Wk5 Topography Of The Brain

Question 1

What does the central nervous system consist of?

Answer 1

Brain, spinal cord and proximal sections of nerves (meninges)

Question 2

What is a méninge?

Answer 2

Proximal sections or segments of nerves found within enclosing membranes

Question 3

What does the nervous system divide neatly into?

Answer 3

Somatic nervous system

Autonomic nervous system

Question 4

What does the neural tube divide into in the 1st stage?

Answer 4

3 primary vesicles
Forebrain
Midbrain
Hindbrain

Question 5

2nd stage of neural tube differentiation

Answer 5

forebrain and hindbrain each differentiate into 2 further secondary vesicles, while the midbrain remains unchanged. This results in 5 secondary vesicles of the embryonic brain, which then develop progressively into what becomes the adult brain

Question 6

What does the forebrain consist of?

Answer 6

Cerebrum and thalamic brain masses

Question 7

What is the cerebrum?

Answer 7

largest part of the brain

it is divided into two bilaterally symmetrical hemispheres that at first sight, appear to be separated by the longitudinal fissure

each cerebral hemisphere is divided into four superficial lobes and two deeper lobes that cannot be seen from the surface of the brain

Question 8

What does the frontal lobe do?

Answer 8

occupies the front of the brain and is largely involved in planning and commanding voluntary movements

Question 9

What is the parietal lobe?

Answer 9

receives and processes general somesthetic sensations (perceptions of the body surface and internal organs)

Question 10

What is the temporal lobe?

Answer 10

receives and processes auditory (hearing) information

Question 11

What is the occipital lobe?

Answer 11

receives and processes visual information.

Question 12

What is deeper to the parietal lobes?

Answer 12

cingulate gyrus or limbic lobe while the insular lies deep to the temporal lobe

Question 13

What are gyri?

Answer 13

Elevations or ridges of brain tissue, separated by depressions known as sulci

Question 14

What are major sulci?

Answer 14

Fissures

Question 15

What are the main separations in the brain?

Answer 15

the central sulcus separates the frontal and parietal lobes while the lateral fissure (archaic: Sylvian fissure) separates the temporal lobe from the parietal and frontal lobes. The parieto-occipital sulcus is not easily seen from the lateral surface. It formally separates the occipital lobe from the parietal lobe: look for the pre-occipital notch and connect it in your imagination to the bend in the lateral fissure. It is more easily seen from the medial surface.

Question 16

Thalamic masses (diencephalon)

Answer 16

The diencephalon is the second part of the forebrain and is almost completely covered by the cerebral hemispheres. In terms of neural function, it is the second most complex brain structure and works as a relay centre between the cerebrum and the rest of nervous system tissues. The major sub-divisions of the diencephalon are the thalamus and hypothalamus. The thalamus is a bilaterally symmetrical, egg-shaped structure sitting on either side of the 3rd ventricle (one of the spaces, or canals within the brain). Its groups of neurones, known as thalamic nuclei act as ‘switchboards’, controlling what sensory information reaches the cerebrum and how motor commands from the frontal lobes are distributed downstream.

Question 17

What is below the thalamus?

Answer 17

Below the thalamus is the hypothalamus (prefix hypo- translates as ‘below’). The hypothalamus extends inferiorly to form the posterior part of the pituitary gland, which directly regulates the release of hormones into the bloodstream. The hypothalamus is the central controller of homeostasis of, for example body temperature, blood sugar, blood volume and osmolarity. The epithalamus (prefix epi- translates to ‘upon or over’) includes the pineal gland, which releases melatonin and is involved in the control of our circadian rhythms.

Question 18

Brain stem

Answer 18

Taken together, the midbrain, pons and medulla constitute what is known as the ‘brainstem’. The brainstem is very important to normal functions of the brain because all neurons that travel anywhere between the cerebral cortex and spinal cord, or back to the cerebral cortex pass along motor and sensory pathways through the brainstem. Secondly, it contains the nuclei of ten of the cranial nerves. Thirdly, it is home to neurones that regulate some of the brain’s basic functions that sustain life such as breathing and regulation of heart rate. Thus, damage to the brainstem can be catastrophic, often leading to loss of life.

Question 19

Midbrain

Answer 19

The midbrain is a relatively small structure connecting the forebrain and hindbrain and lies above and below the cerebral aqueduct, which joins the 3rd and 4th ventricles allowing cerebrospinal fluid (CSF) to flow between them. It is associated with vision, hearing, motor control, sleep and wakefulness, and state of arousal.

Question 20

Hindbrain

Answer 20

The hindbrain consists of the pons, medulla oblongata, and cerebellum. The pons is connected to the cerebellum by the cerebellar peduncles and it coordinates activities of the cerebellum. It also houses neurones that are important in regulating respiratory rhythm. The medulla not only contains fibre tracts running to and from the forebrain, midbrain, and spinal cord, but also contains neurones that regulate respiration, and that regulate a range of autonomic functions from heart rate to vomitting.

Question 21

Cerebellum

Answer 21

The cerebellum is connected to the dorsal side of the brainstem and is involved in fine motor control and memory of motor skills such a ‘riding a bike’ etc. It is designed along similar lines as the cerebrum in that it has two hemispheres split by a central fissure, and has a highly folded surface, However, it does not have gyri and sulci; its parallel grooves are more like the pleats of an accordion

Question 22

Meninges

Answer 22

The brain, spinal cord and central segments of some nerves are completely covered by 3 separate but closely apposed layers of connective tissue envelopes collectively known as the meninges.

The dura mater (latin: tough mother) is the most superficial and most fibrous layer. It is strong and protects the brain from physical trauma. It is within the cranium and is formed from two distinct layers as follows:

• The periosteal layer of dura, also known as the endosteal (meaning: inside bone) layer is most superficial and is actually derived as the endosteal membranes of the cranium. Naturally, it adheres tightly to the inner plates of bones of the skull. This membrane is only found intracranially and not within vertebrae surrounding the spinal cord.

The meningeal layer is the true meninges and covers the brain and spinal cord. It is generally tightly adhered to the endosteal layer intracranially, except where there are dural venous sinuses, which receive blood from cerebral veins, and CSF from subarachnoid space via the arachnoid granulations. Here, the two layers of dura mater are separated from one another.

The arachnoid (meaning: web-like) layer lies deep to the dura mater. It does not follow the contours of the brain and therefore does not generally enter the sulci, the exception being the longitudinal fissure. It is not as fibrous and tough as the dura mater; instead, its role is to hold the liquid cerebrospinal fluid (CSF) in the subarachnoid space. The function of CSF is to provide cushioning and buoyancy to the brain and thereby protect it from any high-energy impacts. The subarachnoid space also contains the cerebral blood vessels that carry oxygenated blood to the brain and veins that drain into the venous sinuses.

The pia mater closely adheres to the surface of the brain like a ‘tight wrap of cling film’ and it covers the surface of gyri and sulci. It cannot be easily separated from the surface of the brain that it covers, due to its thinness. It defines the boundary line that separates the central nervous system from the peripheral nervous system. The arachnoid and pia mater fuse at S2 at the caudal end of the spinal canal.

Question 23

Blood brain barrier and CSF

Answer 23

A distinguishing feature of tissues of the CNS is that they are contained within a strictly regulated and specialised environment, unlike any other in the body. This exclusive environment is made possible by three anatomical features: the meninges, blood brain barrier (BBB) and cerebrospinal fluid (CSF). The meninges provide the envelope in which CNS tissues and its blood vessels are encased, whilst the BBB ensures that the blood vessels of the CNS and the blood they contain are sealed, so preventing their contents from directly communicating with CNS tissues and vice-versa. Figure 10 shows how this sealing of CNS blood vessels is achieved by a highly selective semipermeable border of endothelial cells, glial cell (astrocyte) end-feet which ensheath the capillary, and pericytes which are embedded in the capillary basement membrane

The endothelial cells of the brain are of the “continuous” type, meaning that the tight junctions between them restrict movement of solutes far more than in other tissues of the body. The BBB restricts passage of pathogens, diffusion of solutes in plasma, and large or hydrophilic molecules into the CSF, but allow the diffusion of hydrophobic molecules (O2, CO2, hormones). Cells of the BBB actively transport products such as glucose and amino acids.

Interestingly, several areas of the brain lack a BBB – the “circumventricular organs”; these are adjacent to the 3rd and 4th ventricles. Circumventricular organs have highly permeable capillaries: some allow rapid detection of circulating signals such as osmolarity by the hypothalamus, others, such as those around the pituitary allow secretion of hormones, while a specialised network of blood vessels known as the choroid plexus is responsible for generating CSF, by a two step process involving ultrafiltration of plasma and active transport which is used to modify ionic composition. You will learn more about these structures and their functioning in BAB next year.