Cells of the Central Nervous System PDF

PSYC211 – Machado – 2 Cells of the Central Nervous System What is the brain made of? Although cognitive neuroscience focuses on the activity of neurons, neurons acc...

PSYC211 – Machado – 2 Cells of the Central Nervous System What is the brain made of? Although cognitive neuroscience focuses on the activity of neurons, neurons account for only about 10% of our brain cells. Glial cells account for about 90% of our brain cells, but their role is mainly supportive. Cognition Communication between neurons in the brain enables our behaviours and thoughts. Cognition depends on the activity of neurons as well as the transmission of information between neurons. Disruption of neuronal communication by damage, disease, or drugs can affect perception, movement, and cognition. Neurons Neurons are the basic signalling unit. Neurons are distinguished by their function, location, and interconnectivity. Neurons receive information, process it, and then transmit the output to other neurons. Function: Communication via electrical and chemical signals Structure: – Dendrites – Soma (or cell body) – Axon – Axon terminals Dendrites Dendrites extend from the cell body. Dendrites are (usually) treelike processes that receive inputs from other neurons. Soma The soma is the central part of the neuron. The soma contains metabolic machinery that maintains the neuron; this machinery is suspended in fluid and surrounded by the cell membrane. Axon The axon extends from the cell body. The axon functions like a telegraph wire, sending electrical impulses to distant sites in the nervous system. Axons vary in length from less than a millimetre to over a metre. Some axons are covered in myelin. Axons often branch. (Image adapted from Cognitive Neuroscience: The Biology of the Mind, page 26, Figure 2.2)  PSYC211 – Machado – 2 Myelin Myelin is formed by oligodendrocytes. Myelin is a fatty substance that surrounds and insulates nerve fibres (i.e., axons). Myelin facilitates the conduction of nerve impulses (i.e., speeds up the signal). Myelin appears white. The importance of myelin becomes obvious when you consider the pathology and symptoms of multiple sclerosis (MS). The pathology associated with multiple sclerosis causes the destruction of myelin. Axon Terminals Axons end at axon terminals. The axon terminal is where the axon comes into contact with another neuron and transmits information to it. This point of contact is called a synapse. Synapse (Image from Neuroscience: Exploring the Brain, p. 35, Figure 2.15) The synapse is where the axon terminal of one neuron contacts another neuron in order to transmit information. The synapse has two sides: presynaptic and postsynaptic (information flows from the presynaptic neuron to the postsynaptic neuron). Pre- and Post-Synaptic Presynaptic = before the synapse, with respect to the flow of information (e.g., axon terminals are presynaptic because they are positioned before the synapse) Postsynaptic = after the synapse, with respect to the flow of information (e.g., dendrites are postsynaptic because they are positioned after the synapse) Synapse At most synapses, information in the form of electrical impulses travelling down the axon is converted in the axon terminal into a chemical signal that crosses the synaptic cleft. The chemical signal is called a neurotransmitter. On the postsynaptic membrane, this chemical signal is converted back into an electrical signal. Neurotransmitters Neurotransmitters are stored in and released from synaptic vesicles within the axon terminal. Different neurotransmitters are used by different types of neurons.  PSYC211 – Machado – 2 Glial Cells (Image adapted from Cognitive Neuroscience: The Biology of the Mind, page 30, Figure 2.7) Glial cells account for more than half of the brain’s volume. Glial cells support neurons. Main types of glial cells in the CNS: – Astrocytes – Oligodendrocytes – Microglia Astrocytes Astrocytes form a barrier between neuronal tissue and blood, called the blood-brain barrier (BBB). The BBB protects the CNS from some molecules in the bloodstream. Oligodendrocytes Oligodendrocytes myelinate axons (myelin is a fatty substance that surrounds the axons of many neurons and speeds neural communication). Oligodendrocytes form myelin by wrapping their cell membranes around the axon during development. Myelin is white; hence, axons are referred to as white matter. The myelination of one axon requires many oligodendrocytes. Microglia Microglia devour and remove debris left by dead or degenerating brain cells. Reference List Bear, M. F., Connors, B. W., & Paradiso, M. A. (1996). Neuroscience: Exploring the Brain. Baltimore, MD: Williams & Wilkins. ISBN 0-683-00488-3 (Figure 2.15 from page 35) Gazzaniga, M. S., Ivry, R. B., & Mangun, G. R. (2002). Cognitive Neuroscience: The Biology of the Mind (Second ed.). New York, NY: W. W. Norton & Company. ISBN 0-393-97777-3 (Figure 2.2 from page 26, Figure 2.7 from page 30)