digestion

• The digestive system provides nutrition, energy balance, intermediary metabolism and mechanism of excretion

  Digestive processes: 

  o Ingestion 

  o Digestion 

  o Absorption 

  o Metabolism 

  o Excretion
• Systems may be described by the complexity of the stomach and large intestine, and its length. A longer GIT indicates more difficult to digest/process food. Species variation in size & complexity of both stomach and large intestine relates to contribution of microbial digestion & fermentation to animal’s energy supply
• Extrinsic (parasympathetic and sympathetic) relates to the CNS and endocrine system which are external to the GI tract

  
  

  Intrinsic (enteric) relates to nervous and endocrine components WITHIN the GI organs. They are autonomous and based on local conditions. The extrinsic and intrinsic systems coordinate, and external influences (sympathetic) are able to override normal functioning
• The enteric nervous system is an intricate system with up to 100 million nerve cells WITHIN the wall of the GI tract.

  Cell bodies are arranged in 2 ganglia (layers): myenteric plexus between inner and outer mm. layers and submucosal plexus in the submucosa.

  Their reflexes operate independently of the CNS  and control motility, fluid movement and blood flow.
• Acetylcholine, Vasoactive intestinal peptide, Nitric oxide and Substance P are all important neurotransmitters in the GIT
• The fight/flight response REDUCES digestion

  1.       Preganglionic factors in the spinal cord at L2-L5 synapse in the stellate ganglion

  2.       Pre-ganglionic fibres release Ach and act on ganglia in the mesentery (mesenteric ganglia)

  3.       Post-ganglionic factors then act on the enteric system by releasing noradrenaline, a digestive suppressor.
• Cranial nervous supply is from the vagus nerve and innervates the oesophagus, stomach small intestine and ceacum

  Sacral nervous supply is from the pelvic nerve and innervates the large intestine.

  Preganglionic fibres of the vagus nerve arise in the medulla and of the pelvis nerve arise in S2-S4

  All preganglionic factors synapse directly onto the enteric nervous system
• somatic innervation of the GIT: Don’t choke or shit yourself

  Vagus nerve acts on pharynx for swallowing

  Pudendal nerve acts on the external anal sphincter
• The oral cavity is defined as the space that extends from the lips to the pharynx, bound laterally by the cheeks. Divided into the oral cavity and the vestibule.
• the functions of the oral cavity are:

  Prehension (grasping and seizing e.g with the lips, teeth and tongue)

  Selection of foodstuffs

  Mastication (focused movement at TMJ)

  Insalivation of food, both for swallowing and in fermentation digestion

  Social role (aggression, defence, greeting, grooming)

  Airway in some species when flow through the nasal cavity is impaired/ incr. resp. dem.

  Vocalisation and communication in some species

  Temperature control (panting)
• During embryonic development, the oral cavity is formed when the stomodeaeum invaginates, causing surface ectoderm to meet with the endoderm derived gut tube (the oral cavity and pharynx are ectoderm, gut tube is endoderm)
• The lips unite at each side if the mouth at the commissures.

  They assist in prehension (especially herbivores) and in the infant animal they form the seal for suckling.

  They are a tactile organ but mobility and sensitivity varies between species, greatest in the horse, little in the ox and pig (mainly just serve to close the mouth)

  The central part of the upper lip (rostrum) may be modified (the nasolabial plate)

  On the ox and horse, the lower lip protrudes with muscle and fat for grazing.

  Orbicularis oris muscle runs round and moves lips. Innervated by Facial nerve (CN VII)
• The cheeks are muscular structures lined with mucous membranes which aid the tongue in positioning food.

  Small in the carnivore as they need to be able to open their mouths wide.

  Extensive (long) in horse, ruminants and rodents (herbivore cheeks) to hold, store and manipulate food during mastication and RUMINATION. Can create a vacuum to drink fluids .

  Ruminants’ cheeks are lined with conical papillae which project caudally and are heavily keratinised.
• the hard palate is the rostral bony surface made up of the incisive, maxillary and palatine bone. It is covered in palatine rugae. Helps to form the food bolus as the tongue compresses food against it. Incisive papilla is found caudal to the incisors (or dental pad) on the hard palate where the incisive ducts open out and connects the nasal and oral cavities for vomeronasal organ which is situated dorsal to the hard palate
• the soft palate is a muscular mucosal shelf which continues from the HP and extends into the pharyngeal cavity. Divides nasopharynx and oropharynx. Prevents food entering nasal cavity. Connects to the tongue via the palatoglossal arches and to the pharynx walls by the palatopharyngeal arches. Raised in swallowing to allow space, lies on the root of the tongue at rest. Mm. involved are levator veli palatini and tensor veli palatini. During vomiting the SP closes off the nasopharynx to prevent entry into nasal cavity. SP is mobile in most species (exceptions include horses and cats)
• there are three types of tonsil:

  Palatine tonsils- protrude into the lumen of the oropharynx in dogs and cats, lie in a similar place but in the submucosa covered by a mucous membrane so they don’t protrude in the horse and ruminants.

  Lingual tonsils- accumulations of lymphatic nodules in the base of the tongue

  Pharyngeal tonsils- accumulations of lymphatic tissue on the dorsal pharyngeal wall. Adenoids in humans for reference.
• The oral floor is formed of the mandible and merging of the mylohyoideus muscles in the median plane which forms a soft tissue sling.

  2x lateral sublingual recesses under the tongue (the bit you can feel under your tongue under the teeth). Remember to check this for oral lesions

  The lingual frenulum is a mucosal fold in the midline under the tongue which forms the attachment.

  On either side of the frenulum there are two sublingual caruncles in which the ducts of the mandibular and monostomatic sublingual salivary glands open onto.
• The tongue acts as a ladle for fluids, is essential for prehension and sorting of solid foods, (mechanically  and chemically), Grooming and  cleansing of wounds, mastication and deglutition of food, suckling in young and aids in thermoregulation e.g. dogs
• the tongue is made up mostly of muscle with interlacing, interdirectional muscle fibres. Has root (caudal attachment of mucosal reflections via the palatoglossal arches), body and free apex and a median groove. Ventral attachment is the frenulum.

  Intrinsic muscles enter at the root from the hyoid apparatus.
• Innervation of the tongue:

  Body and apex (rostral 2/3rds)

  • Lingual branch of mandibular nerve- general sensory

  • Chorda typani (branch of facial nerv)- taste

  
  

  Root: 

  •  Glossopharyngeal nerve 

  •  Slight input from vagus?

  
  

  Motor innervation of intrinsic and extrinsic muscles all by the hypoglossal nerve
• The oral cavity is lines with oral mucosa (a mucous membrane) which do not contain a muscularis mucosa like the rest of the GI tract.-Submucosa

  -Epithelium

  -Lamina propria
• the oral epithelium are stratified squamous epithelium which are keratinised in areas of greater wear. The cells risk desiccation and wear from course diets. The main function of the cells is protection, with surface layers able to be worn off without damaging the basement membrane. Keratinisation can also resist desiccation. Continuous with the skin at the mucocutaneous junction (lips).
• the oral cavity lamina propria is a layer of connective tissue of dense collagen which connects the epithelium to the submucosa and other underlying structures. It is capillary rich and acts as a support to the epithelium as BVs do not cross up through the BM. In highly mobile areas (floor, SP) the submucosal connection is loose. In immobile areas (HP, dental arcades) where it overlies bone it is attached by fibrous mucosa, which anchor it to the periosteum.
• the oral cavity submucosa contains loose collagenous tissue with larger blood vessels that the lamina propria, lymphatics, nerves and small accessory salivary glands (both serous and mucous) known as labial, buccal or lingual glands depending on location. Supports and anchors mucosa to underlying musculature or bone
• General histology of the tongue: most of the tongue is made up of skeletal muscle bundles and some loose connective tissue. Muscle fibres lie in three planes at right angles to eachother which allows for a wide range of movement.

  Ventral surface= stratified squamous epithelium

  Dorsal surface= keratinised SS epithelium. The LP has raised projections known as lingual papillae The oral cavity is lines with oral mucosa (a mucous membrane) which do not contain a muscularis mucosa like the rest of the GI tract. The oral cavity is only epithelium and its underlying lamina propria.
• Mechanical papillae:

  Facilitate movement of digesta in the oral cavity and offer some protection. Filliform are the most numerous and are slender keratinised SS epithelia with a thick stratum corneum. Found in all species and gives the tongue its velvety appearance. Conical papillae are large filiform which are found at the root of the tongue in many birds, dog, cat and pig. In cattle they are also on the torus inguae, lips and cheek.
• Sensory/gustatory papillae contain taste buds. (Fu&C) Fungiform papillae are scattered in among the filliform at the apex and body. Dome shaped and covered in non-keratinised SS epithelium- taste buds on the upper surface. Circumvallate papillae located just rostral to the root of the tongue. Large and flattened, surrounded by a moat. Taste buds are in the lateral walls, facing the moat.
• Sensory/gustatory papillae contain taste buds. (Fo &L) Foliate papillae are parallel folds of mucosa located at the lateral borders of the tongue at palatoglossal arch level. Taste buds are located in the epithelium but are absent in ruminants. Lenticular papillae are found in ruminants, where they form flattened lentil‐shaped projections, often mechanical but in cattle they are gustatory. Gustatory serous glands empty into the clefts, secreting a watery fluid dissolves food constituents and washes the sensory hairs allowing new tastes to be recognised.
• Structure of a taste bud: Ellipsoid clusters of gustatory chemoreceptor cells are found in high  numbers within gustatory papillae on the tongue, but they can also be found scattered in  other parts of the mouth and pharynx. 

  The taste bud is a barrel‐shaped organ containing spindle‐ shaped cells, extending the full thickness of the epithelium. The taste pore is the communicating area with the external environment.

  In terms of histology, gustatory cells stain light and sustentacular cells stain dark.
• There are 5 tastes in mammals: sweet, sour, bitter, salt, umami (savoury/meaty, associated with protein and carbs). Each taste is detected by its own receptor type: one cell, one taste.

  Taste is crucial for appetite (bad taste, you don’t want to eat it) as it helps evaluate the nutritional value and quality of a meal. High energy foods will taste umami and an animal will be warned away by sour or bitter flavours. Adequate or excess intake of salt is detected by salt sensors.
• Dentine is a calcified tissue, similar to but harder than bone (70% inorganic matter). Produced throughout life by odontoblasts and makes up the bulk of the tooth. Porous and SENSITIVE. Made up of tubules, each containing an odontoblastic process and a sensory nerve.

  Primary dentine is present when the tooth erupts.

  Secondary dentine is deposited through life as the tooth gets used, (pulp chamber subs. shrinks w/ age)

  Tertiary (reparative) is laid down quickly when there is trauma to the tooth, but it is not organised the same so easily discolours and often appears brown or yellow.
• enamel is the hardest tissue in the body- 96% inorganic material, formed of parallel highly calcified rods cemented together.

  Produced by ameloblasts BEFORE the tooth erupts, covering the crown, and cannot be replaced.
• The periodontal ligament connects the tooth (and its surrounding cementum) to the bone. It is made up of gingival, cervical and oblique fibres. which each secure the tooth in a different direction for optimum security.
• Dental pulp is the soft centre of the tooth which contains blood vessels, lymphatics, nerves and various cell types including odontoblasts. Each root has a pulp canal, and a common pulp chamber in multi-rooted teeth.

  The pulp is open in young animals at the root apex; by one year of  age, this open canal has reduced to an apical delta of 10‐20 small passages that contain vessels and nerves running from the pulp to the surrounding tissues in the periodontal space.
• Cementum is an avascular calcified connective tissue which covers the root within the gums. Produced by cementoblasts. Softer than dentine at about 45-50% inorganic matter. Thickest near the root of the tooth. Constantly produced in life and acts as an anchor to the periodontal ligament, attaching the tooth to its socket.

   

  The cemento-enamel junction is the transition area between cementum and enamel at the neck of the tooth. it is also the point where the gingiva changes from free to attached.
• Gingiva/gums are made up of tough epithelial tissue which forms a cuff around each tooth.

  Free gingiva is around the tooth surface and the attached gingiva is bound tightly to the underlying periosteum of the alveolar bone. The gingival sulcus (little pocket where the free gums meet the teeth) should be less than 1mm deep in cats and 4mm deep in dogs.
• The periodontium describes the supporting structures of the tooth which hold it in place. This includes the cementum, the periodontal ligament, the alveolar bone and the gingiva

  The periodontal space is the narrow gap that exists between the tooth and the alveolar bone which contains the periodontal ligament, blood vessels and lymphatics.
• Sharpey's fibres are fibres of the periodontal ligament which unite the alveolar bone on one side  and the cementum on the other in a meshwork of inter‐woven branches which lock the tooth in place and act as shock absorbers
• The tooth alveolus is the area of the jaw which accommodates a tooth, a.k.a the dental socket. They show up in radiographs as a white line called the laminar dura. The area of bone surrounding the neck of the tooth is the alveolar margin.

  The specialised attachment of the tooth to its socket is classified as a fibrous joint 

  sometimes referred to as gomphosis
• There are 3 stages to embryonic tooth development:

  1.       Bud stage- areas corresponding with the deciduous dentition bud off from the dental lamina

  2.       Cap stage- in the deepest portion of each bud, a concavity forms. The EO is now comprised of the outer enamel epithelia (OEE), inner enamel epithelia (IEE) which line the developing cavity and the stellate reticulum (SR) within the cap

  3.       Bell stage- the beginning of this stage is marked by the appearance of a 4th layer, the stratum intermedium (SI) between the IEE and SR