Lecture 2_ Neurons, neuroglia, and neuronal protection PDF

Cellular components, neuronal protection, and development Unit 1 Lecture 2 1 Table of contents 1. Internal organization of nervous tissue 2. Functional units of the system: neurons and neuroglia 3. Neural protection 4. Brain development...

Cellular components, neuronal protection, and development Unit 1 Lecture 2 1 Table of contents 1. Internal organization of nervous tissue 2. Functional units of the system: neurons and neuroglia 3. Neural protection 4. Brain development 2 smooth brain 1. Internal organization of nervous tissue Nervous tissue has two shades This is best examined in a cross-section of the brain or spinal cord. There are distinctive areas in these organs identified by their shade: grey or white. matter superficial cerebrum outsidefold grey white matterdeep In the cerebrum, grey matter occurs superficially while the white matter occurs more deep. In the spinal cord, it is opposite. Grey matter occurs deep (butterfly shape), while the white matter occurs superficially. 3 1. Internal organization of nervous tissue Gray matter versus white matter in the brain neuronalcell unmyelinatedaxondendrites 1. Grey matter contains neuronal cell 2. White matter lies deep to the bodies, unmyelinated axons, and cerebral cortex and contains dendrites. In the cerebrum, it includes: myelinated and unmyelinated axons Superficial layer called cerebral cortex connecting areas of grey matter to each other and other parts of the Deeper structures called basal nuclei CNS. 4 1. Internal organization of nervous tissue A closer look at grey matter in the brain The folds of the cerebral Along with the cerebral cortex, more neuronal cortex are called gyri, and cell bodies occur deeper in the brain in regions these increase the surface called basal nuclei: of thatcontrolmovementemotionreward collection neurons area of the brain (more Caudate nucleus Lentiform nucleus neurons, more processing power). 5 1. Internal organization of nervous tissue Gray matter versus white matter in the spinal cord Grey matter still contains the cell bodies of neurons but occurs in the middle of the organ in a butterfly-shape with dorsal and ventral top bottom horns. The white matter contains myelinated and unmyelinated axons and surrounds the grey matter. Axons in the spinal cord are anatomically organized into three paired columns. Posterior or dorsal Lateral Anterior or ventral 6 1. Internal organization of nervous tissue An overview of grey matter Grey matter not only contains the cell bodies of motor neurons and interneurons but also short, unmyelinated axons. Where do we find grey matter in the CNS? The cerebral cortex, basal nuclei, and center portion of the spinal cord. Where do we find grey matter in the PNS? Technically, in the ganglia, as these contain neuronal cell bodies. 7 1. Internal organization of nervous tissue An overview of white matter White matter is a collection of myelinated (and unmyelinated) axons. axons Where do we find white matter in the CNS? Deep to the cerebral cortex but superficial to the grey matter in the spinal cord. This matter is organized as columns in the spinal cord and further divided into tracts. Where do we find white matter in the PNS? White matter also refers to the peripheral nerves (myelinated axons), which leave the brain, brain stem, and spinal cord. 14min 8 2. Functional units of the system Cell types: the neuron These are the basic functional units of the nervous system and are specialized to transmit electrical impulses called action potentials, allowing communication between neurons and other cell types to coordinate bodily activities. All neurons have three major cellular structures: Cell body contains the nucleus and most organelles. Dendrites are projections off the cell body that receive signals. Axon(s) is(are) projections off the cell body that transmit incoming messages away from the cell. 2. Functional units of the system Classifying neurons by their function Sensory neurons collect sensory information and transport them to the CNS. *All* are unipolar neurons. rarelybipolar if mentioned Motor neurons transmit motor information from the CNS to the PNS to synapse on effectors. All are multipolar neurons. fullyinsidebrain spinalcord Association, or interneuronspconnect sensory and motor neurons and are only found in the CNS. All are multipolar neurons. 10 2. Functional units of the system Transmission of signals Consider the direction of an electrical impulse: dendrites to cell body to axon. Why is this important? Because neurons can only transmit signals in a single direction. A neuron sending signals towards the CNS carries sensory information, while a neuron carrying signals outside the CNS carries motor information. 11 2. Functional units of the system notimportant Let’s consider events across the membrane… A graded potential caused by a chemical or mechanical stimulus can disrupt the resting membrane potential on a patch of neuronal cell membrane… … if this graded potential is strong enough, it can trigger the opening of voltage-gated channels on the axolemma (at the axon hillock). If this is the case, an action potential is produced. 12 2. Functional units of the system Cell types: neuroglia Different types of neuroglia are found in the CNS and PNS: In the CNS, we find astrocytes, oligodendrocytes, microglia, and ependymal cells. In the PNS, we find satellite cells and Schwann cells. 13 3. Neural protection Why do the nervous organs need protection? Because nervous tissue is very fragile, and homeostasis relies on the communicative roles of neurons. This tissue is vulnerable to physical, chemical, and biological damage. How does the body protect these organs against threats? Bones and fatty cushions Meningeal layers Cerebrospinal fluid (CSF) Neuroglia (provide cellular barriers ) 27mi 14 3. Neural protection: bone Bones are a critical line of defense Bones offer protection against exposure to mechanical stresses, like blunt trauma, chemical stresses, like carcinogens, and biological stresses, like microbes. The skull protects the brain and brainstem, while the vertebral column protects the spinal cord. Adipose cushioning between the spinal cord and vertebrae help cushion 6 this organ. The peripheral nerves are partially protected as they leave the vertebral column through the intervertebral foramen. 15 3. Neural protection: meninges How do the meningeal layers protect these organs? These layers help anchor the brain to the surrounding bone and act like a seatbelt to attenuate mechanical trauma caused by hits to the head. One of the meningeal layers creates an elastic dural fold that extends between the EE hemispheres into the longitudinal fissure T Eikaese E and between the brainstem and cerebellum to help decelerate the movement of the 1 qetbg brain inside the skull. 16 3. Neural protection: meninges These cover the brain and spinal cord The meninges are composed of three specialized connective tissue layers: Dura mater: most superficial Arachnoid mater: middle layer back ilyt n IE itacnoi Pia mater: deepest layer making contact cellbodiesof the neurons with brain and spinal cord tsheriggsfttheooabrain Minor differences between the layers around either organ… The pia mater and arachnoid mater are organized very similarly around the brain and spinal cord, but the dura mater is organized differently. Let’s examine these layers more closely. 17 3. Neural protection: meninges Dura mater around the brain This outermost layer is composed of two layers. The outer layer is fused with the periosteum (no space between bone and dura), while the inner layer forms the dural folds. notaroundPigal Between the two layers of the dura mater is the dural sinus, into which nutrient-poor CSF drains, eventually entering the venous circulation via the jugular veins. Elias shiftightEgs thin Pg in tovenfirst baked in 18 3. Neural protection: meninges Dura mater around the spinal cord The inner and outer dura mater layers are fused, so they’re considered a single layer. We find no dural folds or dural sinus around the spinal cord. Instead of being anchored to bone, spinal cord dura mater is attached to adipose tissue, which cushions the organ. This space between bone and dura mater, composed of adipose, is called the epidural space. 19 3. Neural protection: meninges Arachnoid mater around the brain and spinal cord This layer is separated I from the dura mater by the subdural space, which contains lymphatic fluid. I 1 iÉIiifI The arachnoid mater is created by a collagen fibre web, below which we find the subarachnoid space, through which cerebrospinal fluid flows. 20 3. Neural protection: meninges Pia mater around the brain and spinal cord This innermost layer is below the arachnoid mater and makes direct contact with the neural tissue. It is highly vascularized. In the spinal cord, but not in the brain, it forms the denticulate ligaments, which anchor the spinal cord to the arachnoid and dura maters. 21 3. Neural protection: CSF How does cerebrospinal fluid protect these organs? As a fluid, it supports the brain’s weight and maintains its shape by resisting compression caused by gravity. By bathing both organs, it cushions them by absorbing shocks. It further supports these organs functionally… It transports nutrients, removes wastes, and transmits compounds that help different parts of the system communicate. What are its physical and chemical properties? It is a clear, mostly colourless fluid containing ions, glucose, urea, proteins, microglia, and some WBCs (but no RBCs). We circulate about 150 mL of CSF, 22 and it’s constantly refreshed and resorbed. 3. Neural protection: neuroglia What are the cellular barriers? These are barriers generated by neuroglial cells, principally the astrocytes and ependymal cells, that protect the nervous tissue from components in the blood. There are two main barriers: makes thingshardtoenterthecus blood-brain barrier blood-cerebrospinal fluid barrier 23 3. Neural protection: neuroglia These barriers are completely intact in the CNS With a few exceptions, like in the hypothalamus and pineal gland, they occur over all external and internal surfaces of the CNS organs. They’re very selective in what they let through O2, CO2, and lipid-soluble compounds can cross the barrier via diffusion, while larger solutes, like amino acids and glucose, and ions, like Na+ and K+, require transporters to cross. Most proteins, toxins, drugs, viruses, and bacteria cannot pass freely. 24 3. Neural protection: neuroglia 1. Blood-brain barrier astrocytesincharge This is a cellular layer between the blood and interstitial fluid of nervous tissue. It is composed of three layers: 1. Endothelial cells making up capillary walls are in contact with the blood. 2. Basement membrane is a layer of anchoring proteins that prevent certain solutes from passing. 3. In the nervous tissue, astrocyte processes wrap around the endothelial basement membrane. 25 3. Neural protection: neuroglia What makes the BBB special? thievinginbefugs prevent Tight junctions occur between the endothelial cells of the capillaries to prevent the passage of (most) molecules between these cells. between endothelial cells capillaries Molecules are transported into the interstitial fluid of neural tissue by crossing through the endothelial cells, the BM, and astrocyte processes. 26 3. Neural protection: neuroglia noquestionsasked onthis 1. Blood-brain barrier: how do molecules get through? 1. Vesicular transport via endocytosis. 2. Active transport and facilitated diffusion. 3. Simple diffusion by lipid-soluble molecules. 27 3. Neural protection: neuroglia 2. Blood-cerebrospinal fluid barrier 1. It occurs in the choroid plexus in the ventricles of the brain (more on this later). 2. It is composed of the endothelial capillary cells, basement membrane, and ependymal cells. 3. Only ependymal cells are bound together by tight junctions, preventing most compounds from passing between 28 these cells. 3. Neural protection: neuroglia 2. Blood-cerebrospinal fluid barrier 4. At the choroid plexi, the capillaries of the pia mater penetrate deeper into the nervous tissue to make contact with the ependymal cells. 5. Molecules cross the BCB via similar transportation mechanisms as in the BBB, including active transport, facilitated diffusion, and simple diffusion. 29 4. Brain development not onmonday quiz 10questions 10mins As we introduce the parts of the brain, we will refer to some components according to their identity in the embryo. To better understand these structures and their names, we will discuss how the brain develops in the embryo and fetus. 30 4. Brain development The embryonic brain 2-Minute Neuroscience: Early Neural development In my 2-Minute Neuroscience videos I explain neuroscience topics in 2 minutes or less. In this video, I discuss early neural development. I likeaworm looks p explain how the neural plate develops into the neural tube, and then the process by which the neural tube forms the brain and spinal cord. I In the embryo, the brain discuss the primary (prosencephalon, mesencephalon, and rhombencephalon) and secondary (telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon) vesicles and what parts of the brain they will eventually form. develops from a tube that For more neuroscience articles, videos, and a complete neuroscience glossary, check out my website at www.neuroscientificallychallenged.com ! slowly differentiates over TRANSCRIPT: Welcome to 2 minute neuroscience, where I simplistically explain neuroscience topics in 2 minutes or less. In this installment I will discuss early time. Check it out. neural development. The development of the nervous system begins at around the third week of embryonic development, when an area of the ectoderm, or the outer tissue layer of the embryo, thickens and forms what is known as the neural plate. Link to video. This plate begins to fold inward, forming a groove called the neural groove. The sides of the neural groove, known as the neural folds, begin to come together. At the end of the third week, the folds will begin to fuse together. By the end of the fourth week, they have completely fused together to form the neural tube, which will eventually become the brain and spinal cord. As the neural tube closes, bulges and bends begin to appear and they gradually become more noticeable. During the fourth week, there are three of these bulges present. They are called the primary vesicles. They are the prosencephalon, which will eventually form the cerebrum. The mesencephalon, which will eventually become the midbrain. And the rhombencephalon, which will eventually become the rest of the brainstem and the cerebellum. This end of the neural tube will form the spinal cord. As the brain continues to develop, two of these vesicles further subdivide to form secondary vesicles. The prosencephalon forms the 31 telencephalon and diencephalon. The telencephalon will become the cerebral hemispheres, the diencephalon will eventually consist of thalamus, hypothalamus, and other structures. The mesencephalon does not subdivide any further and will become the midbrain. The rhombencephalon will subdivide into the metencephalon and myelencephalon. The metencephalon will become the pons and the cerebellum, and the myelencephalon will become the medulla. And the end of the neural tube will remain the spinal cord. 4. Brain development The embryonic brain Early in development, the embryo is simply a hollow ball of cells. Eventually, these cells develop into the germ layers, which give rise to all the tissues of the body. One of these layers will develop into the nervous system. It does this first by creating a tube called the neural tube. 33 4. Brain development The embryonic brain Inside the neural tube, there is a fluid- filled internal cavity called the neurocoel. This neurocoel will form the chambers of the adult brain where cerebrospinal fluid will flow. These chambers become the ventricles of the brain and central canal of the spinal cord. 34 4. Brain development The embryonic brain At the head portion of the neural tube, three areas enlarge to designate the three primary brain vesicles: 1. Prosencephalon, or the forebrain forebrain 2. Mesencephalon, or the midbrain midbrain 3. Rhombencephalon, or the hindbrain hindbrain 35 4. Brain development 0 Primary vesicles Secondary vesicles Adult structures telencephalon cerebrum Prosencephalon diencephalon epithalamus, thalamus, and term developmental hypothalamus Mesencephalon midbrain metencephalon pons and cerebellum Rhombencephalon myelencephalon medulla oblongata 36 closetospinalcord myelinsheats 4. Brain development The embryonic brain As you can tell, the primary and secondary vesicles refer to neural structures in the developing embryo. In the adult brain, you will find certain structures named after these embryonic vesicles. 37 4. Review What should you keep in mind… 1. Gray matter and white matter is differently organized in the brain and spinal cord. 2. Neurons are the basic cellular unit of nervous tissue, as changes in membrane potential can trigger electrical signals, i.e. action potentials. 3. Since nervous tissue is so fragile, it needs to be protected, and bone, fat, cerebrospinal fluid, meningeal layers, and cellular barriers made by the neuroglia offer protection against physical, chemical, and microbial threats. 38