Anatomy

Before delving into the various parts of the body and investigating their structure and function, there are a few terms that are critical to understanding anatomy as a subject. These definitions will certainly be taught in anatomy class, however, knowing them early on will enhance understanding of the topics below as well as provide a solid theoretical foundation for an array of knowledge that will be learnt in the future. The given terminology is essentially simplified so that it can be understood even by non-medical personnel.

Image 1 - The Directional References and Planes of the BodyFrom: http://pre.weill.cornell.edu/mri/pages/book/chapter_3.html

Directional References & Bodily Planes

Directional References - Depicted in Image 1

These guide the visualization and comprehension of complex human anatomy.

  1. Anterior / Ventral - Towards the front of the body;

  2. Posterior / Dorsal - Towards the back of the body;

  3. Superior / Cranial - Towards the upper portion of the body, nearing the head (thus 'cranial');

  4. Inferior / Caudal - Towards the lower end of the body; further from the head (thus 'caudal');

  5. Proximal - Closest to the point of origin of an anatomical structure;

  6. Distal - Away from the point of origin of an anatomical structure;

  7. Medial - Nearest to the midline of the body,

  8. Lateral - Further from the midline of the body.

The Planes of the Body - Depicted in Image 1

The planes of the body help in the identification of anatomical structures and in pinpointing their specific location (when described in conjunction with other indications (directional references, cavities etc).

  1. Sagittal Plane - A vertical plane that divides the body into into left and right segments (not halves as it does not necessarily run via the midline of the body). This is also known as the lateral plane;

  2. Coronal Plane - A vertical plane that divides the body anteriorly and posteriorly (front and back) as it lays between 2 sides of the body. This is also known as the frontal plane;

  3. Median Plane - A vertical plane that runs via the midline of the body dividing it into 2 halves (similar to the sagittal plane, except the median plane always refers to 2 equal left and right segments as it cuts via a midline);

  4. Axial Plane - A horizontal plane that divides the body into upper and lower parts. This is also known as the transverse plane.

The Cavities of the Body - Depicted in Image 2

'By the broadest definition, a body cavity is any fluid-filled space in a multicellular organism. However, the term usually refers to the space where internal organs develop, located between the skin and the outer lining of the gut cavity.' Body cavities can have one or many organ systems within them depending on the area they cover.

  1. The Ventral Cavity - The largest cavity in the body towards the front (as the name suggests) that can be subdivided into an anterior part, i.e. the thoracic cavity, and a posterior part, i.e. an abdominopelvic cavity. This division is facilitated by the diaphragm - a thin layer of skeletal muscle.

  2. The Thoracic Cavity - The upper cavity within the large ventral cavity that is host to the major organs of the respiratory and cardiovascular systems (heart and lungs), as well as other organs such as the oesophagus. 'The thoracic cavity is lined by two types of mesothelium, a type of membrane tissue that lines the ventral cavity: the pleura lining of the lungs, and the pericardium lining of the heart.'

  3. The Abdominopelvic Cavity - The lower cavity within the large ventral cavity that is host to abdominal organs, i.e. mainly from the digestive system alongside some endocrine glands (adrenal and pancreas), and pelvic organs, i.e. from the reproductive system alongside the bladder/rectum etc. 'The abdominopelvic cavity is lined by a type of mesothelium called the peritoneum.'

  4. The Dorsal Cavity - This cavity is located towards the back of the body, thus dorsal, and houses organs such as the brain and spinal cord. It is divided into 2 sub-cavities - the anterior portion known as the cranial cavity and the posterior portion known as the vertebral cavity. 'The meninges is a multi-layered membrane within the dorsal cavity that envelops and protects the brain and spinal cord.'

  5. The Cranial Cavity - This is a cavity within the skull's frontal portion. 'This cavity contains the brain, the meninges of the brain, and cerebrospinal fluid.'

  6. The Vertebral Cavity - This is the narrowest cavity in the body with the spinal cord, its meninges, and the fluid between them.

The Fornix

The fornix is the main output tract of our limbic system and is situated in the basal forebrain. 'The fornix originates in the hippocampus, where it emerges from a collection of fibers called the fimbria.' It is a curved structure that stretches to the anterior portion of the thalamus as the commissure tract then leads the fibers of the fornix to varying sections of the forebrain, the farthest being the mammillary bodies.

Its anatomical structure is made up of commissural (transverse) fibers, i.e. white matter. It serves as a connection between the corresponding parts of both the right and left hemispheres. The fornix is named after its arch-like shape as shown in Image 1.

Image 1 - Shape of the Fornix within the BrainFrom: https://virtualpsychcentre.com/fornix-or-trine-anatomy-and-function/

It is known to act as a connection between the following -

  • The hippocampus (associated with memory) and mammillary bodies (associated with recollective memory) at the ends of the anterior fornix;

  • The hippocampus and the nucleus accumbens (the brain's reward system which is responsible for high concentration dopamine surges) and septal nuclei (a localized area for olfactory function);

  • The mammillary bodies and the anterior thalamic nuclei. 'The anterior nucleus of thalamus (ANT) is a key component of the hippocampal system for episodic memory.'

Image 2 - Parts of the Fornix From: https://human-memory.net/fornix/

Though seemingly complex, the anatomy of the fornix can be simply understood by looking at its 4 main parts -

The crus - It is made of 2 crura at opposing ends towards the posterior of the cerebrum which join together to then form the fornix's main body.

The body - 'The body of fornix begins by joining together of the two crura. They join in the midline, just under the corpus callosum, to form the body of fornix. '

The commissure - 'The commissure of fornix is the collection of transverse fibers that connect the two crura of the fornix.' It transfers hippocampal output to both cerebral hemispheres.

The columns - The anterior part of the fornix further divides into 2 columns that extend through grey matter to the base of the brain, i.e. into the mammillary bodies via a semicircular curvature.

The parts of the fornix can be seen in Image 2.

The function of the fornix, apart from acting as a connective component, is to aid in cognitive function, memory consolidation, and recollection, emotion, and motivation. This is as the fornix activates alongside other cerebral structures. It is especially vital for memory formation as 'it serves as the conduit for theta rhythms and acetylcholine, as well as providing mnemonic representations to deep brain structures that guide motivated behavior, such as when and where to eat.' Acetylcholine is a neurotransmitter relaxant that functions under the parasympathetic nervous system and regulates responses such as blood vessel dilation, slowing heart rate, relaxing muscles, and increasing bodily secretions. Theta waves are of low frequency (3 to 8Hz) and induce the brain's 'autopilot' state - one of sheer relaxation. These 2 factors are associated with reducing stress and anxiety, boosting healing processes and immune response; all phenomena that increase the efficiency of the body's functioning.

Damage to the fornix has resulted in the onset of anterograde amnesia, i.e. the inability to create new memories. 'Research using in vivo imaging methods has linked fornix pathology to cognitive aging, mild cognitive impairment, psychosis, epilepsy, and, importantly, Alzheimer's Disease.' Neurodegeneration of the fornix is associated with cognitive impairment as well as memory loss and usually acts as a precursor to hippocampal degeneration.

Further research into the localization of parts of the fornix suggests that 'the left fornix primarily carries verbal memory information, while the right carries visuospatial memory information. In addition, the medial fornix carries fibers from the caudal hippocampus, which processes exteroceptive signals and integrates object recognition within a spatial context.' The lateral fornix is presumed to connect with fibers from the anterior portion of the hippocampus, and thus be associated with emotion, motivation, memory, and learning.

The brain is a highly complex organ with numerous anatomical structures working in sync to execute proper social, cognitive, and behavioral function. It is thus vital to understand these structures and their workings before looking at how dementia affects them.

The brain receives approximately 20 to 30% of inspired oxygen by a dense vascular network surrounding it. The brain has 2 hemispheres – the left that is responsible for language and logical processing, speech function, arithmetic ability and writing skills; the right side of the brain controls spatial perception, artistic and musical comprehension, and creativity. It is noteworthy that the left hemisphere controls the right side of the body and vice versa. The 2 hemispheres are connected by a structure called the corpus collosum. The hemispheres of the brain are asymmetrical, with the right hemisphere being slightly larger. Alongside this, there are variations in chemical concentrations on both sides of the brain alongside differences in white and grey matter distribution. The brain can be broadly categorized into 3 parts – the cerebrum, the cerebellum, and the brainstem. The cerebrum is the largest, consisting of both the hemispheres of the brain. It is responsible for execution of high order functions such as the interpretation of sensory signals, regulating speech and emotions, aid in learning and memory, as well as fine motor control. The cerebellum lays at the base of the cerebrum and is responsible for controlling balance, posture, coordination, and movement. The brainstem connects the spinal cord to the cerebrum and cerebellum, and also controls automatic homeostatic mechanisms such as breathing, digestion, heart rate, body temperature, sleep cycles, swallowing, sneezing, and coughing.

Further distinction between the parts of the brain can be seen by looking at the 4 lobes each hemisphere has – the frontal lobe, the parietal lobe, the occipital love, and the temporal lobe. Each of these lobes is localized in function, i.e., they are associated, but not causally linking, to certain functions. The lobes of the brain and their functions are listed below.

The frontal lobe – This lobe ‘is generally where higher executive functions including emotional regulation, planning, reasoning and problem solving occur;’ it is associated with judgement, problem solving, comprehension, reasoning, planning, as well as speech since Broca’s area lies in the posterior frontal gyrus, i.e., an infolding in the cortex towards the back of the frontal lobe. This lobe also aids in the mediation of emotional and behavioural responses, essentially forming the basis of one’s personality. Lastly, this lobe is known to influence intellectual capacity and self-awareness.

The parietal lobe – It holds the sensory strip, which is why the processing of ‘signals from vision, hearing, motor, sensory and memory’ occurs in the parietal lobe. It is also responsible basic visuospatial perception and for the sense of touch, pain, and temperature. The parietal lobe also plays some part in the construing and comprehension of words.

The occipital lobe – This lobe is responsible primarily for visual processing - of colour, light, and movement - as it holds the primary visual cortex. ‘The primary visual cortex, also known as V1, receives visual information from the eyes. This information is relayed to several secondary visual processing areas, which interpret depth, distance, location and the identity of seen objects.’

The temporal lobe – This lobe is known to play part in memory and recall as its medial portion is close to the hippocampus, as well as sequential organization, and lingual ability as it holds Wernicke’s area. ‘The temporal lobe contains the primary auditory cortex, which receives auditory information from the ears and secondary areas, and processes the information so we understand what we’re hearing.’ Alongside hearing, it also performs some visual functions – such recognition and differentiation.

Apart from these 4 lobes, the cortex refers to the surface of the cerebrum, and has several infoldings that make it uneven, thus increasing its surface area. A larger surface area is attributed to the capacity to perform high order functions. ‘The cortex contains neurons (grey matter), which are interconnected to other brain areas by axons (white matter). The cortex has a folded appearance. A fold is called a gyrus and the valley between is a sulcus.’

Deeper structures within the brain are – the hippocampus, the hypothalamus, the thalamus, the limbic system with the amygdala, the pituitary and pineal glands, and the basal ganglia. The anatomical structure, position, and function of these parts are explained below.

The hippocampus – It ‘is a complex brain structure embedded deep into temporal lobe. It has a major role in learning and memory.’ This also makes it highly neuroplastic as neurogenesis and synaptogenesis, i.e., the creation of new neurons and synapses, occurs at a high rate here when a particular synaptic pathway is fired regularly. The hippocampus is also known to be associated with the consolidation of long-term memory via rehearsal.