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Central Nervous System

Contents

Introduction

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.

Medial view of the brain
Recommended video: Medial view of the brain
Structures seen on the medial view of the brain. The images show a midsagittal section of the brain.

Embryology

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.

Brain

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

Brain stem

Midbrain

Tectum

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.

Cerebral Peduncles

Cerebral crusThese 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.

Pons

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.

Medulla

This is the lowermost part of the brainstem. It is continuous with the spinal cord at the level of the foramen magnum.

Medulla oblongata - medial view

Limbic lobe

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.

Insula

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.

Hippocampus

Hippocampus - lateral-left viewThe 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.

Cerebrum

Frontal lobe

Frontal lobe - lateral-left viewThe 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.

Parietal lobe

Parietal lobe - lateral-left viewThese 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.

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.

Occipital Lobe

The occipital lobe contains the primary visual cortex and association visual areas.

Basal Ganglia

Lentiform Nucleus

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).

Caudate Nucleus

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.

Caudate nucleus - dorsal view

Amygdala

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

Internal Capsule

Internal capsule - cross-sectional viewThis 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.

Thalamus

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.

Pineal Gland

This is a small gland that releases melatonin, the hormone responsible for the regulation of sleep wake cycles.

Pineal gland - medial view

Hypothalamus

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.

Pituitary gland

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.

Cerebellum

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.

Cerebellum - medial view

Cerebrocerebellum

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.

Spinocerebellum

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.

Vestibulocerebellum

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.

Spinal Cord

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.

Spinal cord - dorsal view

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.

Spinal Tracts

Descending pathways

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.

Ascending pathways

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.

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Show references

References:

  • Frank H.Netter MD: Atlas of Human Anatomy, 5th Edition, Elsevier Saunders, Chapter 1 Head and Neck.
  • Chummy S.Sinnatamby: Last’s Anatomy Regional and Applied, 12th Edition, Churchill Livingstone Elsevier.
  • Richard L. Drake, A. Wayne Vogl, Adam. W.M. Mitchell: Gray’s Anatomy for Students, 2nd Edition, Churchill Livingstone Elsevier.
  • Elliiot L.Manchell: Gray's Clinical Neuroanatomy: The Anatomic Basis for Clinical Neuroscience.

Author, Review and Layout:

  • Shahab Shahid 
  • Uruj Zehra
  • Catarina Chaves

Illustrators:

  • Cerebral crus - Paul Kim
  • Medulla oblongata - medial view - Paul Kim
  • Hippocampus - lateral-left view - Paul Kim
  • Frontal lobe - lateral-left view - Paul Kim
  • Parietal lobe - lateral-left view - Paul Kim
  • Caudate nucleus - dorsal view - Paul Kim
  • Internal capsule - cross-sectional view - Paul Kim
  • Pineal gland - medial view - Paul Kim
  • Cerebellum - medial view - Paul Kim
  • Spinal cord - dorsal view - Rebecca Betts
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