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Nervous System Part 1 Nervous System • The nervous system is one of the two control systems of the body – The other being the endocrine system – These two systems have a lot of interaction! • Made of neurons and neuroglia – Nervous tissue! Nervous System Organization Neurons • There are three classifications of neurons – Sensory Neurons – Interneurons – Motor Neurons • Interneurons form the central nervous system (CNS) • Sensory and motor neurons form the peripheral nervous system (PNS) Neuron Structure • Neurons have three general structures: – Soma (cell body) – Axon (signal transmission) – Dendrite (signal reception) • Some neurons look rather different but all have these three parts in some way Central Nervous System • Composed of the brain and spinal cord • Contains many tracts (bundles of axons) leading from one area to another – These are all interneurons, signaling each other and storing information – This is how we think and remember! Peripheral Nervous System • The PNS consists of organs called nerves which are sheaths of neuronal axons covered in connective tissue – There are 12 pairs of cranial nerves and 31 pairs of spinal nerves • Peripheral nerves sending signals towards the brain are afferent nerves, while nerves sending signals to the body are efferent nerves – Afferent nerves make up our senses (chapter 12, not covered in this class) Cranial & Spinal Nerves • When a nerve ends at a particular body part it is said to innervate that region • The cranial nerves project out of the brain and innervate the body directly from the brain – Include the optic nerve (eye) and the vagus nerve (organs) • The spinal nerves branch out from the spinal cord and each innervates a region of the body – Named for which vertebral gap they come out of Cranial vs. Spinal Nerve • Both afferent and efferent nerves (sensory and motor neurons) are bundled in each cranial and spinal nerve • The spinal cord contains the cell bodies (somas) of the PNS neurons in clusters called ganglia (sing. ganglion) – Only the axons stick out into the rest of the body – The axon of the neuron can be regrown if the soma is intact! Examples of Cranial vs. Spinal Nerve Activities Action Potentials • Recall: Sodium inrush depolarizes a cell membrane, before potassium rushes out and repolarizes the membrane again • This brief flicker back and forth of voltage triggers adjacent voltage-gated ion channels to open • Propagates the action potential as a wave of ions rushing across the membrane Speed of Conduction • Action Potentials may be sped up if there is myelin on the axon • Myelin is a dense, fatty material, with gaps called Nodes of Ranvier • Myelin conducts the voltage from one Node of Ranvier to the next much more quickly • Only some neurons (e.g. skeletal motor neurons) are myelinated More on Myelin • Heavily myelinated nervous tissue appears white, while unmyelinated tissue appears gray – White matter vs. Gray matter • If myelin is stripped off (by, for example, the immune system) there are no voltage-gated ion channels underneath it and the neuron fails to send signals – This is the cause and effect of Multiple Sclerosis, which paralyzes myelinated neurons while leaving other neurons intact Release of Neurotransmitters • Once the action potential reaches the end of a neuron, voltagegated calcium channels open and allow calcium into the neuron – The neuron attaches to another neuron or to another organ at a synapse • The calcium stimulates the release of synaptic vesicles into the synapse – These vesicles contain neurotransmitters that have an effect on the target cell That’s our show! • Next class: last new material before exam #2!