New Zealand Brain Bee Challenge - Cells of the Nervous System PDF

Brain Bee 🧠🐝 SECTION 1: Cells of the Nervous System The Nervous System consists of: (1) Nerve cells (Neurons), and (2) Glial cells (or Glia). 1.​ Neurons Neurons, or nerve cells are in charge of transmitting and integrating information in the brain. In the body, neurons coordinate and regu...

Brain Bee 🧠🐝 SECTION 1: Cells of the Nervous System The Nervous System consists of: (1) Nerve cells (Neurons), and (2) Glial cells (or Glia). 1.​ Neurons Neurons, or nerve cells are in charge of transmitting and integrating information in the brain. In the body, neurons coordinate and regulate activity. Each neuron is an independent unit whose shape and structure can vary quite widely. A neuron consists of the following 4 major components: ​ Dendrites: ​ A number of finely branched structures called dendrites that receive inputs from other neurons. ​ Cell Body: ​ The cell body, or soma, contains the nucleus and other organelles. ​ The cell body is also the place where all the dendrites converge and their electrical inputs are combined. ​ Axon: ​ A single (often long) process known as an axon which often branches to form terminals that contact other neurons or other cells. ​ Terminals: ​ A single (often long) process known as an axon which often branches to form terminals that contact other neurons or other cells. 2.​ Gilia Older books estimated that Glial cells make up about 90% of the Central Nervous System. However current estimates are that the ratio is more likely to be 1:1 between Glial cells and neurons. This ratio can vary between different parts of the brain but rarely gets greater than a 4:1 ratio of Glia : Neurons. The term neuroglia means "nerve glue" and that comes from the belief that these cells functioned to hold the nerve cells together in a coherent structure like the brain or spinal cord. So they were thought to function as structural support for neurons. Glia functions include: ​ Providing structural support in the nervous system. ​ Nourishing neurons by supplying nutrients sourced from the blood. ​ Creating a barrier between the blood and the brain so toxic substances can't get into the brain (known as the Blood-Brain Barrier, BBB). ​ Scavenging debris and dying nerve cells. ​ Providing a sheath for some neurons to form an "insulation" called myelin. ​ Repairing the nervous system after injury. ​ Controlling the local environment (the microenvironment) around neurons, mopping up extra ions and other chemicals like neurotransmitter chemicals, to help maintain neurons in an optimal state for function. This list of functions indicates that glial cells act to maintain an optimal environment for neuronal function. We can see then, that glial cells keep neurons safe and protected, well-supplied and free to move where they need to! Types of Gilia Modern evidence shows that glial cells may also play an important role in how electrical information is transmitted in the brain, and may be actively involved in the transmission of waves of electrical activity across brain areas. The waves of electrical activity appear likely to play a major role in modulating how efficiently neurons carry out their function. There are about 5 different types of glial cells each with specialised functions: 1.​ Astrocytes Multipurpose cells. They are actually a class of cell, with different ones in the class doing different things. 1.​ Astrocytes deliver nutrients from the blood to neurons and take waste products in the opposite direction. 2.​ Astrocytes control the chemical environment, like ions and transmitters, around neurons and other brain cells (the "extracellular environment" or "microenvironment") to keep them healthy and regulate metabolism. 3.​ Astrocytes are also responsible for controlling capillary blood flow, which in turn modulates the flow of chemicals between blood and neurons - this makes them crucial little housekeepers! 4.​ Astrocytes play an important role in repair and scarring of nerve cells in the Central Nervous System (the CNS consisting of the brain and spinal cord), following injury to these structures. 5.​ Astrocytes provide a buffer reserve of "fuel" for cells as they contain glycogen (a large molecule made of glucose subunit molecules) which they can break down to release glucose during periods of high demand by neurons. 6.​ Oligodendrocytes and Schwann cells ​ Both cells provide a myelin sheath around the axon of some neurons (this helps insulate the axon, increasing the speed of information transmission). ​ Providing a helpful assist to astrocytes, Oligodendrocytes and Schwann cells contribute towards maintaining an optimal extracellular environment (keeping in good working order the area around the neuron - a bit like tidying up the garden around your house!) 7.​ Microglia Microglia are specialised immune cells and have the important role of making up the main active immune defence mechanism in the central nervous system. They are a particular type of macrophage (special white blood cells) that are only present in the central nervous system 1.​ They scavenge cell debris from dying neurons and other glial cells and are the "first responders'' when there is damage to the Central Nervous System (CNS). The scavenging actions of microglia can be both good and bad - it prevents the buildup of toxic waste substances but may also contribute to neurodegeneration (nerve degeneration), e.g., in Multiple Sclerosis. 2.​ They constantly and rapidly reorganise their shape by changing their processes to allow them to very efficiently scan the local environment to identify any insults to the CNS. While this happens, their cell body doesn't change shape, so that it doesn't disturb local neuronal circuits. 3.​ Microglia also play important roles in regulating the development of the brain after birth, and in brain plasticity in adulthood. 8.​ Ependymal cells These cells line the brain’s fluid spaces (the ventricles) to form a slight barrier between the fluid spaces and the cells (like a picket fence with gaps between the pickets), while producing cerebrospinal fluid. Ependymal cells have cilia, little hair-like organelles that face the cavity of the ventricles. The cilia time their movement to direct cerebrospinal fluid and are also able to influence the distribution of neurotransmitters to neurons. ​ Some ependymal cells can divide and form neurons through the life of a cell, allowing neuroregeneration to occur. ​ These support cells can provide an environment that protects axon stumps from degeneration after damage, allowing alternate neuronal connections to grow and restore function. Essentially the brain can replenish a portion of dead neurons with these guys! EDUCATION PERFECT TASK  1.​ General Organisation of the Nervous System The nervous system is divided 2: 1.​ CENTRAL NERVOUS SYSTEM (CNS) 2.​ PERIPHERAL NERVOUS SYSTEM (PNS). These terms describe two regions where nerves are located: within the area of the brain and spinal cord (the CNS) or everywhere else in the body (the PNS). The PNS can be further divided according to where neurons are located: ​ Neurons located in deep internal structures (the gut, the heart, the lungs, the blood vessels etc.) make up the AUTONOMIC NERVOUS SYSTEM. ​ Neurons located in more superficially located structures (such as the skeletal muscles of the body, the eye, the ear etc.) form part of the SOMATIC NERVOUS SYSTEM. The PNS is connected to the CNS through AFFERENT and EFFERENT neurons. ​ Afferent neurons run from the periphery (the body) to the CNS, carrying information to the CNS. ​ Efferent neurons run in the opposite direction, from the CNS to the periphery, carrying information to body parts. Nuclei and Ganglia ​ In the CNS nerve cell bodies are located in groups in structures called nuclei or fields (areas like the thalamus, putamen, or substantia nigra). ​ In the PNS nerve cell bodies are located in groups in structures called ganglia (eg. “Superior Cervical Ganglion”). Nerve clumps ​ From these clumps of nerve cell bodies (nuclei or ganglia), each neuron has an axon that runs to somewhere else. Generally the axons of neurons run from these areas, as bundles of nerve axons known as nerve tracts (i.e., nerve pathways), to other such areas or tissues, to then make contact with other cells. Recap: The plug of the power cord has a number of pins (generally, in Australia and NZ, three pins). If you were to open up the plug and look inside, you’d see each pin had a wire running from to the device that you want to connect to the power. Imagine you have a cable connected to the mains that you need to slot into your laptop (or mobile phone) to charge it up. ​ The plug end (the “nucleus”) has a number of pins (each being a “nerve cell body”). ​ Each pin has a wire (an “axon”) and all the wires run together as a single power cable (the “nerve tract”). ​ The cable terminates in the clip end that fits into the laptop or mobile phone power socket. ​ From this termination (the “synapse” that you will see in a later part of this module) power flows into the lap top (another “nucleus”). Classifying Neurons: Functional classification of neurons Class Location Function Sensory Neurons Afferent Neurons Transmit information from sensory organs to the CNS. Motor Neurons Efferent Neurons Transmit information from the CNS to control muscles. Transmit information from one neuron to the next Interneurons: neuron. (a) Over short distances (e.g. between neurons within (a) Local Interneuron Only within the CNS the one nucleus) (b) Projection (or tract) (b) Over long distances (e.g. between neurons located interneuron in two different nuclei) CNS and almost every organ in Release hormones into the blood supply to influence Neuroendocrine Cell the body distant target organs.  How do neurons work? Imagine you're walking along the street and step on a nail. Ouch! How does your brain process that? 1.​ First you withdraw the foot that encounters the object. For that to happen, information about the painful stimulus has to travel from the foot to the spinal cord via a sensory neuron. 2.​ This neuron then has to pass messages on to the muscles that allow retraction (flexion) of that leg, i.e. to motor neurons. 3.​ At the same time, you have to move the other leg to maintain your balance. For this to happen, the sensory input has also to be transmitted to motor neurons to the muscles of the other leg. This does not occur directly between the incoming sensory neuron and the motor neurons to the other leg. Instead, the information is transmitted to an intermediate neuron that then relays it to the motor neurons to the other leg, i.e., it is relayed through interneurons. 4.​ The final class of neuron – not involved in the above actions - is a group of neurons, found in the brain, which release hormones into the blood supply to influence tissues in various parts of the body. These are neuroendocrine cells. EDUCATION PERFECT TASK 1.​ Neuronal Structure For all neurons, despite their variety of shapes and sizes, we can identify the following components which have particular functional significance: ​ Dendrites, where neurons receive most of their input. The dendrites constitute the major input region of neurons. ​ A cell body, with organelles for synthesis and/or processing of proteins, lipids etc. ​ An axon, to transmit information, generally extending from the cell body. In some neurons the axon is sheathed (myelinated) by particular glial cells. The axon constitutes the conducting region of neurons. ​ An axon hillock or initial segment. This is known as the integration region, generally interposed between the cell body and the axon. The integration region collects all the information that a neuron receives before determining if there is sufficient excitation to relay that excitatory information along the conducting region. ​ A "skeleton" consisting of micro- and neuro-filaments for rigidity, and microtubules for transport. These components extend the entire length of the neuron, from cell body through axon to the terminals, giving the neuron a structure - we can think of it like a building’s scaffolds or a car's chassis! ​ Finally, a neuron has axon terminals at specialised regions called synapses, where it contacts other cells in a method that doesn't even involve touch! These synapses are known as the output regions of neurons. Dendrites and their spines ​ The dendrite is an extension of the cell body; it receives most (but not all) of the inputs (synapses) to a neuron. ​ Dendrites possess small protrusions called spines which are extensions of the dendrite themselves. ​ It is at these spines that excitatory input is provided by other neurons, and the spines are the region through which chemicals flow into the cell when it is activated. A dendrite can change their spine's shape because of the presence of actin, a protein that can contract and elongate (and this is also the protein that allows muscle to contract). This is also seen to occur when a neuron changes its function: ​ Dendritic spines are classed as either thin, mushroom or stubby and these appear to have different functions: ​ Thin spines are also known as ‘learning spines’ and they have been shown to grow when a new task is being learned. ​ The larger mushroom spines are known as ‘memory spines’ and are formed from the thin spines when learned tasks are remembered. ​ At different stages of life, as well as in different disease states, different proportions and density of spines is observed, for example alcoholism is associated with a decrease in spine density. Studies in rats have found associations between spine density and learning and memory, e.g., spine density increases with learning, and behavioural measures of memory can be linked to changes in spine density. Cytoskeleton In the same way as our skeleton gives our body structure, the inside of the neuron has a cell skeleton (cytoskeleton) along its entire length. The cytoskeleton is made up of a lattice work of microfilaments and neurofilaments to provide a framework and for structural rigidity. Imagine the wooden frame of a house - it's there to keep everything stable and upright! The cytoskeleton also consists of microtubules for transport. ​ The regular transport system of a neuron by bulk flow is very slow (0.2 – 2.5 mm/day) down the cytoplasm of the neuron, and so we have microtubules to help. ​ Microtubules allow faster transport. ​ Transport includes nutrients and waste products and the chemical neurotransmitters used to transmit information from the neuron’s terminals to the next cell(s) in the chain. Packages of nutrients or wastes are carried like along a conveyor belt along the outside of the microtubule: Many forms of brain injury – such as Alzheimer’s disease and traumatic brain injury (due to sporting or vehicular accidents) – appear to involve damage to the cytoskeleton of neurons. This results in damage to the neuronal structure and to the accumulation of organelles and particles normally transported by the microtubules. The accumulation of organelles and particles in one area results in swelling of the axon and eventually in disconnection and lesioning of the axon (axotomy) - this can lead to neuronal death. Myelination of the Neuron As we covered earlier in this lesson, axons are conductible functional components of neurons. What that means is that axons carry (conduct) information that needs to be relayed or to control the next cell in the chain. In some neurons the axon is covered by a myelin sheath. The sheath is formed by glial cells: Schwann cells (in PNS) or oligodendrocytes (in CNS). These nerves are called myelinated nerves, and they're super fast! In myelinated axons, the glial cell wraps itself around a part of the axon and winds itself tighter and tighter. As it does so it squeezes all its own content out to the outermost winding, so that all the inner windings of the sheath consist only of the cell membrane of the glial cell. This sheath is a poor conductor of electricity and so acts as a good insulator of the axon. It’s like the sheath around a copper wire carrying electricity to your electrical appliances! Like that sheath, it prevents leakage of current. In neurons this speeds up the flow of information. The Synapse The synapse is where a neuron contacts another cell to relay information or commands. The synapse is therefore the functional output component of a neuron. It delivers all the messages! In vertebrates, the cells are separated by a gap, known as the synaptic cleft of about 20 nanometers width The neuron uses chemical neurotransmitters to carry the information or commands. We will look further into this in the next module. Information is sent from the signalling neuron (the presynaptic neuron’s terminal) to the target cell (the postsynaptic cell’s receptors) using these transmitters. EDUCATION PERFECT TASK 1.​ SECTION 1: Summary 1.​ The nervous system consists of nerve cells (neurons) and glial cells. ​ Neurons are in charge of transmitting and integrating information in the brain. In the body, neurons coordinate and regulate activity. ​ Neurons have three major components: dendrites; cell body and an axon. ​ Glial cells make up about (90%) of the CNS and act as a support system, including supplying nutrients, removing waste, scavenging debris and creating protective barriers. ​ There are five different types of glial cells: astrocytes; oligodendrocytes; Schwann cells; microglia and ependymal cells. 2.​ The nervous system is divided into a central nervous system (CNS) and a peripheral nervous system (PNS). ​ Nerves in the brain and spinal cord are part of the CNS, while nerves everywhere else are part of the PNS. ​ The PNS is further split into two subsystems, autonomic and somatic. The CNS and PNS are connected to each other via afferent and efferent neurons. ​ Nerve cell bodies are organised in the CNS into groups called nuclei, while cell bodies in the PNS are organised into groups called ganglia. These clumps of nerve cell bodies have bundles of axons that run to other areas, known as nerve tracts.  3.​ The functional classification of neurons to answer 'who, what and where' Class Location Function Transmit information from sensory organs to Sensory Neurons Afferent Neurons the CNS. Transmit information from the CNS to control Motor Neurons Efferent Neurons muscles. Transmit information from one neuron to the Interneurons: next neuron. (a) Over short distances (e.g. between neurons (a) Local Interneuron Only within the CNS within the one nucleus) (b) Projection (or (b) Over long distances (e.g. between neurons tract) interneuron located in two different nuclei) CNS and almost every Release hormones into the blood supply to Neuroendocrine Cell organ in the body influence distant target organs. 4.​ A neuron will also have a cytoskeleton consisting of micro- and neuro-filaments for rigidity, and microtubules for transport. These components extend the entire length of the neuron, from the cell body through the axon to the terminals, giving the neuron a structure. Axon terminals are specialised regions called synapses, where it contacts other cells in a method that doesn't even involve touch, sending chemical neurotransmitters between neurons. These synapses are known as the output regions of neurons. 5.​ Myelination involves glial cells wrapping themselves around parts of the axon and winding themselves tighter and tighter. As they do so they squeeze all their own content out to the outermost winding, so that all the inner windings of the sheath consist only of the cell membranes of the glial cells. This sheath is a poor conductor of electricity and so acts as a good insulator of the axon.It’s like the sheath around a copper wire carrying electricity to your electrical appliances! Like that sheath, it prevents leakage of current. In neurons this speeds up the flow of information.