THE NERVOUS SYSTEM Homeostasis Strand
Introduction In general, a nervous system has three overlapping functions : 1. Sensory input conduction of signals from sensory receptors to integration centres 2. Integration information is interpreted and associated with proper responses 3. Motor output conduction signals from the processing centre to effector cells
Organization of The Nervous System
Structure of the Neuron
Nervous System Cells - the CNS uses two types of cells : glial - non-conducting cells - often surround neurons - functions : hold neurons in place supply nutrients and oxygen to neurons insulate one neuron from another destroy / remove carcasses of dead neurons
neurons - functional units of nervous system - three different kinds 1. sensory (afferent neurons) - sense and relay info. from environment to CNS - located in clusters called ganglia - i.e. photoreceptors, chemoreceptors, baroreceptors
2. interneurons (association neurons) - connect sensory neurons to motor neurons - interpret info.
3. motor (efferent neurons) - relay info. to effectors - muscles, organs, glands
The simplest nerve pathway is called the reflex arc.
The Electrochemical Impulse - around 1900 : nerve impulses are an electrochemical message created by the movement of ions through the nerve cell membrane Inside A Nerve Cell - neurons have a large supply of both positive and negative ions inside and outside the cell (rare in cells of other types) - the electrochemical gradient is caused by an unequal concentration of positive ions across the membrane - NOTE : neg. ions play only a small role since they are mainly large and cannot cross the membrane
- the electrochemical impulse takes place in four basic steps : 1. The Resting Potential - K +, concentrated inside the membrane, want to diffuse out - Na +, concentrated outside the membrane, want to diffuse in - diffusion is unequal - however, because the resting membrane is 50X more permeable to K +, the membrane maintains an external (outside) pos. charge - the exterior of the membrane becomes pos. relative to the interior (neg.) - this resting potential is about -70mV
2. Depolarization - a nerve impulse can be initiated in a neuron by mechanical, chemical, thermal, or electrical stimulation - when excited, the nerve cell membrane becomes more permeable to Na + (than K + ), their ion gates open, K + gates shut, and Na + rushes in by diffusion and charge attraction - this change in potential is called the action potential and lasts about 3 ms - when an action potential occurs, the axon is said to be depolarized (the interior is now pos. relative to the exterior) - the resting potential changes from -70mV to about +40mV
3. Repolarization (and Hyperpolarization) - K + channels open in the membrane and K + ions diffuse out along a concentration gradient, starting repolarization - at the same time, sodium channels in the membrane close, preventing any further influx of Na + ions - the Na + gates are slow to respond to the changes in voltage - in fact, so many K + ions leave that the charge on the inside of the membrane becomes more negative than it was originally (referred to as the undershoot, or hyperpolarization)
4. The Na+/K+ Pump - the Na + /K + pump restores the resting polarized membrane by transporting Na + from the inside to the outside, and K + from the outside in (3Na + : 2K + ) - this is a form of active transport - adjoining areas of the nerve membrane become permeable to Na + and the action potential moves away from the site of origin - the electrical disturbance causes Na + channels to open in the adjoining area of the nerve cell membrane and a wave of depolarization moves along the nerve membrane
Important Notes nerves conducting an impulse cannot be activated until the condition of the resting membrane is restored (the refractory period) different neurons have different threshold levels smaller diameter axons result in faster impulse transmission the response is all or none greater stimulus intensity produces more frequent impulses greater stimulus intensity produces impulses in more neurons
The Synapse - small (~20 nm) spaces between neurons are called synapses - in axon endplates there are chemicals called neurotransmitters - when the action potential reaches a terminal, it depolarizes the terminal membrane, opening calcium (Ca2+) channels in the membrane - calcium diffuse into the terminal and cause neurotransmitter release - they are released when the nerve impulse reaches the end of the axon and they diffuse across this space - these neurotransmitters, released from the presynaptic membrane, depolarize the dendrites of the postsynaptic membrane
- the nerve impulse slows (slightly) at each synapse - excitatory neurotransmitters open Na+ channels on dendrites, thereby increasing the membrane potential toward threshold - inhibitory neurotransmitters open K+ channels on dendrites, thereby decreasing the membrane potential away from threshold - chemicals are released by the postsynaptic membrane to destroy neurotransmitters after they have completed their function - why is this important?
- examples of neurotransmitters : acetylcholine inhibitory or excitatory, depending on receptor excitatory on most muscle cells, inhibitory on cardiac muscle low levels linked to Alzheimer s disease serotonin regulation of mood, appetite, sleep, memory, learning mostly inhibitory effects LSD binds to serotonin receptors and blocks inhibitory effect which leads to hallucinations (trippy!) low levels linked to depression dopamine normally released when a need is filled, causing a feeling of pleasure or satisfaction low levels linked to Parkinson s disease schizophrenia possibly linked to high levels norepinephrine can excite and inhibit low levels linked to depression
The Autonomic Nervous System all autonomic nerves are motor nerves that regulate the organs of the body without conscious control smooth and cardiac muscle, organs of gastrointestinal, cardiovascular, excretory, and endocrine systems convey signals that regulate the internal environment (as opposed to the somatic n.s. which carry signals in response to external stimuli)
made up of two distinct, and often opposing, units : the sympathetic nervous system generally increase energy consumption and prepare an individual for action the parasympathetic nervous system enhance activities that gain and conserve energy
ANS animation : Paul Andersen, I challenge you! You re a dork. Copycat.