Medical

Chief Medical Officer/Chief Surgeon
The Chief Medical Officer is a senior staff-level position, minimum held is a lieutenant who is a Doctor of Medicine with advanced surgical skills. Chief medical officers were also referred to as the "ship's surgeon". Chief medical officers have been seen with a variety of ranks. They can hold any rank in the lieutenant grades, but are quite often seen as lieutenant commanders. Chief medical officers are eligible to take the Bridge Officer's Test for promotion to full commander. At this point, they are considered capable of assuming command of a starship. CMOs aboard a starship or starbase are directly in charge of the sickbay or infirmary, commanding medical department personnel and activities. The CMOs' primary duties are the oversight and maintenance of the overall health and physical fitness of crew and passengers. Officers likely to be assigned as the CMO over long-duration missions of exploration, have scientific training in disciplines outside of medicine, allowing them to take on major, extended research projects; or to better participate in a wider variety of encounters. In addition to the normal duties of an officer, the CMO also has the power to relieve an officer or crewman, including a superior one, of their duties if in the officer's professional judgment, he or she is medically unfit. Typically, the CMO is expected to have the support of a senior command officer or detailed medical documentation to justify this course of action.
http://memory-alpha.org/en/wiki/Chief_medical_officer

Surgeon
A Surgeon is a medical doctor who specializes or is proficient in performing surgery on patients. Minimum rank held is a Lieutenant Junior Grade who is a Doctor of Medicine with advanced surgical skills.
http://memory-alpha.org/en/wiki/Surgeon

Physician
A Physician, also known as a medical doctor or medical practitioner, is an individual who practices in the field of medical science. It is a physician's duty to save the lives of ailing patients, while at the same time preserve the life of healthy patients. It is also a requirement of a medical practitioner to have a certain sensitivity to properly address a patient. As a physician, it's often their job to give people unpleasant news, to tell them they need surgery, or they can't have children, or that they might be facing a difficult illness. Although they are qualified for surgery, Physicians often are more involved in the diagnostics, pathology, pharmacology and non-surgical procedures. Physicians are usually the supervisors of actual recovery and for intake of patients, continueing care, etc. Minimum rank held for Chief Physician (under CMO) is a Lieutenant who is a Doctor of Medicine with surgical skills. Other Physicians are minimum Chief Warrant Officers.
http://memory-alpha.org/en/wiki/Physician

Pathologist
Pathology is the study encompassing the origin, nature, and course of diseases. A doctor who studies pathology is known as a pathologist. Surgeon and Physicians will have training in pathology, but very few ships or starbases have specialized diagnostic professionals.
http://memory-alpha.org/en/wiki/Pathologist

Dentist
A Dentist provides the professional care of a persons teeth. Most Starfleet medical personnel are trained in numerous fields, including dentistry. At least one member of the surgical staff usually has an oral surgery sub-specialty. Usually one of the physicians is qualified in general dentistry. Very few ships or outposts have specialized dentistry professionals, but most large starbases do.
http://memory-alpha.org/en/wiki/Dentist

Psychiatrist
The pressures of extended space travel could result in psychological problems that would require professional care. A psychiatrist is trained in the medical field of psychiatry. Qualified psychiatrists served aboard starships during the 23rd century. As with the chief medical officer, the ship's Chief Psychiatrist has the power to relieve other officers and crewmembers of duty if he or she feels that their patient is suffering from a condition that may hinder their ability to perform their duties effectively. The Chief Psychaitrist is minimum of a Lieutenant, other Psychiatrists are minimum Chief Warrant Officers. Psychiatrists report to the Chief Psychiatrist, who reports to the Chief Medical Officer.
http://memory-alpha.org/en/wiki/Psychiatrist

Counselor
A Counseloris trained in psychology, diplomacy and morale. The counselor is responsible for the emotional and mental well-being of the crew, civilian staff and guests. Ship's counselor also had a diplomatic role, advising in first contact and other diplomatic situations. As with the chief medical officer, the ship's counselor has the power to relieve other officers and crewmembers of duty if he or she feels that their patient is suffering from a condition that may hinder their ability to perform their duties effectively. Unless the Counselor is also a Psychiatrist, he cannot prescribe medicine or surgery, she would have to refer them to a psychiatrist, doctor or surgeon. The Chief Counselor is minimum of a Lieutenant, other Counselors are minimum Chief Warrant Officers.
http://memory-alpha.org/en/wiki/Counselor

Surgical Nurse
A Surgical Nurse is a health care practitioner who assists a surgeon or physician and is responsible for an individual's health and well-being. Most Nurses can also function as Med-Techs and Medics. Surgical Nurses are also trained in physical therapy and responsible for said physical therapy and other recovery processes under the supervision of Physician or Surgeon. Minimum rank Chief Surgical Nurse is Chief Petty Officer. Other Surgical Nurses are a minimum of Ablesman.
http://memory-alpha.org/en/wiki/Nurse

Nurse
A nurse is a health care practitioner who assists a doctor and is responsible for an individual's health and well-being. Most Nurses can also function as Med-Techs and Medics. Nurses are also trained in physical therapy and responsible for said physical therapy and other recovery processes under the supervision of Physician or Surgeon. Nurses are a minimum of Crewman.
http://memory-alpha.org/en/wiki/Nurse

Medic
A medic is an individual trained to administers emergency medical aid where needed. This can be an individual trained to administer emergency aid as a primary or secondary profession. This is different that when someone calls "Medic", they mean anyone with medical training, Medic, Nurse or Doctor. Medics are a minimum of Apprentice.
http://memory-alpha.org/en/wiki/Medic

Med-Tech
Med-Tech are trained to administer tests, operate equipment and other duties peripheral to the actual care of patients. Most Med-Techs are trained as Medics. Med-Techs are a minimum of Recruits.

Enlisted Personnel usually enter service with a minimum Med-Tech rating and rise up thru the ranks thru the acquisition of Ratings, tested in Service.

After successful experienced service as a Med-Tech, they may acquire Medic Rating.

After successful experienced service as a Medic, they may acquire Nurses Rating.

After successful experienced service as a Nurse, they may acquire Surgical Nurse Rating.

After successful experienced service as a Surgical Nurse, they may acquire Chief/Head Surgical Nurse Rating.

After successful experienced service as a Chief/Head Surgical Nurse, they must attend Starfleet Medical Academy.

After completion of Starfleet Medical Academy, they may serve as Physicians aboard a Starship or Starbase.

After successful service as a Physician, they may acquire the Surgical Rating and become a Surgeon. Physicians are required to successfully complete Dentistry, Pathology, Pharmacology and Pyschiatry Ratings before they are permitted to acquire the Surgical Rating. Physicians may also opt to take a Psychiatry Specialty Rating and serve as Ship's Psychiatrist.

After successful service as Surgeon, they may acquire the Chief Medical Officer Rating and become a Chief Medical Officer.

Med-Techs and Medics usually report to Nurses, who in turn report to the Surgical Nurse, who reports to the Physicians, who report to the Surgeons, who report to the Chief Medical Officer, who reports to the Captain, NOT the Executive Officer. This is the only department that does not report to the Captain thru the Executive Officer.

Counselors and Psychiatrists report to the Chief Psychiatrist, who reports to the Chief Medical Officer.

Positions

1 Medical Command:
Chief Medical Officer (Chief Surgeon) - Minimum Rank: O3 Lieutenant
2 Surgical
Assistant Chief Medical Officer (Assistant Chief Surgeon) - Minimum Rank: O3 Lieutenant
Senior Duty Surgeon - Minimum Rank: O2 Lieutenant Junior Grade
Junior Duty Surgeon - Minimum Rank: O2 Lieutenant Junior Grade
3 Physician
Alpha Shift Physician - Minimum Rank: O3 Lieutenant
Beta Shift Physician - Minimum Rank: W2 Chief Warrant Officer
Gamma Shift Physician - Minimum Rank: W2 Chief Warrant Officer
Delta Shift Physician - Minimum Rank: W2 Chief Warrant Officer
4 Psychiatry
Chief Psych/Counselor - Minimum Rank: O3 Lieutenant
Beta Shift Psych/Counselor - Minimum Rank: W2 Chief Warrant Officer
Gamma Shift Psych/Counselor - Minimum Rank: W2 Chief Warrant Officer
Delta Shift Psych/Counselor - Minimum Rank: W2 Chief Warrant Officer
5 Surgical Nurse
Chief Surgical Nurse - Minimum Rank: E7 Chief
Assistant Chief Surgical Nurse - Minimum Rank: E6 Specialist
Senior Duty Shift Surgical Nurse - Minimum Rank: E5 Technician
Junior Duty Shift Surgical Nurse - Minimum Rank: E4 Ablesman
6 Nursing
Alpha Shift Nurse - Minimum Rank: E6 Specialist
Beta Shift Nurse - Minimum Rank: E5 Technician
Gamma Shift Nurse - Minimum Rank: E4 Ablesman
Delta Shift Nurse - Minimum Rank: E3 Crewman
7 Medic
Alpha Medic - Minimum Rank: E5 Technician
Beta Medic - Minimum Rank: E4 Ablesman
Gamma Medic - Minimum Rank: E3 Crewman
Delta Medic - Minimum Rank: E2 Apprentice
7 Med-Tech
Alpha Med-Tech - Minimum Rank: E4 Ablesman
Beta Med-Tech - Minimum Rank: E3 Crewman
Gamma Med-Tech - Minimum Rank: E2 Apprentice
Delta Med-Tech - Minimum Rank: E1 Recruit