ANAT Week 5.docx

Week 5 **Module: Nervous System 1 - Introduction** **Learning Outcomes** --------------------- By the end of this module, you should be able to: **LO1**: Describe the basic functions of the nervous system **LO2**: Describe the structural and functional organisation of the nervous system **LO3*...

Week 5 **Module: Nervous System 1 - Introduction** **Learning Outcomes** --------------------- By the end of this module, you should be able to: **LO1**: Describe the basic functions of the nervous system **LO2**: Describe the structural and functional organisation of the nervous system **LO3**: Describe the basic characteristics, structure and types of neurons **LO4**: List the types of neuroglia and briefly describe their major functions **Nervous System Functions** ---------------------------- ***LO1: List the basic functions of the nervous system*** The nervous system is the master controlling and communicating system of the body. Every thought, action, sense, and emotion reflects its activity. The nervous system cells communicate via electrical and chemical signals, which are rapid and specific and usually cause almost immediate responses. The endocrine system assists the nervous system in the regulation of body functions and maintaining homeostasis. Its chief functions are to monitor, integrate and respond to information in the environment. These three functions are overlapping, as shown on the image below. Click on the hotspots to learn more about each function. The nervous system is extremely complex and can be divided both structurally and functionally. Let's start with the structural organisation of the nervous system. **Structural Organisation: Central and Peripheral Nervous System** ------------------------------------------------------------------ ***LO2: Describe the structural and functional organisation of the nervous system*** Anatomically, the nervous system can be divided into the central nervous system (CNS) and peripheral nervous system (PNS). The CNS includes the brain and spinal cord, while the PNS includes the cranial and spinal nerves, ganglia, and networks of nerves. Click on the hotspots on the image below to learn more about the CNS and PNS. You may like to watch the 3D animation below demonstrating the CNS and PNS components of the nervous system. **Functional Organisation: Sensory and Motor Nervous System** ------------------------------------------------------------- ***LO2: Describe the structural and functional organisation of the nervous system*** Now let's look at the functional organisation of the nervous system. The nervous system has two functional divisions -- the sensory nervous system and the motor nervous system. Look at the simplified diagram below and consider the following. **The sensory (afferent) nervous system** is responsible for receiving sensory information from sensory receptors and transmitting this information to the CNS. The term 'afferent' means 'inflowing', indicating that the sensory nervous system is responsible for **input**. Nerves of the PNS transmit sensory information, and certain parts of the brain and spinal cord (CNS) interpret it. There are two subdivisions of the sensory nervous system - somatic and autonomic (visceral). Click on each subdivision below to learn more about them. **Somatic Sensory Subdivision** ------------------------------- The **somatic sensory subdivision** includes the general somatic senses such as touch, pressure, vibration, temperature and proprioception (sensing position/movements of joints/limbs) and the special senses such as vision, hearing, balance and smell. These functions are considered **voluntary** because we have some control of them, and we tend to be conscious of them. **Autonomic (Visceral) Sensory Subdivision** -------------------------------------------- The **motor (efferent) nervous system** is responsible for transmitting motor impulses from the CNS to effectors (muscles or glands). The term 'efferent' means 'outflowing', indicating that the motor nervous system is responsible for **output.** Parts of the brain and spinal cord (CNS) initiate nerve impulses, which travel through motor nerves that in turn transmit these impulses to effector organs. There are two subdivisions of the motor nervous system - somatic and autonomic (visceral). Click on each subdivision below to learn more about them. **Somatic Motor Subdivision** ----------------------------- The **somatic motor subdivision** conducts nerve impulses from the CNS to skeletal muscles, causing them to contract. This is a **voluntary** nervous system because it is under our conscious control. **Autonomic (Visceral) Motor Subdivision** ------------------------------------------ Now let's make sure you understand the sensory and motor nervous systems and their subdivisions. Click on the hotspots on the diagram below for a summary of each of their functions. Now let's take a step further and discuss more precise details regarding the functional subdivisions of the nervous system. On the diagram below, note that the CNS (brain and spinal cord) receives afferent input from the PNS (all other nervous tissue outside the CNS) and sends efferent output to the PNS. Therefore, there is a reciprocal connection between the CNS and PNS. There are three functional subdivisions of the PNS - somatic, autonomic and enteric. Click on each subdivision below to learn more about them. **Somatic Nervous System (SNS)** -------------------------------- This is a **voluntary** nervous system that has both sensory (afferent) and motor (efferent) components, as described earlier. It deals with structures such as skin, skeletal muscles and joints and includes all special senses except taste. **Autonomic Nervous System (ANS)** ---------------------------------- This is an **involuntary** nervous system that also has both sensory (afferent) and motor (efferent) components, as describe earlier. It deals with structures such as smooth muscle, cardiac muscle and glands, as well as the special sense of taste. It contains two distinct subdivisions: sympathetic ANS and parasympathetic ANS. Both of these divisions include sensory (afferent) and motor (efferent) fibres that provide sensory input and motor output, respectively, to the CNS. The effects of the sympathetic and parasympathetic ANS are opposing. Activation of the **sympathetic ANS** leads to a state of overall elevated activity and attention -- the so-called 'fight or flight' response. In this response, blood pressure and heart rate increase, gastrointestinal activity decreases, etc. Activation of the **parasympathetic ANS** produces the opposite effect -- the so-called 'rest and digest' response. In this response, blood pressure and heart rate decrease, gastrointestinal activity increases, etc. **Enteric Nervous System (ENS)** -------------------------------- This system can also be considered as part of the ANS because it includes nerve plexuses (myenteric and submucosal) that are located within the intestinal wall and control digestive functions (which are **involuntary**, visceral). These functions include gut peristalsis and the movement of water and electrolytes across the intestinal wall. The ENS functions through local reflex activity and also receives input from, and provides feedback to, the sympathetic and parasympathetic ANS. Now that we have looked at the structural and functional subdivisions of the nervous system, let's look at the cell types found within the nervous system. **Neuron Characteristics and Structure** ---------------------------------------- ***LO3: Describe the basic characteristics, structure and types of neurons*** ### **Neuron Characteristics** The nervous system consists mostly of nervous tissue which is highly cellular, where cells are tightly packed and intertwined. There are two distinctive cell types comprising nervous tissue -- neurons and neuroglia or glial cells. Neuroglia are non-excitable cells that support and protect neurons. Neurons are the basic structural unit of the nervous system. They generate, transmit and receive nerve impulses and have several special characteristics. Click on the hotspots below to learn more about these characteristics Now let's look at the structure of a neuron. ### **Structural Components of a Neuron** Neurons are typically large and complex cells. Although they come in different shapes and sizes, they share certain basic structural components. They all have a cell body, an axon and one or more slender processes called dendrites. The plasma membrane of neurons is the site of electrical signalling, which is important for cell-to-cell interactions. Click on the hotspots on the image below to learn more about the main structural components of a neuron. Now, let's check your understanding of neurons. Have a go at identifying the structures labelled on the photomicrograph of a large motor neuron below. **Neuron Types** ---------------- ***LO3: Describe the basic characteristics, structure and types of neurons*** Neurons vary in morphology and location. They can be classified according to either their structure or their function. Structurally, neurons can be classified according to the number of processes extending from their cell body. There are three structural types of neurons -- multipolar (have many processes - one axon and many dendrites), unipolar (have a single short process) and bipolar (have two processes). The two most common types are multipolar and unipolar neurons, while bipolar neurons are rare in humans. Click on the hotspots on the images below to learn more about these three structural types of neurons. Functionally, neurons can be divided into three types -- sensory neurons, motor neurons and interneurons. Click on each type of neuron below to learn more about them. **Sensory neurons** ------------------- Sensory neurons are afferent neurons that transmit nerve impulses from sensory receptors to the CNS. Most sensory neurons are unipolar, except for those in the retina of the eye and in the olfactory epithelium in the nasal cavity, which are bipolar. **Motor neurons** ----------------- Motor neurons are efferent neurons that transmit nerve impulses from the CNS to effectors (muscles or glands). The cell bodies of most motor neurons are located in the CNS, especially in the spinal cord, and their axons travel in the cranial or spinal nerves to the effectors. All motor neurons are multipolar. **Interneurons** ---------------- Interneurons are association neurons that are located exclusively within the CNS. They receive impulses from many other neurons and carry out the integrative function of nervous system. All interneurons are multipolar. **Neuroglia** ------------- ***LO4: List the types of neuroglia and briefly describe their major functions*** Now that we have looked at neurons, let's look at the other cell type comprising nervous tissue -- neuroglia or glial cells. There are six types of neuroglia -- four types are located within the CNS and two types are located within the PNS. Study the two images below demonstrating the cellular organisation of nervous tissue in (a) the CNS (in this case the brain) and (b) the PNS (in this case a spinal nerve). Click on the hotspots on the image to learn more about the six types of neuroglia. **Module: Nervous System 2 - Brain Part 1** =========================================== **Learning Outcomes** --------------------- By the end of this module, you should be able to: **LO1**: Identify the major parts of the brain **LO2**: Identify and describe the cerebral cortex and its major lobes, and the main fissures, sulci and gyri that differentiate the lobes **LO3**: Describe the basic functions of the major lobes of the brain **LO4**: Identify the cerebellum and describe its structure and functions **LO5**: Understand the basic embryonic development of the brain **LO6**: Describe and identify the brain stem (midbrain, pons and medulla) and describe its basic functions **LO7**: Identify the components of the diencephalon (thalamus, hypothalamus and epithalamus) and describe their basic functions **LO8**: Identify the components of the basal ganglia (caudate nuclei, putamen and globus pallidus) and describe their basic function **LO9**: Describe the basic functions of the limbic system **Major Parts of the Brain** ---------------------------- ***LO1: Identify the major parts of the brain*** The brain is responsible for vital functions of your body. Controlling your breathing, blood pressure, all our sensations, execute movement, and how we think and feel. Many of these discoveries were made quite recently in the 19th and 20thcentury, off the impetus of anatomist Thomas Willis, in the 17th century. In order to understanding what the brain does and how it works, we first need to recognize the major parts of the brain, diving into a bit of depth into the cerebral cortex, the major lobes, and main fissues/sulci and gyri that differentiate the lobes. The brain consists of three major parts: the cerebrum, the cerebellum, and the brainstem. These structures work together to govern a wide range of physical and cognitive processes, essential for human functioning. **Cerebrum** ------------ ***LO2: Identify and describe the cerebral cortex and its major lobes, and the main fissures, sulci and gyri that differentiate the lobes*** ***LO3: Describe the basic functions of the major lobes of the brain*** The cerebrum is the largest and most superior part of the brain. The cerebrum consists of two hemispheres, left and right hemispheres, split by a central fissure called the longitudinal fissure (seen from a superior view of the brain in the diagrams below). Although each hemisphere has special features and responsibilities, there are many common parts, such as the major lobes. It's outermost layer of nerve cell tissue is covered by gray matter, and is referred to as the cerebral cortex. Deep in the brain, there are groups of gray matter called nuclei (clusters of neurons that work together to perform a specialised function). Gray matter's critical role is in information processing. White matter on the other hand are mainly myelinated axons (think very fine electrical wires that are exceptionally well insulated so they don't lose speed of conduction). These axons are surrounded by myelin sheaths, which is a fatty substance giving it a white-ish colour. White matter serves to connect and facilitate communication between different parts of the brain and we categorise these into 3 major categories: **Commissures** --------------- Nerve fibres that connect the left and right cerebral hemispheres and a great example of this is the corpus callosum. **Association Fibres** ---------------------- Nerve fibres that connect different regions of the brain on the same cerebral hemisphere. An example of this is short association fibres that facilitate connection within a lobe or long association fibres connection different lobes. **Project Fibres** ------------------ Nerve fibres that connect the cerebral cortex with other parts of the brain, and the classic example would be the internal capsule connecting the cerebral cortex with the brainstem and spinal cord. On each hemisphere, there are 4 major lobes visible from the outside. They are the frontal, parietal, occipital, and temporal lobes, named after the bones they lay under. The final two lobes we will learn is the insula lobe, requiring the temporal lobe to be retracted as seen in the figure below (lateral view of the brain) and the limbic lobe can be seen midsection. Click on the hotspots below for a brief overview of the responsibilities of each of the lobes. ### **Sulci** The cerebrum is covered by folds/ridges (gyri) and shallows depressions between folds (sulci), which demarcates special regions of the brain and lobes respectively. Special note that some of the larger sulci are sometimes named fissures, for example the Central and Lateral sulci are interchangeably called Central and Lateral fissures. ### **Gyri** Gyri function to significantly increase the surface area of the brain. They contain important nerve cell bodies and dendrites (the receiving end of a nerve cell). Three key gyri to note here are: **Precentral Gyrus** -------------------- This is the primary motor area of the cerebral cortex controlling voluntary movements. **Postcentral Gyrus** --------------------- This is the primary somatosensory area of the cerebral cortex receiving and processing all somatosensory information (touch, nociception, temperature, proprioception). **Superior Temporal Gyrus** --------------------------- This contains the Wernicke's area which allows us to interpret language by recognising speech. **Inferior Frontal Gyrus** -------------------------- Broca's area, critical for speech production, is located in this gyrus. ### **Homunculus** A special feature about the Primary Motor Cortex and Primary Somatosensory Cortex is the mapped (or topographic) representation of the body along the precentral gyrus and postcentral gyrus. Experiments were performed by scientists stimulating specific areas of these gyri to work out what parts of the human body moved (precentral gyri) and where in the body they felt sensation (postcentral gyri). Now take a deeper look at the diagram below. Can you see that the face, hands and fingers, which are relatively small parts regions of the body have a much greater representation (aka. more devoted brain cells) on the cortex than say the entire trunk. Because of this distorted representation of the human body, which we call 'homunculus', we have much more ability for both very fine motor skills and greater sensory inputs from different areas of the body. **Cerebellum** -------------- ***LO4: Identify the cerebellum and describe its structure and functions.*** The cerebellum, located posterior to the brainstem and beneath the occipital lobe of the cerebrum, is a highly organised structure composed of folded grey matter. Despite occupying only about 10% of the brain\'s volume, it contains at least half of all its neurons. Like the cerebrum, it has two hemispheres with a distinctive look of many highly organised ridges and grooves. From a midsection view, some people describe its appearance like a leaf from a tree. **The cerebellum does have a crucial role in:** ----------------------------------------------- Due to its important functions in motor control, balance and co-ordination, the consumption of alcohol highly affects the functioning of the cerebellum. Taking a closer look at the structures around the cerebellum. We see the **medulla oblongata** and **pons** anteriorly which are key parts of the brainstem, and the tentorium cerebelli which is an extension of the dura mater (we will talk about a little later in Part 2 of the Brain) separating the occipital lobes and the cerebellum. Focussing on the the cerebellum's structure, it has a number of similarities with the cerebrum. - - - - **Embryonic Development** ------------------------- ***LO5: Understand the basic embryonic development of the brain*** Before we learn more about the last major part of the brain, the brainstem, we first need to have a good appreciation for the fascinating embryonic development of the brain. All the way from a neural tube to full development into the adult brain. Watch this short 2min video and take particular note of the new terms diencephalon and mesencephalon as these will be used in the remainder of this Brain module. This will allow you to fully understand why certain structures are named or referred to in said ways. Embryonic Development Question **Brainstem and Diencephalon** ------------------------------ ***LO6: Describe and identify the brainstem (midbrain, pons and medulla) and describe its basic functions*** ***LO7: Identify the components of the diencephalon (thalamus, hypothalamus and epithalamus) and describe their basic functions*** **Brainstem** ------------- The brainstem is a vital structure located at the base and centrally in the brain, connecting the cerebral hemispheres with the spinal cord. It consists of three main regions: the midbrain or mesencephalon, and the pons and medulla oblongata which arise from the rhombencephalon (hindbrain). Overall, the brainstem serves as a vital relay centre for transmitting sensory and motor signals between the brain and spinal cord, as well as regulating basic life-sustaining functions such as breathing, heart rate, and consciousness. **Midbrain** ------------ Located above the pons, the midbrain is involved in various sensory and motor functions. It contains nuclei that control **visual and auditory reflexes**, **eye movements**, and **coordination of movements**. Additionally, the midbrain houses structures such as the substantia nigra a part of the Basal Ganglia, which plays a role crucial role in movement regulation and is implicated in Parkinson\'s disease. **Pons** -------- Positioned above the medulla oblongata, the pons serves as a bridge connecting different regions of the brainstem and facilitating communication between the cerebrum and cerebellum. It contains nuclei involved in regulating sleep, respiration, facial movements. **Medulla Oblongata** --------------------- Situated at the lowest part of the brainstem, the medulla oblongata controls essential functions such as breathing, heart rate, blood pressure regulation, and reflexes such as swallowing, coughing, and vomiting. It contains nuclei responsible for relaying sensory and motor information between the brain and spinal cord. **Diencephalon** ---------------- Now that we've learnt the 3 main parts of the brain, there are a few pieces of the puzzle left to fill in from an overarching perspective. One of these is the diencephalon which we mentioned a little earlier, and the Basal Ganglia. The diencephalon is a region of the brain located between the cerebral hemispheres and the midbrain, playing a crucial role in sensory processing, homeostasis, and the regulation of various physiological functions within the body. It consists of three main structures: the thalamus, hypothalamus, and epithalamus. ### **Thalamus** The thalamus, one in each hemisphere of the brain connected by the intermediate adhesion, acts as a relay station for sensory information, transmitting signals to the cerebral cortex. It is responsible for: There are over 50 distinct nuclei in the thalamus. In this diagram are some of the major ones categorised into the ventral, posterior and medial group. There's no need to memorise the individual nuclei or their groupings, but keep this diagram handy when any particular nuclei are mentioned involved in information relaying pathways (centres and tracts that connect the CNS with body organs and systems) in future modules. ### **Hypothalamus & Epithalamus** Situated below the thalamus, the **hypothalamus** regulates vital functions such as temperature (thermoregulation), thirst, hunger, and circadian rhythms, as well as controlling your body's hormones through the endocrine system. Often considered as the homeostasis centre of your body. Structurally, the nuclei within the hypothalamus are arranged in four regions: preoptic (most anterior), supraoptic, tuberal and the mammillary. The key take home here is identifying the 3 parts and location of the diencephalon and their basic functions. You do **NOT** need to memorise the nuclei or specific parts of the thalamus, hypothalamus and epithalamus of this unit. We have included it here only because some students like to make associations between specific structure and their function. If this helps you learn that's fantastic, but if it does not, don't try to memorise these. Situated above and behind the thalamus, the epithalamus (coloured in red) consists of a number of parts, two of which are the pineal gland and the habenula. The pineal gland secretes melatonin, which promotes sleepiness and sets the body's sleep-wake cycles. The habenula has connections to the limbic system and other parts of the brain, with primary functions associated with regulating mood, reward processing, and stress responses, as well as a role in olfaction (smell). **Basal Ganglia** ----------------- ***LO8: Identify the components of the basal ganglia (caudate nuclei, putamen and globus pallidus) and describe their basic function.*** The Basal Ganglia, found deep within the white matter of the cerebral hemispheres, are a group of interconnected nuclei. These nuclei are the caudate (green), putamen (blue), and globalus pallidus (light purple and purple). Working intimately with other key nuclei in the region, for example the substantia nigra, and subthalamic nucleus, they play a critical role in regulating voluntary motor control, movement coordination, and cognitive functions. You will only need to be able to identify the 3 structures depicted below, as a whole (not its different parts). Dysfunctions in the basal ganglia are associated with movement disorders such as Parkinson\'s disease, Huntington\'s disease, and dystonia, as well as cognitive impairments. Now try to complete this activity labelling the listed nuclei on the brain dissection. **Limbic System** ----------------- ***LO9: Describe the basic functions of the limbic system*** Narration: We have introduced the location and basic function of the limbic lobe at the beginning of this module. However, the function of emotions, memory, behavioural regulation, pain, pleasure and motivation is a complex interplay of many more regions of the brain than just the limbic lobe, which we term the limbic system. The primary components of the limbic system are: the limbic lobe (which is predominantly the cingulate gyrus), basal ganglia, thalamus, hypothalamus, amygdala, and hippocampus. These structures are typically around the upper part of the brain stem and the corpus callosum, on the inner border of the cerebellum and the floor of diencephalon. Each of these centres have multiple responsibilities but are often remembered by their primary function. Complete this matching activity below to discover the primary function of each of these primary components of the limbic system. **Module: Nervous System 3 - Brain Part 2** =========================================== By the end of this module, you should be able to: **LO1**: Describe the anatomy, anatomical arrangement and histology of the meninges surrounding the brain and spinal cord **LO2**: Name and describe the ventricles of the brain **LO3**: Explain the formation, circulation and functions of cerebrospinal fluid **LO4**: Descrbe the blood supply and venous drainage of the brain **LO5**: Describe the histological layers of the cerebral and cerebellar cortices Welcome back to Brain module, part two. In this second part of the module, you will appreciate the many mechanisms that protect, supply and and heal the brain, namely the layers of membrane around the brain called meninges, the ventricles and cerebrospinal fluid, the vascular supply and venous drainage of the brain. Finally, we will dive into the basics of the histological layers of the cerebral and cerebellar cortices, to have a deeper appreciation of the components involved in the seemless neural functions we spoke about in Part 1 of the Brain. ***LO1**: Describe the anatomy, anatomical arrangements and histology of the meninges surrounding the brain and spinal cord* The meninges are three layers of protective connective tissue membranes that surround and encase the brain and spinal cord. These layers are called dura (Latin for 'tough' or 'hard'), arachnoid (Latin for 'spider'), and pia (Latin for 'soft') mater, from outermost to innermost as you can see from the diagram below. Mater means 'mother' in Latin, so it may not come as a surprise that this implies a primary function of meninges is to protect the brain and spinal cord. Acting as a physical barrier, like a cushion, it absorbs external forces and prevents direct trauma. The meninges also serve other critical roles, including **Containment** --------------- Meninges help contain and support the brain and spinal cord, preventing excessive movement within the skull and vertebral canal. **Cerebrospinal Fluid (CSF) Circulation** ----------------------------------------- Between the arachnoid mater and pia mater is the subarachnoid space, which contains cerebrospinal fluid (CSF). **Barrier Function** -------------------- The meninges contribute to the blood-brain barrier. This is a protective barrier that controls the passage of substances between the bloodstream and the brain tissue, maintaining a stable environment for optimal neural function **Immunological Function** -------------------------- The meninges contain immune cells that can respond to infections or abnormalities, contributing to the defence mechanisms of the brain and spinal cord The associated spaces between the meninge layers are the epidural, subdural and subarachnoid spaces. Recognising thesespaces will become clinically important when discussing disease and injury. For example, the subarachnoid space which we spoke previously being the space where bleeds in the brain often occur as that's where the arteries lie, whether this be from direct trauma or a stroke. We will now dive a little bit deeper into each meningeal layer and the significance of each of the spaces. The cranial dura mater is made of dense irregular connective tissue, making it very tough. It consists of two layers: **Endosteal or periosteal layer** --------------------------------- Lining the internal surface of the skull **Meningeal layer** ------------------- Covers the brain itself -- creates folds (e.g. falx cerebri in the diagram above) that divide the cranial cavity and prevent the brain from moving within the skull In the diagram below, you will appreciate that venous sinuses, channels where oxygen-depleted blood from cerebral veins drain into, lie between these two dura mater layers. We will come back to dural venous sinuses later in this module. Some major cranial dural folds, or sometimes referred to as dural partitions, are: **Falx cerebri** ---------------- Separates the two cerebral hemispheres **Falx cerebelli** ------------------ Separates the two cerebellar hemispheres **Tentorium cerebelli** ----------------------- Separates the cerebellum from the posterior cerebral hemispheres In the spinal cord, spinal dura mater is continuous with the meningeal layer of the cranial dura mater, so it only has one layer. The epidural space is found between the vertebrae and the dura mater in the vertebral canal. Another difference between the skull and vertebral canal is that the epidural space in the vertebral canal is filled with fat to cushion the spinal cord, while there is really no epidural space in a disease-free skull unless there is a pathology. In other words, the epidural space in the skull is a potential space. Cranial **arachnoid mater** has finger-like projections called arachnoid villi (arachnoid villus -- singular) that project into the dural venous sinuses and allow CSF to enter the bloodstream. These arachnoid villi act as one-way valves to drain and maintain normal CSF pressure within the cranial cavity. Cranial **pia mater** is the innermost layer and is well and best vascularised. Between the pia and arachnoid mater is the **subarachnoid space**, which contains CSF. In the spinal cord, the pia mater have lateral extensions, which are thickenings of the pia mater. This is to form the **denticulate ligaments** that anchor and protect the spinal cord. Match the labels to their corresponding structure on this cadaveric image -- this is a tough one. ### **Functional Meninges** Interference with the drainage of CSF from the ventricles into the subarachnoid space causes excess CSF to accumulate in the ventricles and increase CSF pressure. In a baby whose skull is not yet fully hardened, having soft cartilaginous areas called fontanelles, the head bulges due to increased pressure. If this is not managed, this pressure can damage nervous tissue and lead to severe consequences. Pressure can be relieved by implanting a drain line called a shunt into the lateral ventricle to drain the excess CSF into the superior vena cava. This can also occur in adults after a head injury, meningitis or a subarachnoid haemorrhage. Because the adult skull bones are fused, there is no extra space for swelling, so nervous tissue is quickly damaged which can be life threatening if not resolved quickly. ***LO2**: Name and describe the ventricles of the brain* The ventricular system is an expanded tube that represents an upward continuation of the central spinal canal into the brain. There are four cerebral ventricles, or cavities, filled with cerebrospinal fluid (CSF). The four ventricles of the brain include: - - - Reference: Fame, R. M., Cortés‐Campos, C., & Sive, H. L. (2020). Brain ventricular system and cerebrospinal fluid development and function: light at the end of the tube: a primer with latest insights. BioEssays, 42(3), 1900186. ***LO3**: Explain the formation, circulation and functions of cerebrospinal fluid.* Cerebrospinal fluid is made up mostly of water, and contains oxygen, glucose, proteins, and white blood cells. The **choroid plexuses**, lined by a specialised epithelium called ependyma, is responsible for producing CSF in each of the ventricles. CSF travels from these ventricles, through the subarachnoid space around the brain and spinal cord. The ventricles and cerebrospinal fluid (CSF) play crucial roles in maintaining the structure and function of the brain. Here\'s why they are important: ***LO4**: Describe the blood supply and venous drainage of the brain* The main arteries supplying blood to the brain include the **internal carotid arteries (\~80%)** and the **vertebral arteries (\~20%)**. The internal carotid arteries arise from the common carotid arteries in the neck and enter the skull through the carotid canal. Within the skull, each internal carotid artery gives rise to two major branches: the anterior cerebral artery and the middle cerebral artery. These arteries supply blood to the frontal, parietal, temporal lobes of the cerebrum and some parts of the diencephalon. The vertebral arteries originate from the subclavian arteries and ascend through the transverse foramina of the cervical vertebrae before entering the skull through foramen magnum. They then merge to form the basilar artery, which supplies blood to the brainstem and cerebellum (e.g. superior cerebellar artery). The basilar artery bifurcates to give rise to the two posterior cerebral arteries, which supply the occipital lobe of the cerebrum and parts of the temporal lobe, midbrain and thalamus. The Circle of Willis, a network of interconnected arteries at the base of the brain and anterior surface of the brainstem, provides collateral circulation and helps maintain blood flow to the brain if one of the major arteries becomes blocked. Our brain's way of having a backup plan! The 7 arteries that are part of this network are marked by the asterisk symbol. These vessels lie in the **subarachnoid space**. It's important to note that this is why you may come across the term 'subarachnoid haemorrhage', when describing the type of stroke someone has sustained. In this case, one of the arteries has haemorrhaged usually due to trauma or a weakening in the artery vessel wall, leading to bleeding in this subarachnoid space around the brain. ### **Dural Venous Sinuses** In nearly every part of the body, the deep veins runs parallel to its associated artery. This is not the same for the brain. While all cerebral arteries enter the brain at the base as we have seen, venous blood is drained from the entire surface of the brain, including the base and from the interior of the brain, following its own course separate to the arteries. Cerebral veins, both superficial and deep sets, are responsible for draining blood into larger venous vessels in the skull called the dural venous sinuses. The crucial sinuses to note are the **confluence of sinuses** and the **transverse sinus** where nearly all the other sinuses drain into, before it reaches the sigmoid sinus and then finally the **internal jugular vein** (IJV). The IJV carries venous blood from the dural venous sinuses, exiting the skull via the jugular foramen and ultimately returning to the heart. The veins that carry blood from the internal jugular vein back to the heart, as shown from the diagram below, will be looked at later in the cardiovascular system module. ***LO5**: Describe the histological layers of the cerebral and cerebellar cortices* The cerebral cortex, the outermost layer of the brain, consists of six distinct histological layers. These layers are organised in a stacked fashion and have distinct cellular composition, made up of special nerve cell types (i.e. small and large pyramidal cells, and stellate cells). You are NOT required to memorise the nerve cell types or the specific function of each of the layers. The message to take away is to appreciate the highly organised manner the cerebral cortex is arranged to maximise and optimise neural function **Molecular Layer (I)** ----------------------- Contains few cell bodies and serves as a site for synaptic connections between neurons (nerve cells). **External Granular Layer (II)** -------------------------------- Contains small pyramidal cells and stellate cells. Involved in: processing sensory information and transmitting it to deeper layers. **External Pyramidal Layer (III)** ---------------------------------- Contains medium-sized pyramidal (multipolar) neurons that project to other cortical areas. Involved in: processing sensory and motor information. **Internal Granular Layer (IV)** -------------------------------- Rich in stellate cells. Involved in: sensory processing. **Internal Pyramidal Layer (V)** -------------------------------- Contains large pyramidal (multipolar) neurons that project to subcortical structures and the spinal cord. Involved in: motor control and output. **Multiform Layer (VI)** ------------------------ Composed of many neuron cell types. Involved in: integrating information from other cortical layers and sending feedback to lower brain regions. Abnormal changes or disruption to the histological layers of the cerebral cortex have significant clinical implications such as in neurological diseases (e.g. Alzheimer\'s disease and Epilepsy), developmental disorders (e.g autism spectrum disorder), or brain tumours. Comparatively, the cerebellar cortex has 3 major histological layers. From outermost to innermost, these are **Molecular Layer** ------------------- Contains the dendrites of the Purkinje cells (large motor neurons), and stellate and basket cells which in turn exhibit inhibitory influence on the Purkinje cells. Involved in: integration of sensory input from various sources, including the spinal cord and brainstem, as well as for the modulation of Purkinje cell activity. **Purkinje Cell Layer** ----------------------- A single layer of Purkinje cell bodies. Involved in: fine-tuning motor coordination and motor learning. **Granular Cell Layer** ----------------------- Packed with nerve cell bodies, predominantly of granule cells (small motor neurons). Involved in: processing and integrating sensory information and is essential for the generation and refinement of motor commands. You are **NOT** required to memorise what cells types are in each of these cerebellar cortex layers. The take home message here is to appreciate the important role of the cerebellum for normal every day functioning, and the Purkinje neurons only found in the cerebellum. They are remarkable for their large, intricately branched, flat dendritic trees, giving them the ability to integrate large amounts of information and learn by remodelling their dendrites. **Further fun fact**: despite the mass of the cerebellum being only \~10% of the whole brain, it contains approximately 50% of all neurons in the brain. It is therefore not surprising that the cerebellum plays a significant role in motor coordination of everyday and complex sophisticated movements. Damage to the cerebellar cortex through a stroke, tumours, multiple sclerosis, or genetic conditions, often leads to **cerebellar ataxia**, resulting in significant impairments in coordination, balance and motor control. - - - - - - - - - - - - - -