• 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