This article describes the anatomy of the head and neck of the human body, including the brain, bones, muscles, blood vessels, nerves, glands, nose, mouth, teeth, tongue, and throat.
The occipital bone joins with the atlas near the foramen magnum, a large hole (foramen) at the base of the skull. The atlas joins with the occipital condyle above and the axis below. The spinal cord passes through the foramen magnum.
Two rows of teeth are supported by facial bones of the skull, the maxilla above and the mandible below. Adults have 32 permanent teeth, and children have 20 deciduous teeth. There are various tooth shapes for different jobs. For example, when chewing, the upper teeth work together with the lower teeth of the same shape to bite, chew, and tear food. The names of these teeth are:
(1) Incisors, there are eight incisors located in the front of the mouth (four on the top and four on the bottom). They have sharp, chisel-shaped crowns that cut food.
(2) Cuspids (or canine tooth), the four cuspids are next to each incisor. Cuspids have a pointed edge to tear food.
(3) Premolars (or bicuspids), the four pairs of molars are located next to the cuspids. They crush and tear food.
(4) Molars, there are twelve molars, in sets of three, at the back of the mouth. They have wide surfaces that help to grind food.
The white visible part of a tooth is called the crown. The rounded upper projections of the back teeth are cusps. The hard white exterior covering of the tooth is the enamel. As the tooth tapers below the gumline, the neck is formed. Below the neck, holding the tooth into the bone, is the root of the tooth. The inner portions of the tooth consist of the dentin, a bonelike tissue, and the pulp. The pulp is a soft tissue area containing the nerve and blood vessels to nourish and protect the tooth, located within the pulp cavity.
A tooth sits in a specialized socket called the alveolus. The tooth is held in location by a periodontal ligament, with the assistance of cementum. Teeth are surrounded by gingiva, or gums, part of the periodontium, support tissue of oral cavity protection. The periodontium includes all of the support membranes of the dental structures surround and support the teeth such as the gums and the attachment surfaces and membranes. These include epithelial tissues (epithelium), connective tissues, (ligaments and bone), muscle tissue and nervous tissue.
There are three sets of salivary glands: the parotid, the submandibular and the sublingual glands. The (exocrine) glands secrete saliva for proper mixing of food and provides enzymes to start chemical digestion. Saliva helps to hold together the formed bolus which is swallowed after chewing. Saliva is composed primarily of water, ions, salivary amylase, lysozymes, and trace amounts of urea.
Tongue
The tongue is a specialized skeletal muscle that is specially adapted for the activities of speech, chewing, developing gustatory sense (taste) and swallowing. The tongue contains two sets of muscles, the intrinsic- involved with shape of tongue, and the extrinsic- involved with tongue movement. It is attached to the hyoid bone. Terms meaning tongue include "glosso" (from Greek) and "lingual" (from Latin).
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Microanatomy
The outer surfaces of the head and neck are lined by epithelium. The protective tissues of the oral cavity are continuous with the digestive tract are called mucosa or mucous membranes. The cells of the inner oral cavity are called the buccal mucosa.
The left subclavian artery and the right subclavian artery, one on each side of the body form the internal thoracic artery, the vertebral artery, the thyrocervical trunk, and the costocervical trunk. The subclavian becomes the axillary artery at the lateral border of the first rib. The left subclavian artery also provides blood to the left upper chest and left arm.
Blood–brain barrier
The blood–brain barrier (BBB) is semi-permeable membrane that controls the capillary leak potential of the circulatory system. In most parts of the body, the smallest blood vessels, called capillaries, are lined with endothelial cells, which have small spaces between each individual cell so substances can move readily between the inside and the outside of the capillary. The endothelial cells of capillaries in the brain lack these spaces. Instead, the endothelial cells fit tightly together to create a tight junction which prevents substances from passing out of the bloodstream.
Specialized glial cells called astrocytes form a tight junction or protective barrier around brain blood vessels and may be important in the development of the BBB. Astrocytes may also be responsible for transporting ions (electrolytes) from the brain to the blood.
Together this set of lymphatic tissue is called the tonsillar ring or Waldeyer's ring.
Nerve supply
The spinal nerves arise from the spinal column. The top section of the spine is the cervical section, which contains nerves that innervate muscles of the head, neck and thoracic cavity, as well as transmit sensory information to the CNS.
The cervical spine section contains seven vertebrae, C-1 through C-7, and eight nerve pairs, C-1 through C-8.
There is the formation of an extensive network of nerve groups or tracts attaching to the spinal cord in arrangements called rami or plexus.
The sensory branches of spinal nerves include: lesser occipital, C-2, great auricular, (C-2 and C-3); transverse cervical, C-2 and C-3; and supraclavicular, C-3 and C-4. These nerve groups transmit afferent (sensory) information from the scalp, neck, and shoulders to the brain.
The motor branches of spinal nerves include: ansa cervicalis, dividing into a superior root, C-1, and an inferior root, C-2 and C-3, and the phrenic nerve, C-3 to C-5, the segmental nerve branches, C-1 to C-5. These nerve groups transmit efferent nerve (motor) information from the brain to muscle groups of the scalp, neck, diaphragm (anatomy), and shoulders.
Damage to a person's spinal cord above C-5 may result in respiratory arrest and death if medicinal aid does not intervene.
Cranial nerves
Twelve pairs of cranial nerves emerge from the brain; these affect movements and sensation, and some special organs such as hearing of parts of the head and neck.
Function
Movements of the neck includes: flexion, extension, (nodding yes), and rotation (shaking head no).
The mouth has evolved to support chewing, (mastication) and swallowing (deglutition), and speech (phonation).
In addition to the teeth, other structures that aid chewing are the lips, cheeks, tongue, hard palate, soft palate, and floor of the mouth.
Endocrine glands
Several glands of the endocrine system are found within the head and neck. Endocrine means that the secretion is used within the body. Endocrine glands are termed as ductless and release their secretions directly into the blood. The endocrine system is under the direct supervision of the nervous system, using the negative feedback principal of homeostasis, to create hormones which act as chemical instant messengers.
The pituitary gland secretes hormones that directly impact the body as well as hormones that indirectly control body functions because they activate other endocrine glands, such as the adrenal cortex (ACTH) and the thyroid gland (TSH). These two glands when stimulated by pituitary hormones then release their own hormones.
The pituitary gland has two lobes, the anterior lobe and the posterior lobe. The anterior lobe secretes: growth hormone (GH), Luteinizing hormone (LH), Follicle stimulating hormone (FSH), Adrenocorticotropic hormone (ACTH), Thyroid-stimulating hormone (TSH), Prolactin (PRL), and the posterior lobe secretes: Antidieuretic hormone (ADH), and Oxytocin. There is an intermediate lobe, in adult humans it is just a thin layer of cells between the anterior and posterior pituitary, nearly indistinguishable from the anterior lobe. The intermediate lobe produces melanocyte-stimulating hormone (MSH).
In the neck are the thyroid and parathyroid glands, that secrete hormones that control metabolism and blood calcium levels. The four parathyroid glands are situated upon the back surface of the thyroid gland.
The respiratory system begins in the head and neck, with air entering and leaving the body through the mouth and nose. The respiratory system involving the head and neck includes:
the nasal cavity for filtering, moistening, and warming the air
These lead down into the lower respiratory tract. A critical junction between the respiratory and digestive systems is the epiglottis, a cartilage flap which shuts during swallowing to prevent aspiration. The epiglottis is normally open to support respiration and shuts during swallowing to prevent food and fluids from entering the trachea, activating the gag reflex or initiates the choking mechanism.
The CNS is protected by the cranium, vertebral column, meninges, cerebrospinal fluid. The spinal cord is an extension of the brain. The spinal cord and the brain stem are joined at the base of the cranium at the foramen magnum. Most of the functions of the head and neck are directly influenced by the brain and transmitted to the PNS via the cranial nerves and spinal nerves of the cervical portion of the spine.
The PNS has two subdivisions
somatic nervous system (SNS). The SNS is associated with the voluntary control of body movements through the action of skeletal muscles, and also the reception of external stimuli.
The facial bones usually form into pairs and then fuse together. As the cranium fuses, sutures are formed that resemble stitching between bone plates. In a newborn, the junction of the parietal bones with the frontal and occipital bones, form the anterior (front) and posterior (back) fontanelle, or soft spots. The separation of the cranial bone plates at time of birth facilitate passage of the head of the fetus through the mother's birth canal, or pelvic girdle. The parietal bones, and occipital bone can overlap each other in the birth canal, and form the unusual looking "cone head" appearance in a newborn when delivered in a natural, or vaginal, delivery.
Teeth
Humans normally will produce two sets of teeth called primary dentition, or deciduous teeth, and secondary dentition, or permanent teeth.
A tooth is the toughest known substance in the body exceeding bones in density and strength. Tooth enamel lends great strength to the tooth structure. The formation of a developing tooth includes the process of dentin formation, (see: dentinogenesis) and enamel formation, (see: amelogenesis). The tooth breaks through the gum into the mouth in a process called eruption. The formation of teeth begins in early fetal development and goes through six stages:
(1) initiation stage, 6th - 7th week
(2) bud stage, 8th wk
(3) cap stage, 9th-10 wk
(4) bell stage, 11th-12th wk
(5) apposition
(6) maturation stage
Clinical significance
Infection
Severe viral infections that affect the mouth, lips, or the oral cavity include:
Oral cancer may have a viral link.
Minor viral infections include: Mumps is a viral infection of the parotid salivary glands. Chicken pox is a viral infection that can spread to the mouth.
Thrush (candidiasis) fungal infection. Tonsillitis is an inflammation of the tonsils and may cause a sore throat and fever. In chronic cases tonsillectomy may be required.
Diseases may be transmitted by contact of the head, mouth, or body fluids, such as Herpes Simplex Virus Type I (HSV-1), Herpes Simplex Virus Type II (HSV-2) genital herpes, which may present as a lesion on the lips, and contactable via skin to skin contact
Autoimmune diseases such as: Crohn's disease of the oral cavity, see reference below.
Careful observation of the oral cavity, teeth and gums may reveal indicators of other medical conditions. For example, a person suffering from the eating disorder, bulimia nervosa may show signs of excessive tooth and gum erosion.
Anaphylactic shock requires advanced medical care immediately; but other first aid measures include rescue breathing (part of CPR) and administration of epinephrine using an EpiPen for immediate administration of epinephrine (adrenaline) to reverse swelling and to keep the respiratory airway (trachea) open.