Neuroanatomy primer

The aim of this post is to clarify some basic neuroanatomy terms and to give you a sound foundation. Before we start, a quick bit of housekeeping, I use the British spelling for neurone (also used by Gray’s Anatomy, The Anatomical basis of clinical practice) as opposed to the American spelling neuron. Abbreviations used include CNS: central nervous system, PNS: peripheral nervous system, SNS: sympathetic nervous system and PSNS: parasympathetic nervous system.

Orientation

One of the first concepts to address is orientation, neuroanatomical descriptions for orthogonal planes are slightly different to the planes used for the rest of the body. For a second I would like you to think about a dog or any other quadruped, standing on all four legs with its head extended (looking forward as it would normally), its neural axis (tip of the frontal lobe to the end of the spinal cord) is virtually horizontal. Therefore rostral mean towards the beak/nose (or the tip of the frontal lobe), caudal towards the tail, ventral towards the belly and dorsal towards the back (spine).

In humans, the standard anatomical position is us standing on two legs, our neural axis goes from inferior to superior, but once we get to our head, unlike the quadrupeds, our head is bent forward (flexed), so at that point our neural axis curves forward (see the red line in Figure 1). So the brainstem is still relatively in line with our spine; however, our brains are tilted forwards. Hence when considering the brainstem and brain, the front of the brainstem is the ventral surface, the back is the dorsal surface (as applies to the spinal cord), the medulla is caudal and the midbrain is said to be rostral (it means towards the beak/nose). Our neural axis is curved forward because compared to a quadruped, our head is bent effectively forward. the top of our brains is the dorsal surface, this is because it is a continuation of the dorsal surface of the brainstem (think about the dog). The bottom of the brain is thus the ventral surface, as it continues from the ventral surface of the brainstem. The frontal lobe of the brain is rostral and the occipital lobe is caudal, see Figure 1.

brainanatomicalaxes

Figure 1 Neural axis, r = rostral, c = caudal, d = dorsal v = ventral

You can relax now, that was the tricky bit and the rest is straightforward. The nervous system can be thought of as a system which allows us to gather information about our internal and external environment, then process that information and react appropriately to it. The cells that constitute the nervous system can be classified into two types, neurones and neuroglial cells. Neurones are the cells that produce, transmit and receive action potentials, the cells have specialised features to enable these functions. There are three types of neurons, multipolar (motor cells), bipolar (special sensory) and pseudounipolar (sensory). Neuroglial cells are supporting cells and essentially take the role of connective tissue cells, which are not found in the central nervous system (CNS). For more information on the types of neuroglial cells and the features of neurones please see a copy of Wheater’s or Junquira’s.

The nervous system can be classified from both a functional (Figure 2) and anatomical (Figure 3) perspective. The following definitions focus on the anatomical perspective and I would recommend reading through them in order.

funcclass.png

Figure 2 Functional classification of the nervous system

antomclass2.png

Figure 3 Anatomical classification of the nervous system

Central nervous system

From an anatomical perspective, the CNS consists of the brain, spine, optic nerve (cranial nerve I) and retina; the brain and spine are subdivided into white and grey matter. The brain can also be divided into its key regions, forebrain, midbrain and hindbrain, these terms are expanded upon later in this list.

Peripheral nervous system

The peripheral nervous system consists of neural tissue outside of the meninges, this includes the peripheral autonomic nervous system (SNS, PSNS and enteric nervous system), all spinal and cranial nerves (including special senses), except cranial nerves I and II. Cranial nerves I and II are anatomical extensions of the CNS.

White matter

White matter consists of neuroglial cells and bundles of myelinated (myelin gives the tissue its white colour) axons (not the bodies of the neurones), these bundles of axons are called tracts and are further subdivided into three types of tracts, projection, commissure and association fibres.

Projection fibres

Projection fibres are tracts (bundles of myelinated axons) which ascend and descend the CNS, connecting cortical regions with subcortical, brainstem and spinal cord nuclei. Examples include your motor (corticospinal, corticonuclear etc) and sensory tracts (dorsal column, spinothalamic etc.)

Association fibres

Association fibres are tracts (bundles of myelinated axons) which connect one cortical area to another cortical area within the same hemisphere. An example includes the cingulum (in the cingulate gyrus), which connects the frontal and parietal lobes with the parahippocampal gyrus.

Commissure fibres

Commissure fibres are tracts (bundles of myelinated axons) which connect one cortical region in one hemisphere to the same cortical region in the opposite hemisphere. Examples include the corpus callosum, anterior and posterior commissures etc.

Grey matter

As I mentioned earlier, the CNS can be subdivided into white and grey matter. Grey matter consists of collections of functionally related neuronal cell bodies embedded amongst neuroglial cells. There are two types of grey matter, cortex and nuclei.

Cortex

Cortex is a type of grey matter (collection of cell bodies) found at the edges of the CNS, therefore the two types are, cerebral and cerebellar.

Nuclei

Nuclei (singular: nucleus) is the other type of grey matter, which consists of a collection of functionally related neuronal cell bodies, surrounded by white matter (unlike cortex which is on the edges of the CNS, nuclei are embedded within the CNS). Examples include the basal nuclei, the thalami and the cranial nerve nuclei, etc.

Ganglia

Are collections of cell bodies of neurones in the PNS, not the CNS (remember nuclei are collections of cell bodies in the CNS). They are NOT examples of grey matter as they are not located in the CNS. Examples of ganglia include the posterior root ganglion (aka dorsal root ganglion), trigeminal ganglion, geniculate ganglion, sympathetic ganglia (para-vertebral ganglia) and parasympathetic ganglia.

Nerve

Is a collection of myelinated and non-myelinated axons in the PNS. Therefore strictly speaking there are no nerves in the CNS, only tracts.

Tracts

A bundle of myelinated axons in the CNS, therefore this is an example of white matter, tracts come in three types: projection, association and commissure fibres.

Spinal cord

The spinal cord is the caudal continuation of the brain stem (medulla oblongata), it starts at the foramen magnum and ends approximately at the lower border of the first lumbar vertebra. As it is part of the CNS, it too consists of white and grey matter. The grey matter is centralised in an H shape (as seen in axial sections of the spine). The central bar of the H is known as the grey commissure, containing the small central canal. The arms of the H are called the anterior and posterior horns, in the thoracic and upper lumbar region, there are also smaller lateral horns (preganglionic sympathetic neurones). Surrounding this grey H are ascending and descending white matter tracts. The size of the H is dependent on the number of muscles innervated, hence the H appears larger in the cervical and lumbosacral regions of the spinal cord, where the upper and lower limbs obtain their innervation respectively.

Spinal nerve

A bundle of myelinated and non-myelinated axons, containing both afferent and efferent axons, formed from the union of an anterior (efferent) and posterior (afferent) root from the spinal cord. Spinal nerves are short and as soon as they exit the intervertebral foramina, they divide into anterior and posterior rami. In total there are 31 pairs of spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal).

Cranial nerve

A cranial nerve is the same as a spinal nerve, except cranial nerves come directly from the brain stem (except cranial nerves I, II and XI) rather than the spinal cord. There are 12 pairs of cranial nerves, I and II are technically not typical cranial nerves, in fact, they are not nerves, but rather tracts (they even have meningeal coverings). See a neuroanatomy text for further details.

Upper motor neurone

Is a motor neurone located in the cortex, whose axon eventually synapses on the cell body of a lower motor neurone (either an interneurone, alpha or gamma motor neurone) located in a cranial nerve nucleus (located in the brainstem) or the anterior horn of the spinal cord.

Lower motor neurone

Is an alpha motor neurone (multi-polar neurone cell) located in a brainstem nucleus or the anterior horn of the spinal cord. Its axon extends out of the brainstem/spinal cord as an efferent root, which then becomes part of a cranial/spinal nerve respectively, lower motor neurones innervate extrafusal muscle fibres (skeletal muscle).

Encephalon

Encephalon means the brain; the brain is split into different regions based on its morphology and evolution. During embryological development the cranial end of the neural tube divides into three primary vesicles, from caudal to cranial you have the rhombencephalon (hindbrain), mesencephalon (midbrain) and prosencephalon (forebrain). As development proceeds, both the rhombencephalon and the prosencephalon each divide into two further secondary vesicles, see below for further details.

Rhombencephalon

This is the caudal-most part of the brainstem, also known as the hindbrain and is subdivided into the myelencephalon (medulla oblongata) and metencephalon (pons and cerebellum).

Mesencephalon

This is the midbrain, the rostral-most part of the brainstem. From the ventral surface, the cerebral peduncles arise and attach the brainstem to the brain. Dorsally located are the rounded surfaces of the superior and inferior colliculi. The cerebral aqueduct (connecting 3rd and 4th ventricles) channels through the dorsal aspect of the midbrain in a rostral-caudal direction.

Prosencephalon

This is the forebrain and is the rostral-most portion of the brain. It is subdivided into the diencephalon and telencephalon.

Diencephalon

This consists of the thalamus, hypothalamus, epithalamus and subthalamus, all these structures surround the third ventricle. Cranial nerve II arises from the diencephalon, the optic tract (cranial nerve II) fibres terminate in the lateral geniculate body of the thalamus.

Telencephalon

The telencephalon makes almost 85% of the entire brain and includes the cerebrum (both left and right cortex), the underlying white matter (e.g. corpus callosum, internal capsule), hippocampus and the basal nuclei (aka basal ganglia). The olfactory nerves (cranial nerve I) arise from the olfactory epithelium and terminate in the temporal and frontal lobes.

References:

  • Clinical Neuroanatomy, Richard Snell
  • Gray’s Anatomy: The Anatomical Basis of Clinical Practice,  Susan Standring

Leave a Reply