A neurologist is a physician who diagnoses and treats disorders of the nervous system which is comprised of the brain, spinal cord and nerves. These doctors do not perform surgery, but refer patients to neurological surgeons when they determine that surgical intervention is necessary.
Some of the conditions that neurologists diagnose and treat are epilepsy, aneurysms, hydrocephalus, Parkinson's disease, multiple sclerosis, stroke, spinal disc herniation, and spinal disease.
In addition to using diagnostic tests like MRI, CT scans, EEG and EMG, neurologists also employ neurological testing to gauge muscle strength and movement, balance, reflexes, sensation, memory, speech, and other cognitive abilities.

Also sometimes referred to as intensivists, critical care specialists are physicians with specialized training in the diagnosis and management of life-threatening conditions. Some of these conditions affect vital organs like the heart and lungs, those that make breathing difficult or impossible, and those that affect entire organ systems, like the renal system.
Critical care specialists are typically found in a hospital's intensive care unit where they monitor patients with life-threatening conditions and make determinations as to the best course of treatment.

Hospitalists are physicians who specialize in the care of patients in the hospital. The majority of hospitalists are board-certified internists and have completed the same training as other internal medicine doctors including medical school, residency and board certification examination.
Hospitalist activities include patient care, teaching, research, and leadership related to hospital care. They have more expertise in caring for complicated hospitalized patients on a daily basis since, unlike other specialists or primary care doctors, they spend most of their day in the hospital.
They often coordinate the care of their patients and act as the central point of communication among the different doctors and nurses involved in the patient's care. They are also the main physician for family members to contact for updates on a loved one.

A pulmonologist is a physician who specializes in the diagnosis and treatment of conditions related to the lungs and respiratory tract.
These specialists are similar to critical care specialists in that their patients often require mechanical ventilation to assist their breathing.
Pulmonologists diagnose and treat patients with conditions such as asthma, cystic fibrosis, asbestosis, pulmonary fibrosis, lung cancer, COPD, and emphysema. Exposure and inhalation of certain toxic substances may also warrant the services of a pulmonologist.
Some of the tools and tests pulmonologists use to diagnose a patient are a stethoscope in order to listen for abnormal breathing sounds, chest X-rays, CT scans, blood tests, bronchoscopy, and polysomnography.

An internist is a physician who focuses on the diagnosis and treatment of conditions that affect the adult population—both acute and chronic.
These doctors are often who adults see as their primary physicians because they treat a broad range of illnesses that do not require surgical or specialist interventions. They also work to help a patient maintain optimal health in order to prevent the onset of disease.
In addition to treating the common cold and flu, internists also treat chronic diseases like diabetes and heart disease.

Anesthesiologists are physicians who are trained to administer anesthetics, which are medicines used to block nerve sensation. Anesthesia can be either local to one specific part of a body, like a tooth, or regional to block feeling to a larger portion of the body, such as during an epidural for child birth. It can also be more general to block sensation to the entire body, resulting in unconsciousness.
Anesthesiologists assist in surgery by determining how much anesthesia is necessary and by monitoring the patient's level of responsiveness and vital signs throughout the procedure. The anesthesia specialist will also bring the patient out of anesthesia and then continue to monitor his or her vital signs post-operation.
Besides assisting in surgeries, anesthesiologists may also treat patients suffering from chronic pain.