LECTURE 4: OVERVIEW OF THE CENTRAL NERVOUS SYSTEM

Question 1

Understand the development of the nervous system from the ectoderm → neural tube → brain & spinal cord. (3 steps but in detail)

Answer 1

1. NEURAL STRUCTURES derived from the ECTODERM.
   - ectoderm is 1 of the 3 primary germ layers in embryonic development.
   - It gives rise to various tissues, including the NERVOUS.
2. The FORMATION OF THE NEURAL TUBE is a critical event in nervous system development. It involves the following steps:

A. INDUCTION OF THE NEURAL PLATE BY THE NOTOCHORD:
- The notochord, a rod-like structure, FORMS ALONG MIDLINE of the developing embryo.
- It SECRETES SIGNALLING MOLECULES that CAUSE the OVERLYING ECTODERM to THICKEN AND FORM THE NEURAL PLATE.

B. ELEVATION AND FOLDING OF THE NEURAL PLATE:
- As development progresses, the LATERAL EDGES OF THE NEURAL PLATE START TO ELEVATE AND FOLD INWARD.
EVENTUALLY, they MEET AT MIDLINE.

c. FORMATION OF THE NEURAL TUBE: The FOLDING PROCESS LEADS TO FORMATION OF THE NEURAL TUBE = HOLLOW STRUCTURE WITHA CENTRAL CANAL.

3. DIFFERENTIATION OF THE NEURAL TUBE:
   ** ROSTRAL (ANTERIOR) end of the NEURAL TUBE develops = BRAIN
   *** CLAUDAL (POSTERIOR) end develops = SPINAL CORD.

As development continues, the neural tube further differentiates into various regions and structures within the brain and spinal cord, giving rise to different regions and cell types.

Question 2

Developmental pathways of neuroanatomical structures from the neural tube.

Answer 2

Neural Tube:
1. Prosencephalon
2. Mesencephalon
3. Rhombencephalon

*Prosencephalon
- Telencephalon = Cerebral hemispheres
- Diencephalon = Thalamus, Hypothalamus, Subthalamus.

*Mesencephalon
- Mesencephalon = Midbrain

• Rhombencephalon
• Metencephalon = Pons, Cerebellum.
• Myelencephalon = Medulla

Question 3

Understand the difference between grey and white matter and where they are located.

Answer 3

1. Grey matter = cell bodies of neurons.

• Grey matter is composed mainly of neuronal cell bodies, dendrites, glial cells, and unmyelinated axons.
• It appears greyish in colour due to the high density of cell bodies and lack of myelin.
• Grey matter is primarily involved in information processing, such as perception, cognition, and motor control.
  It contains neural circuits responsible for integrating and processing sensory information, initiating and coordinating motor responses, and higher-order cognitive functions.
• Grey matter is located on the outer layer of the brain (cerebral cortex) and within deep brain structures (e.g., basal ganglia, thalamus) as well as the inner region of the spinal cord (grey horn).

2. White matter = myelinated axons of neurons.

-White matter consists mainly of myelinated axons, which are nerve fibres surrounded by myelin sheaths.
- The myelin sheaths give white matter its whitish appearance.
- White matter acts as a communication network, transmitting signals between different areas of grey matter within the CNS.
- White matter is primarily found beneath the grey matter of the cerebral cortex, within the inner regions of the brain (e.g., subcortical structures, such as the corpus callosum), and in the outer regions of the spinal cord (white matter columns).

Question 4

Explain the Meninges:

Answer 4

1. DURA MATER (SUPERFICIAL).
   - toughest, outermost layer of the meninges.
   - It is a thick, fibrous membrane that provides strong protection for the brain and spinal cord.
   - composed of dense collagen fibers and is highly resistant to stretching or tearing.
   - two layers in most areas: the outer periosteal layer, which attaches to the inner surface of the skull, and the inner meningeal layer. These layers are usually fused together,
   *** but they can separate to form dural sinuses, which are spaces that allow blood to drain from the brain.
2. ARACHNOID MATER (INTERMEDIATE)
   - delicate, web-like layer located beneath the dura mater.
   - composed of thin connective tissue and lacks blood vessels.
   - It forms a protective barrier between the dura mater and the innermost layer, the pia mater.
   - Beneath the arachnoid mater lies the subarachnoid space, which contains cerebrospinal fluid (CSF). The CSF helps to cushion the brain and spinal cord against impact and provides nutrients and waste removal.
3. PIA MATER (DEEP).
   - innermost layer of the meninges.
   - the thin, delicate membrane that closely adheres to the surface of the brain and spinal cord, following their contours.
   - contains numerous blood vessels that supply oxygen and nutrients to the underlying neural tissue.
   - It is responsible for delivering nutrients and removing waste products from the central nervous system.

Question 5

Explain the Ventricular System:
What is it composed of?
Ventricles, Channels & foramina

Answer 5

Ventricles:
* Lateral ventricles
* Third ventricle
* Fourth ventricle

Channels & foramina:
* interventricular foramina
* cerebral aqueduct
* median aperture of 4th ventricle
* lateral apertures of the 4th ventricle
* central canal of the spinal cord

Question 6

Explain the Circulation of Cerebrospinal Fluid (9 steps)

Answer 6

1. Lateral ventricles
2. → interventricular foramina
3. → third ventricle
4. → cerebral aqueduct
5. → fourth ventricle
6. → median aperture + lateral apertures of fourth ventricle
7. → central canal of spinal cord + subarachnoid space
8. → arachnoid granulations
9. → Dural venous system.

Question 7

What are the Limbic System structures? 6
What is the Limbic System?

Answer 7

Limbic structures refer to a group of interconnected brain regions that play a crucial role in various functions related to EMOTIONS (regulate and integrate emotional responses) MEMORY (formation and retrieval,) and BEHAVIOUR (behaviour and motivation)
- They form a complex network that influences our emotional experiences, learning, and decision-making processes.

STRUCTURES:
1. HIPPOCAMPUS: The hippocampus is a seahorse-shaped structure located in the medial temporal lobe. It is vital for the formation and retrieval of memories, particularly declarative memories, which are conscious and explicit memories about facts and events.

2. AMYGDALA: The amygdala is an almond-shaped structure located deep within the temporal lobes. It plays a key role in processing emotions, particularly fear and the formation of emotional memories. The amygdala is involved in evaluating threats, emotional responses, and the regulation of emotional behavior.
3. HYPOTHALAMUS: The hypothalamus is a small but crucial structure located at the base of the brain. It is involved in regulating numerous physiological functions, including the autonomic nervous system, body temperature, hunger, thirst, sleep, and sexual behavior. The hypothalamus also plays a role in emotional responses and the release of various hormones.
4. CINGULATE GYRUS: The cingulate gyrus is a curved structure that lies above the corpus callosum. It is involved in processing emotions, cognitive control, decision-making, and the regulation of pain.
5. FORNIX: The fornix is a bundle of nerve fibers that connects the hippocampus to other limbic system structures, such as the hypothalamus and mammillary bodies. It plays a role in memory formation and retrieval.
6. MAMMILLARY BODIES: The mammillary bodies are a pair of small round structures located in the hypothalamus. They are involved in memory processes, specifically in the formation and retrieval of spatial and episodic memories.

Question 8

BASAL NUCELI STRUCTURES: 4
What is it?

Answer 8

Basal nuclei, also known as basal ganglia,
- are a group of SUBCORTICAL STRUCTURES located deep within the CEREBRAL HEMISPHERES of the brain.
- They play a crucial role in motor control, cognition, and emotion regulation.

STRUCTURES:

1. Striatum: The striatum is the largest component of the basal ganglia and consists of two main structures: the caudate nucleus and the putamen. The striatum receives inputs from various regions of the cerebral cortex and is involved in motor planning, initiation, and execution. It plays a role in the control of voluntary movements, procedural learning, and habit formation.
2. Globus pallidus: The globus pallidus is divided into two segments: the external segment (GPe) and the internal segment (GPi). It receives input from the striatum and serves as an output nucleus of the basal ganglia. The globus pallidus is involved in regulating the activity of the thalamus and, therefore, modulating motor activity.
3. Subthalamic nucleus (STN): The subthalamic nucleus is a small nucleus located beneath the thalamus. It receives inputs from the globus pallidus and projects to the globus pallidus. The subthalamic nucleus plays a role in facilitating or inhibiting motor activity and is involved in the regulation of movement.
4. Substantia nigra: The substantia nigra is divided into two parts: the substantia nigra pars compacta (SNc) and the substantia nigra pars reticulata (SNr). The substantia nigra is involved in the production of dopamine, a neurotransmitter that plays a crucial role in motor control and reward-related processes. The degeneration of dopaminergic neurons in the substantia nigra is associated with Parkinson’s disease.

Question 9

BRAINSTEM STRUCTURES: 6.

Answer 9

The brainstem is the lower part of the brain that CONNECTS THE SPINAL CORD TO THE HIGHER REGIONS OF THE BRAIN.
It is composed of several structures that play critical roles in VITAL FUNCTIONS: REGULATING BREATHING, HEART RATE, and CONSCIOUSNESS.
- They facilitate communication between the brain and the rest of the body, integrate sensory and motor signals, and regulate vital autonomic processes.

1. Medulla Oblongata: The medulla oblongata is located at the base of the brainstem and connects the brain to the spinal cord. It is responsible for regulating essential autonomic functions, including controlling respiration, heart rate, blood pressure, and digestion. The medulla also serves as a relay centre for sensory and motor pathways between the brain and spinal cord.
2. Pons: The pons is situated above the medulla oblongata and acts as a bridge connecting different parts of the brain. It contains various nuclei that contribute to functions such as sleep regulation, facial movement, and sensory relay.
3. Midbrain (Mesencephalon): The midbrain is the uppermost portion of the brainstem. It plays a crucial role in sensory processing, motor control, and maintaining consciousness. Two prominent structures within the midbrain are:a. Tectum: The tectum consists of the superior and inferior colliculi. The superior colliculi are involved in visual processing and orienting responses, while the inferior colliculi are responsible for auditory processing.b. Cerebral Peduncles: The cerebral peduncles are bundles of nerve fibres that carry signals between the brainstem and cerebral cortex. They are involved in motor control and transmit motor commands from the cortex to the spinal cord.
4. Reticular Formation: The reticular formation is a network of nuclei that spans the entire brainstem. It plays a crucial role in regulating arousal, attention, and sleep-wake cycles. The reticular formation also modulates pain perception and controls various autonomic functions.

Question 10

Vascular Supply of Brain derived from where?

Answer 10

1. Anterior circulation derived from internal carotid a.
2. Posterior circulation derived from the vertebral-basilar system.
3. Anterior and posterior circulations join to form an arterial circle.

Question 11

Explain simply: Venous Drainage of the Brain

Answer 11

Dural venous + cavernous sinuses → internal jugular vein.