The Neuromuscular System (Part 1) – Basic Anatomy of the Brain

While you revise for your Neuro/Sport Sci/Kinesiology/Physio etc etc undergrad degree, you may come across the large and intricate set of wirings that we call our neuromuscular (NM) system! Or you may be a brain enthusiast while being unsure where to start? Allow me to spark up your own set of wires as we visit each organ and structure within this series of articles dedicated to the NM system.

Simply put: The NM system is the journey from brain to muscle.

Let me try that again in a few more words: The NM system is the journey of electrochemical information (in the form of motor & sensory neurons and neurotransmitters) from the central nervous system (CNS) to the peripheral nervous system (PNS).

Let’s start off with the king of all; the brain.

Gross Anatomy of the Brain

At a gross level, your brain can be divided as the cerebrum, cerebellum and brain stem (Figure 1).

Figure 1. Brain – Primary Regions

The cerebrum (forebrain) is the region of your brain responsible for the most advanced and complex tasks (e.g. cognition & movement), and is further subdivided into the telencephalon and diencephalon. The telencephalon includes the cerebral cortex (aka gray matter), subcortical white matter and basal nuclei (ganglia), while the diencephalon beneath, consists of the thalamus and hypothalamus.

Quick tip: If you’re wondering why the differentiation between white and gray matter? White describes the glistening view of a freshly sectioned brain, representing the high amounts of lipid-rich myelin and lack of neuronal cell bodies and synapses. (We will discuss neurons and myelin soon) (Figure 2).

Grey white matter
Figure 2. a) Gray matter found in the cerebral cortex followed by subcortical white matter. b) This pattern is inversed for the spinal cord.

The brain stem is the connection or relay centre between the brain and spinal cord and consists of the midbrain (mesencephalon), pons and medulla oblongata (relay centre for autonomic function). Finally, the cerebellum, just like the brain (hence named ‘mini brain’) is split into two hemispheres known as the right and left cerebrocerebellum. It’s main role is played within the coordination of movement through the intergration of efferent (from brain/motor) and afferent (to brain/sensory) neurons from the motor cortex and spinal cord respectively.

It is also important to note that you are currently viewing the brain in the sagittal plane (left side) and while it is identical in many ways on the opposite side, the brain is anatomically divided into the right and left hemispheres (Figure 3). These are connected via a bundle of fibers called the corpus callosum and plays a role in cross communication (the right hemisphere controlling the left side of the body and vice versa).

Figure 3. Dorsal view of brain -Right & left hemishpheres connected via the Corpus Callosum.

Another way of helping you to remember the brain in it’s simplest anatomical locations is to remember it via the various lobes, cortices and what lies therein..

Brain cortices and lobes
Figure 4. Cortices, lobes and association areas of the brain.

The brain is split into four major lobes known as the

  1. Frontal lobe – Containing the pre-motor and primary motor cortex (voluntary movement).
  2. Parietal lobe – Seperated from the frontal lobe via the central sulcus and consists of the somatosensory cortices.
  3. Temporal lobe – Seperated from the frontal lobe by the lateral fissure and consists of the primary auditory cortex.
  4. Occipital lobe – Seperated from the parietal lobe by the parietooccipital sulcus and consists of the primary visual cortex.

Finally, to complete our journey around the gross anatomy of the brain, it’s structure of multiple creases allows for higher function by increasing overall surface area (i.e higher amount of neurons within the skull). These specific folds are named gyri (singular: gyrus) while the groove in between each fold are named sulci (singular: sulcus).

Quick fact: Neuroscientists and neurophysiologists that attempt to understand the brain using brain stimulation, follow techniques based on ‘brain mapping‘. While there are various methods used in the study of brain mapping such as Broddman areas, the most commonly taught and understood map of the brain revolves around the motor and somatosensory cortex. This is known as the motor and sensory homonculus, and depicts what region of the cortex activates which region of the body (Figure 5).

Motor sensory homonuculus
Figure 5. Motor and sensory homonculus. Notice how legs and toes require regions further within the cortex representing deeper levels of stimulation for distal muscle groups.

Deep Brain Structures

To connect information between each of the cortices, white matter tracts are embedded deeply within the brain between the right and left hemishpheres. These tracts are collectively known as the basal nuclei (or more commonly known as the basal ganglia, though ‘nuclei’ is the term commonly given for a group of neurons within the CNS). The basal nuclei consists of the striatum (caudate nucleus & putamen), subthalamic nucleus, substantia nigra and globus pallidus (Figure 6).The basal ganglia works alongside the cerebellum to aid and coordinate fine movement.

Figure 6. Basal Ganglia – Sagittal view

Other deeper brain structures include (but are not limited to):

  • Thalamus – Gatekeeper/relay station of most sensory information to and from the cerbral cortex. Makes up most of the mass of the diencephalon.
  • Hypothalamus – Directly above the brain stem and responsible for autonomic control (to restore homeostasis) or releasing hormones; either to the pituitary gland (to secrete further hormones) or the secretion of it’s own (oxytocin and vasopressin).
  • Pituitary gland – Situated below the hypothalamus as two lobes (anterior and posterior pituitary) and a major contributor to the endocrine system. Controlled by the hypothalamus through ‘releasing hormones‘ and secretes further hormones thereafter. While the posterior pituitary is connected via neuroendocrine pathways to aid in the secretion of oxytocin and vasopressin, it cannot synthesize hormones itself. On the other hand, the anterior pituitary is responsible for the secretion AND synthesis of hormones but is not connected to the hypothalamus via neural pathways. Rather it is connected by a specific set of blood vessels named the hypophyseal portal system.
  • Pineal gland – Part of the ‘epithalamus‘ and unlike many of the deeper structures, it is not symetrically seen in both cerebral hemispheres, but just sits exactly on the midline of the brain. While it is currently known that no neurons leave this gland, it is however responsible for the secretion of melatonin, therefore playing a role in  the sleep-wake cycle/circadian rhythm.
Figure 7. Midsagittal view of brain – Deeper structures
  • Hippocampus – Found in both cerebral hemispheres (hippocampi) and named after ‘seahorse‘ in Greek to represent it’s shape upon removal (Figure 8). Plays a key role in the limbic system and formation of memories. Has been seen to signifcantly deteriorate in neurodegenerative conditions such as Alzheimer’s; also found in retired and active athletes who have undergone signifcant head trauma.
  • Amygdala – Found in both cerebral hemispheres (amygdalae) and also a key component of the limbic system, responsible for emotion, fear and pain processing. It may work alongside other regions of the limbic system such as the hypothalamus to initiate the ‘fight or flight’ response, thus releasing hormones such as adrenaline and thereby increasing heart and breathing rates. This has also been seen to signifcantly deteriorate in retired and active athletes who have undergone signifcant head trauma.
Hippocampus seahorse
Figure 8. Representation of the Hippocampus upon removal (left) and location of Amygdala and Hippocampus (right) in relation to other previously mentioned regions.

Ventricles and Cerebrospinal Fluid

Ventriculi (latin for ‘belly‘) within the brain are interconnected cavities forming a network or pathway for within which cerebrospinal fluid (CSF) is secreted and travels around the brain and spinal cord. The brain consists of four ventricles:

  • Lateral ventricles – C shaped chambers found deep within each cerebral hemisphere.
  • Third ventricle – Connected to the lateral ventricles via the interventricular foramen or foramen of Monro and found within the midline of the diancephalon.
  • Fourth ventricle – Connected to the third ventricle via the cerebral aqueduct and found within the small gap between the cerebellum and brainstem.

From here, the pathway for CSF continues through the central canal down the brainstem towards the spinal cord and the subarachnoid space above (Figure 9).

Figure 9. Pathroute of CSF

How is CSF also secreted by these ventricles you may ask..

The ventricles are lined by ependymal cells which form the membrane known as the choroid plexus. These cells are a particular type of glial cell (to be covered later) which secrete CSF of an approximate amount of 0.5L per day. This is generally well regulated by the body (through replenishment/resorption) and plays the role of keeping the brain suspended in an upright fashion within the cranium. It also provides a soft cushioning around and within the brain to partially protect it from injury (slowed rate of accelerational forces) and aids in the removal of waste products towards the bloodstream for renal filtration. Lack of CSF could lead mechanical injury and damage of brain tissue, while heightened levels of CSF could lead to ventricular enlargement, impingement of brain structures and hydrocephalus (skull enlargement).

That concludes my summary of some basic neuroanatomy for today! Stay tuned for the following articles/topics within the NM System series:

  • Spinal Cord anatomy and the Corticospinal tract.
  • Neuronal and Glial Cells
  • Neurotransmission & Membrane Potentials: Excitatory or Inhibitory?
  • Motor Units – Recruitment & Synchronization.
  • NM Contraction (Skeletal Muscle anatomy & Sliding Filament theory).

If you found this useful in anyway, please feel free to drop a comment, give the article and share to all your friends and family. Feel free to connect with me also via Twitter  @Nasiruddin4595 or Instagram @Physcombat