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Neurons and the Nervous System What is the nervous system? It coordinates a range of operations in our everyday lives. It receives and analyses sensory information from the sensory receptors of our bodies, processes information, and responds in various ways to that information. What is the nervous system made of? Nerve cells or neurons Let’s play, “Which part are we talking about?” Small branching projections distributed throughout the cell body which convey nerve impulses toward the cell body DENDRITES Let’s play, “Which part are we talking about?” The space between two neurons The synaptic cleft Let’s play, “Which part are we talking about?” Branch off from the axon and convey nerve impulses to the synaptic cleft (space between 2 neurons) Nerve endings or axon terminal Let’s play, “Which part are we talking about?” Contains the nucleus which holds the dna of the cell and produces proteins necessary for the proper functioning of the cell The cell body Let’s play, “Which part are we talking about?” Gel-like substance that holds all of the major organelles of the cell Cytoplasm Separates the interior of the neuron from the outside world and contains features that allow nerve impulses to be transported from one nerve to the next. Cell membrane Let’s play, “Which part are we talking about?” Located along the axon and allows nerve impulses to travel more quickly along the axon by insulating it. Myelin sheath Ex. MS, and toddlers Let’s play, “Which part are we talking about?” Which parts are a part of the synapse? The synaptic cleft, the pre-synaptic neuron and the post-synaptic neuron The Nervous System is Divided into 2 Parts CNS: Central nervous system Coordinates the activity of all parts of the body PNS: Peripheral nervous system Connects the central nervous system to the limbs and organs How do we process and respond to information from the outside world? Sensory neurons, which are part of the PNS, detect changes in the world around us and then transmit that information to neurons in the CNS (brain and spinal cord), where the information gets processed and integrated with information sent from other neurons. Once processed, our CNS sends messages back to the PNS via motor neurons which tell our muscles or glands to respond to the sensory information in some way. Ex. Picking a book that just fell on the floor. More on Neural Communication Although neurons can transmit information to and from the brain as quickly as 200 miles/hour, our reactions to events are not instantaneous. In order to protect us from dangers, reflexes allow for quicker responses. The physical response happens before our brain receives the message. What part of the body is most involve in reflexes? Spinal cord Why are concussions so dangerous? Neurons are amitotic What does that mean? They do not undergo mitosis, which is another word for cell division and replication So once a neuron dies, it can never be regenerated More on reflexes Baby reflexes: baby reflexes (moro reflex) baby robot arms Neuronal Communication Involves both Chemical and Electrical Messages Within a neuron, the message is transmitted electrically by something called an action potential Between neurons, the message is transmitted chemically via neurotransmitters through the synaptic cleft. Action Potentials At rest, the fluid inside a neuron has an excess of negatively charged ions. i.e. a negative resting potential When a neuron is in its resting state, sodium channels are blocked, thus keeping excess positive ions out of the cell. When a nearby neuron fires an action potential, this triggers some of the sodium channels at the beginning of the axon to open, thus bringing in positively charged ions into the cell. The charge inside that part of the axon then becomes positive, or depolarized. More on Action Potentials This depolarization then causes the next set of sodium channels to open, thus depolarizing the next part of the axon, and this continues like a domino effect. Axons also have channels that pump sodium back out of the cell, to restore the negative resting potential so that it becomes ready again for another action potential. Strength of an Action Potential A neuron’s action potential is all-or-nothing, meaning the strength is not affected by how strong the signal was. No matter how excitatory a signal is, the neuron will always fire with the same intensity. However, the stronger the stimulus, the more neurons there are that will fire Ex. A slap will trigger more neurons to fire than a tap **action potential video The Synapse Axon terminal Presynaptic cell Synaptic cleft synapse Post-synaptic cell At the Synapse When the action potential reaches the axon terminal, it triggers the release of chemical messengers, called neurotransmitters, into the synaptic cleft. These neurotransmitters travel across the synaptic cleft and bind to receptors on the dendrites of the postsynaptic cell. This binding takes place like a key fitting into a lock. *video: neural synapse