Central Nervous System
The central nervous system is central to the functioning of the human body. It comprises of several important structures, essential for bodily functioning. The coordination of its various parts, as well as the interaction with the peripheral nervous system, enable our bodies to work as a unit, with descending control from the master organ i.e. the brain, as well as feedback from our peripheral receptors and senses. This article will discuss the embryology, anatomy, and clinical relevance of the central nervous system.
The central nervous system itself arises from the neural tube (which first forms during the third week of foetal life). At the early stage of development, there are three distinct layers forming. These layers lie together and form the trilaminar germ disc and lie on the floor of the amniotic sac. They are composed of the following layers:
- Endoderm: This layer gives rise to the liver, the pancreas, the gastrointestinal tract, as well as the respiratory tracts.
- Mesoderm: This layer is very important for the viability of the foetus. It represents the origins of the musculoskeletal, urinary, reproductive, and cardiovascular systems.
- Ectoderm: This contributes to the central and peripheral nervous systems. It also gives rise mainly to the skin.
The brain is the master organ and controls the functioning of all the body systems. On the surface, it is comprised of the cerebrum with all of its gyri and sulci. Deeper we have the basal ganglia, the thalamus and the pituitary gland . There are many more structures that form part of the brain. It is essential for:
- muscle movement
- basic continence functions
- higher functions such as thoughts and emotions
- whole body functions such as hormonal balance and ageing
This is referred to as the roof of the midbrain. It has on its posterior surface, the inferior and superior colliculi. These allow for auditory and visual reflexes, respectively.
These are paired Roman pillar-like structures. They connect the brainstem to the cerebrum. It you take an axial section through the midbrain you will see a heavily pigmented region known as the substantia nigra. This area is responsible for transmitting dopamine to the basal ganglia. Anterior to the substantia nigra is the crus cerebri. These transmit the primary motor tract fibres down from the primary motor strip to the spinal cord.
The pons lies between the midbrain above and the medulla below. It also lies anterior to the cerebellum. It contains numerous cranial nerve nuclei that control tears, taste, swallowing, sleep as well as eye movement. It appears as an anteriorly bulging structure.
This is the lowermost part of the brainstem. It is continuous with the spinal cord at the level of the foramen magnum.
The limbic lobe is a belt of tissue that runs around the corpus callosum . Its major functions include memory and emotion. The posterior parts of the orbitofrontal cortex, the anterior section of the insula, and the temporal pole have functions akin to the limbic lobe. Hence, they are referred to as paralimbic areas.
The insula is a segment of cerebral cortex that shields the basal ganglia, and lies within the recesses of the lateral sulcus. It has an important role in motor control, emotions and body homeostasis.
The hippocampus is a bunny rabbit shaped section (on sagittal view) that has functions in memory formation as well as special navigation (taxi drivers have very well developed hippocampuses as they have excellent spatial awareness). It lies within the temporal horn of the lateral ventricle.
The fibres from the hippocampus will merge together at the posterior section and form an arch known as the fornix . This is connected to the corpus callosum above by a membrane known as the septum pellucidum. These run forwards and terminate in the mammillary (or ‘nipple like’) bodies that then project to the thalamus via the mamillothalamic tract. This then projects to the cingulate gyrus that completes the loop by sweeping back around the corpus callosum to join the hippocampus.
The frontal lobe contains the following:
- the orbitofrontal cortex, which is the main area of inhibition of impulsive behaviors
- the pre-central gyrus i.e. the primary motor cortex
- the Broca’s area (on the left side), which enables us to form words. Broca’s homologue on the right side enables us to interpret body language.
These lobes lie on the superoposterior surface of the brain and are the main site of visual interpretation. They also have a crucial role in the pursuit eye movements we perform e.g. following an object across the horizon, as well as the saccades which draw our eyes to different parts of an object. It also contains the post central gyrus of primary sensory strip. Wernicke’s area lies in the boundary between this lobe and the temporal lobe.
The temporal lobe lies just under the lateral fissure on each cerebral hemisphere. It contains the transverse temporal gyri, which interpret auditory information. The left temporal lobe enables us to understand words and comprehend information.
The occipital lobe contains the primary visual cortex and association visual areas.
This nucleus resembles a cone, and has an outer dark section known as the putamen, and an inner section known as the globus pallidus (which itself is divided into an internal and external segment).
This is a c shaped nucleus with a head, body and tail. They lie on the medial surface of the frontal horn of the lateral ventricles, and are responsible for cognitive function.
The amygdala is an olive sized section of cortical matter that receives input from the olfactory nerves. Thus corresponds to danger detection, and fear.
Other Deep Brain Structures
This is a band of white matter that contains the fibres of the corticospinal tract descending down from the primary motor strip. It contains ascending and descending neurons. Its posterior limb separates the thalamus medially from the pallidus laterally, and its anterior limb separates the caudate nucleus anteromedially from the lentiform nucleus.
These are two large nuclei that function as the gateway to the cerebral cortex. The two thalami are said to ‘kiss’ across the third ventricle.
This is a small gland that releases melatonin, the hormone responsible for the regulation of sleep wake cycles.
This is a region of the brain that lies above the pituitary gland and produces small quantities of hormones e.g. Growth hormone releasing hormone and thyroid releasing hormone. This then signals to the pituitary for hormone production e.g. growth hormone, thyroid-stimulating hormone.
This is the master endocrine organ. It has an anterior and posterior part. The anterior part releases Growth hormone, thyroid stimulating hormone, luteinizing hormone and follicle stimulating hormone, ACTH. The posterior pituitary releases vasopressin and oxytocin.
This name comes from the word for ‘Little brain.’ It has three functional lobes, and sits in the posterior cranial fossa. It is the organ responsible for our coordination as well as the smoothness of our movements.
This region of the cerebellum ensures that movements can be performed both accurately and smoothly. Anatomically it is composed of the dentate nuclei, the cerebellar hemispheres, and the posterior lobe. This region will give rise to the crucial superior cerebellar peduncle, which is the major outflow of the cerebellum. From here, the axons synapse in the contralateral thalamus, and from here, to the motor and pre motor areas of the frontal lobe.
Anatomically, this is composed of the vermis and paravermal region as well as a large part of the anterior lobe. As such, it receives constant sensory (proprioceptive) feedback from the spinal cord.
The vestibulocerebellum is anatomically referred to as the flocculonodular lobe. It comprises of two floccules that lie in the cerebellar hemispheres, and the nodule of the vermis (a worm like structure in the midline). The cerebellum is connected to the vestibular nuclei of the brainstem, and utilizes the information from the vestibular division of the Vestibulocochlear nerve from the inner ear, to help maintain balance and posture. The cerebellum also enable is to adjust the position of our eyes and keep them steady with changing head position i.e. the vestibulo-ocular reflex.
The human spinal cord is divided into 31 nerve segments. These segments further sub divide into a ventral (motor) root and a dorsal (sensory root). Both these roots will combine, in order to form a mixed spinal nerve. From here, the each nerve will then subdivide again, in order to form a large ventral ramus (that form the major plexi of the body, and innervate the limbs and anterior trunk), and a small dorsal ramus (mainly sensation and a few back muscles). At each segmental level, the dorsal root ganglion has within it the cell bodies of sensory neurons. The neurons innervate a patch of skin i.e. a dermatome. As well as this, they also have the sensory neuron cell bodies of the joints, ligaments, and tendons.
An axial section of the spinal cord will reveal an H shaped region of grey matter. This has both input (sensory from the dorsal horns), and output (motor via the ventral horns). Each ventral horn contains longitudinal columns of motor neurons. These are responsible for innervating a functionally related group of muscles. The dorsal roots receive input from the dorsal root ganglia. Around this grey core, is a thick band of well myelinated white matter. These white matter tracts allow segmental levels to connect, and also transmit via the long tracts passing to and from the brain.
Corticospinal tract: The voluntary movement tract. It arises from the primary motor strip of the frontal lobe, passes through the posterior limb of the internal capsule, descends down the midbrain and pons and decussates at the junction of medulla and spinal cord. It then supplies voluntary motor function to the contralateral part of the body.
Lateral Spinothalamic Tract: Pain and temperature are transmitted through the brain via this tract. The pain impulses are transmitted to the spinal cord via A delta and C fibres.
The Dorsal Column pathway: This pathway transmits proprioception and vibration to the brain from the limbs.
There are many more spinal tracts but the basic three are covered here.
A stroke is a vascular event disrupting blood flow to the brain. This may be ischaemic (blockage by an embolus), or haemmorhagic (a bleed in the cerebral arteries. A stroke affecting the blood vessel on the right side of the brain will cause hemiplegia on the left side of the body and vice-versa. This is due to the decussation of the corticospinal tract.
Degradation of the substantia nigra means that dopamine is not produced as much. This results in an inability of the dopamine to ‘disinhibit’ the inhibition of basal ganglia; hence a greater amount of effort is required to perform a movement. This results in:
- slow movements that are difficult to initiate
- masked facies
- shuffling gait
- lack of emotion
If Broca’s area in the inferior frontal gyrus of the brain is damaged by a stroke or otherwise, forming speech becomes difficult. The person will understand speech but will find it impossible to form it.
Sub-acute combined degeneration of the cord
This is a condition caused by vitamin B12 deficiency. Symptoms include gradually worsening weakness and numbness in the legs, arms and trunk, as well as visual and mental state changes in the late stage of the disease.