Brain Anatomy

(click pictures to enlarge) FIG. 1

    The brain is the centre of the nervous system and as such is an incredibly complex organ. It is required to receive process and respond to sensory information; control motor function; maintain homeostasis whilst all the time coordinating our perception and thought. Each area of the brain is responsible for separate tasks as part of its overall function, though it is interesting to note that in cases of certain damage, different areas may take on some of the roles of the defunct brain matter.
The Human brain is divided into three main areas:

• The Hind brain (rhombencephalon)
• The Mid-brain (mesencephalon)
• The Forebrain (prosencephalon)

    The forebrain and the hindbrain are also divided down the sagittal plane into hemispheres. These hemispheres have lobes which are derivative of specific functions.

The Hind brain/Rhombencephalon

 

FIG. 2

    The  Hind brain/Rhombencephalon is located within the posterior fossa of the cranial cavity. The Rhombencephalon has the following functions:

• Attention and Sleep
• Autonomic Functions
• Complex Muscle Movement
• Conduction Pathway for Nerve Tracts
• Reflex Movement
• Simple Learning

It is made up of a number of several important structures. These include:

•  The Myelencephalon

1. Medulla Oblongata

FIG. 3

    The Medulla Oblongata is the part of the brain following on immediately from the spinal cord. The  medulla is responsible for a number of autonomic functions such as:

• Breathing
• Conduction Pathway for Nerve Tracts
• Digestion
• Heart Rate
• Swallowing
• Sneezing
• Defecation

     The medulla oblongata is continuous with the spinal chord and part of what is often known collectively as the brain stem. The medulla is so vital to life that diseases effecting it are often fatal.

    2. Lower part of the 4th ventricle

FIG.4

     The fourth ventricle is part of a network of cavities (the ventricular network funnily enough) The fourth ventricle runs between the sinus of sylvia and the obex and is filled with cerebrospinal fluid.

• The Metencephalon

1. Pons

FIG.5

    The Pons measures roughly an inch in length and lies between the Mid-brain and the Medulla Oblongata. It contains nuclei that deal with sleep, respiration, swallowing, bladder control, hearing, equilibrium, taste, eye movement, facial expressions, facial sensation, and posture. It’s functions consist of:

• Relaying sensory information between the cerebrum and cerebellum reticular formation.
• Regulating awareness and sleep.

        2. Cerebellum

FIG.6

    Sometimes referred to as the “little brain” the Cerebellum is inferior to the cerebral hemispheres and has the appearance of being a separate entity to the rest of the brain. This is partially due to its surface appearance; in contrast to the cerebral hemispheres its surface is covered in closely spaced parallel grooves.  It is composed primarily of white matter with the exception of its surface of densely folded grey matter. The cerebellum is mainly involved in motor control; processing information from other areas of the brain, the spinal cord and sensory receptors to fine tune movement. Recent research suggests it also plays a role in the processing of certain temporal stimuli such as language and music. Functions:

• Fine movement coordination
• Balance and Equilibrium
• Muscle tone
• Basic memory and learning

        3. Rhombencephalic isthmus

     The Rhombencephalic isthmus is a constricted portion immediately adjoining the mid-brain and including the superior peduncles of the cerebellum, the anterior medullary velum, and the upper part of the fourth ventricle.

The MidBrain/Mesencephalon

    

FIG.7

     The Midbrain/Mesencephalon is superior to the Pons and inferior to the cerebral hemispheres. The dorsal portion of the midbrain is known as the tectum, it is responsible for reflexes relating to hearing and sight (e.g. the eye movement, pupil size, lens shape). The ventral portion of the midbrain is known as the tegmentum, it is essentially a complex network of neurones responsible for unconscious homeostatic and reflexive pathways. The midbrain also contains the crus cerebri which is composed of nerve fibres connecting the cerebellum and the cerebral hemispheres. Interestingly studies have shown that the midbrain of vertebrates may also be related to outbreaks of aggression.

The Forebrain/Prosencephalon

FIG. 8

     The Forebrain is superior to both the hindbrain and the midbrain as well as being the most anterior. It has major roles in  the following actions:

• Mastication
• Directs sensory impulses through the body
• Equilibrium
• Vision
• Eye movement
• Facial sensation
• Hearing
• Phonation
• Intelligence
• Memory
• Personality
• Respiration
• Salivation
• Swallowing
• Smell
• Taste

     The Forebrain is divided into two primary structures:

• Telencephalon (Cerebrum)

1. Cerebral cortex

FIG.9

    The cerebral cortex is the folded outer portion of the brain, in humans it is between 2-4mm thick. Its has the highest levels of uninsulated grey matter of any area of the brain. The cortex forms forded bulges (thus significantly increasing the area without increasing the volume) called gyri; so much so that more than 2 thirds of the brain lie in these folds (know as sucli). As well as being divided into portions covering the right and left hemispheres, the cerebral cortex can be subdivided according to differences in lamination:

a. Isocortex/Neocortex/Neopallium

    The isocortex is the outer layer of the cerebral cortex. It contains the most highly evolved stratification and organisation of any part of the human brain. It is made up of 6 layers of differentiated neurone cells of specified orientation. It should be noted that some functional areas of the isocortex are exceptions, lacking specific layers. The isocortex is (evolutionarily speaking) the newest area of the human brain and as such is the site of the majority of higher functions of the brain.

   The isocortex is divided into a number of topographically distinct areas or lobes. Each lobe is associated with particular processes.

i. Frontal lobe

FIG.10

    The Frontal lobe is the most anterior of the lobes and is additionally superior to the temporal lobe. This area of the brain is associated with many of the traits associated with personality (e.g ability to comprehend future results of actions), learning, impulse control, and prioritising actions. It is host to most of the brain’s dopamine receptors (these are the primary feedback through which learning is rewarded). This part of the brain is associated with emotion and subsequent modification in order to fit established social conventions.

ii. Temporal lobes

FIG. 11

   The temporal lobes are inferior to the frontal and parietal lobe and anterior to the occipital lobe. Studies suggest they are the primary part of the brain involved in declarative memory; damage to the temporal lobes can result in an inability to form memory after the event (anterograde amnesia). They contain the hippocampus (long-term memory) and are involved with auditory and higher visual perception (e.g. facial recognition).

iii. Parietal lobe

FIG. 12

    The parietal lobe is anterior of the occipital lobe, posterior of the frontal lobe and superior of the temporal lobes. The border between the frontal lobe and the parietal lobe is marked by the central sulcus. The border between the occipital lobe and the parietal lobe is marked by the parieto-occipito sulcus and the border between the temporal lobe and the parietal lobe is marked by the lateral sulcus. The parietal lobe coordinates information from multiple senses in order to establish spatial orientation. This is an area responsible for a number of higher cognitive processes such as speech and visual perception. It is also the area where the sensations of pain and touch are processed.

iv. Occipital lobe

FIG.13

      The Occipital lobe is the most posterior of all the main lobes of the brain. Anatomically this region contains most of the visual cortex (Brodmann area 17) and damage to the occipital lobes results in homonomous vision loss (i.e. the effect is the same in both eyes). The occipital lobes are where shape, colour, and like the temporal lobes, facial recognition take place. Projections from the occipital lobe to the superior temporal-parietal area are important for perceiving motion of objects.

b. Allocortex/heterogenetic cortex

    The Allocortex has fewer layers than the isocortex and is considered to be the “ancient” part of the brain. Allocortex structures include the olfactory cortex (concerned with the sense of smell) and the hippocampus (or archicortex).

c. Paleocortex

    An obsolete definition of this term exists describing paleocortex as an intermediate between allocortex and isocortex structures. This definition has not changed significantly except that nowadays the olfactory bulb is considered to be paleocortex.

2. Basal ganglia/nuclei

FIG.14

     The basal ganglia are a component of the corpus striatum and are in essence a set of interconnected nuclei within the brain. Information from the cerebral cortex passes to the basal ganglia where it is processed and then relayed back via the thalamus. There are a plethora of connections and pathways within and although the basal ganglia have long been implicated in motor function; it is known this is not there sole function, though the exact action in relation to behaviour control have yet to be properly established. Evidence suggests that during learning, basal ganglia and medial temporal lobe memory systems are activated simultaneously and that in some learning situations competitive interference exists between these two systems. One theory suggests the basal ganglia decides which out of a number of possible actions the cortex may be planning, actually gets executed. Fitting this with idea that dopamine is used as a reward system for learning. The main components of the basal ganglia are:

1. Striatum - This area in particular is thought to play a critical role in memory and learning.
2. Pallidum
3. Substantia nigra (functionally related)
4. Subthalamic nucleus (functionally related)
5. Caudate nucleus
6. Lenticular/leniform nucleus 

• Diencephalon

FIG.15

    The diencephalon relays information between many different parts of the brain and controls a number of autonomic functions in the peripheral nervous system. It is also know to work in conjunction with the limbic system to generate and manage emotions as well with the endocrine and nervous systems. The diencephalon is involved in the following functions:

• Directing Sense Impulses Throughout the Body
• Autonomic Function Control
• Endocrine Function Control
• Motor Function Control
• Homeostasis
• Hearing, Vision, Smell, and Taste
• Touch Perception

  Structures that make up the diencephalon include:

1. Hypothalmus

     This structure works with the pituitary gland and the peripheral nervous system in maintaining homeostasis.

       2. Thalamus

     This is a limbic structure and is involved in perception/regulation of motor functions.  As a regulator of sensory information, the thalamus also controls sleep and awake states of consciousness.

        3. Epithalamus

      This structure is primarily concerned with connecting the limbic system to other parts of the brain. It contains the pineal gland which uses produces melotonin as part of its role in the modulation of sleep patterns.

FIG.1, 2, 4, 5, 6, 7,8,9,10,11,12,13- Adapted from Wikimedia

FIG. 3 – Wikimedia

FIG. 14 – Public domain

FIG. 15 – Wikimedia