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The Nervous System Note.docx

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York University
PSYC 1010
Rebecca Jubis

November 22 2011 - The Nervous System pp. 177 to 203 NERVOUS SYSTEM: the body’s neuron-based master control system; it works together with the endocrine system - 90% of the nervous system consists of glia cells which help maintain the proper concentrations of vital ions in the fluid around neurons and assist in the formation of connections between brain neurons Neurons are the communication lines of the nervous system SENSORY NEURONS: a neuron that collects information about a stimulus and relays it to the spinal cord and brain INTERNEURON: a neuron in the spinal cord or brain that receives sensory input, processes it, and sends signals to other neurons MOTER NEURONS: a neuron that relays signals from interneurons to muscles and glands – effectors that carry out responses DENDRITE: a short, branching extension of a neuron that receives incoming signals (“input zones”) AXON: the extension of a neuron that conducts signals away from the neuron  Endings are known as “output zones” where messages are sent to other cells Properties of a neuron’s plasma membrane allow it to carry signals - Neurons can response to certain stimuli by producing an electrical signal - In a resting neuron, the gated sodium channels are closed and the plasma membrane allows only a little sodium to leak inward. The membrane is more permeable to potassium; therefore there is s a concentration gradient across the membrane. Following the rules of diffusion, sodium tends to move in and potassium tends to move out. - The cytoplasm next to the membrane is more negative than the fluid outside of it. The steady charge difference across the membrane is -70 millivolts. The minus indicates that the cytoplasm side of the membrane is more negative than the outer side of the membrane (known as resting membrane potential) Nerve impulses = Action potentials - When an adequate signal reaches a resting neuron’s input zone the sodium gates open and sodium rushes into the neuron. As they flow in, the cytoplasm next to the plasma membrane become less negative. When the voltage difference across the neuron plasma membrane shifts by a minimum amount called the threshold, the result is an action potential - When the threshold level is reached the opening of sodium gates doesn’t depend on the strength of the stimulus anymore Action potentials travel away from their starting point - Each action potential propagates itself, moving away from its starting point. This occurs because the changes in membrane potential leading to an action potential don’t lose strength. A neuron can’t “fire” again until ion pumps restore its resting potential - The area of the cell’s plasma membrane can’t receive another signal until its resting membrane potential is restored - Sodium is always leaking into the neuron (down an electrochemical gradient) and potassium is always leaking out (down its concentration gradient) because the inside of the cell is more negative than the outside and the negatively charged proteins in the cytoplasm help create this electric gradient SODIUM POTASSIUM PUMP: a carrier protein through which active transport moves potassium ions into a neuron and sodium ions outward with the energy from ATP Action potentials are “all-or-nothing” - Every action potential in a neuron spikes to the same level above threshold as an all-or-nothing event. - If threshold is not reached, the disturbance to the plasma membrane will fade away as soon as the stimulus is removed - Each spike lasts for about a millisecond. - Halfway through the action potential, potassium channels open so they can flow out and restore the original voltage difference across the membrane. - After the resting membrane potential has been restored, most potassium gates close and sodium gates are in their initial state (ready to be opened when a stimulus arrives) How neurons communicate 1. Action potentials flow along the axon of a motor neuron to neuromuscular junctions, where an axon terminal forms a synapse with a muscle fibre 2. The axon terminal stores chemical signalling molecules called neurotransmitters inside synaptic vesicles 3. Arrival of an action potential causes exocytosis of synaptic vesicles, and neurotransmitter molecules enter the synapse 4. The plasma membrane of the muscle fibre has receptors for the neurotransmitter 5. Binding of the neurotransmitter opens a channel through the receptor. The opening allows ions to flow into the receiving cell Neurotransmitters can excite or inhibit a receiving cell - Exciting signals help drive the membrane toward an action signal - Inhibiting signals have the opposite effects - Neuromodulators can magnify or dampen the effects of a neurotransmitter o Endorphins inhibit nerves from releasing substance P, which conveys information about pain Neurotransmitter Examples of effects Acetylcholine (Ach) Causes skeletal muscle contractions; affects mood and memory Epinephrine and norepinephrine Speed heart rate; dilate the pupils and airways to lungs; slow GI tract contractions; increase anxiety Dopamine Reduces excitatory effects of other neurotransmitters; roles in memory learning, fine motor coordination Serotonin Elevates mood; has a role in memory GABA Inhibits the release of other neurotransmitters Competing signals are “summed” up - Signals compete for control of the membrane potential at the trigger zone - EPSPs (excitatory postsynaptic potentials) depolarize the membrane, they bring it closer to the threshold - IPSP (inhibiting postsynaptic potentials) hyperpolarize the membrane, they drive it away from threshold or help keep the membrane at its resting level - Synaptic integration is a process in which the competing signals arriving at a neuron are summed up before the neuron responds (sort of like adding up the pros and cons if a certain course of action). This occurs when neurotransmitter molecules from more than one presynaptic cell reach a neuron’s input zone at the same time. Neurotransmitter molecules must be removed from the synapse - The flow of signals through the nervous system depends on the rapid, controlled removal of neurotransmitter molecules from synapses - Certain drugs can block the reuptake of particular neurotransmitters Nerves are long-distance lines - A nerve consists of nerve fibres (long axons of sensory neurons and/or motor neurons) - In the central nervous system, nerves are called nerve tracts - Each axon has an insulating myelin sheath that consists of glial cells which allows action potentials to propagate faster. The unsheathed sodium channels are voltage sensitive, making the action potential jump from node to node - In the central nervous system, glial cells called oligodendrocytes form the myelin sheath. In the rest of nervous system, glial cells called Schwann cells form the sheath Reflexes are the simplest nerve pathways - A reflex is a simple, programmed movement in response to a stimulus. It is always the same and takes place with conscious effort - In the simplest reflexes, sensory neurons synapse directly on motor neurons In the brain and spinal cord, neurons interact in circuits - In the nervous system, sensory nerves relay information into the spinal cord, where they form chemical synapses with interneurons. - Many interneurons synapse with motor neurons, which carry signals away from the spinal cord and brain Overview of the nervous system - Central nervous system is made up of the brain and spinal cord - Peripheral nervous system consists of 31 pairs of spinal nerves and 12 pairs of cranial nerves. At some places, cell bodies of several neurons occur in clusters called ganglia. o This is organized into somatic and autonomic subdivisions - Nerves that carry sensory information to the central nervous system are called afferent nerves - Nerves that carry motor messages away from the central nervous system to muscles and glands are called efferent nerves The peripheral nervous system consists of somatic and automatic nerves - Somatic nerves carry signals related to movements o
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