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The Nervous System

Functions of the Nervous System Sensory Input, Integration, and Motor Output
Sensory Input The body receives stimuli through millions of receptor cells that monitor changes taking place within and outside of the body
Integration The information received from the sensory input needs to be interpreted and decisions must be made as what to do
Motor Output The interpretation causes a response by the body by activating effector organs (muscles and glands)
Central Nervous System The brain and spinal cord; the true command center. Interprets incoming information and dictates what is done based on past experiences, reflexes, and current conditions.
Peripheral Nervous System The part of the nervous system that is outside the CNS. Consists of nerves that extend from the brain and spinal cord. Serves as a communicator that links body to CNS.
Sensory (afferent) division Convey impulses to the CNS from receptors all over the body
Motor (efferent) division Transmits signals from the CNS
Somatic nervous system Voluntary, nerve fibers that conduct impulses from CNS to skeletal muscles
Autonomic nervous system Involuntary, consists of visceral nerve fibers that control the smooth muscles, cardiac muscles, and glands
Sympathetic division Mobilizes body systems during emergencies. Activates the body to cope with stressors.
Parasympathetic division Oversees digestion, elimination, and glandular function. Promotes non-emergency functions
The Reflex Arc Many situations call for immediate action. To help avoid injury, reflex arcs provide a means for withdrawal from dangerous stimuli. Eventually, it reaches the brain, but it takes less time if processed by spinal cord without going to brain first
Neuroglia Neurons work very closely with non-nervous supporting cells called collectively neuroglia or glial cells. There are 6 types, all with a unique function, that provide support for the neurons in order for them to function properly
Astrocytes Star shaped, most abundant and versatile glial cells, support and bracing for the neurons, connection to blood capillaries and aid in the blood-brain barrier formation, control the chemical environment, possibly aid in the concerted impulse transmission
Microglia Small, oval shaped with long thorny processes, can sense injured/damaged neurons, migrate toward them, and transform into macrophages to "clean up". Very important since immune cells are not able to cross into the CNS
Ependymal cells Squamous to columnar in shape, often ciliated, line cavities of CNS where beating action helps circulate cerebrospinal fluid
Oligodendrocytes Fewer branches than astrocytes, found along thicker branches in the CNS, form the myelin sheath, Myelin insulates the axon and allows for very quick impulse transmission
Satellite cells PNS: cells that surround the neuron cell body, function not exactly known but possibly to help regulate the external chemical environment
Schwann cells PNS: Wrap around and form the myelin sheaths around nerve fibers in the PNS, similar function as the oligodendrocytes but in the PNS and only myelinate one axon
The Neuron Structural unit of the nervous system, conduct the electrical messages all over the body
Special characteristics of neurons Extreme longevity, Amitotic, very high metabolic rate
Extreme Longevity They will last you a lifetime if well taken care of (proper nutrition and limited damage)
Amitotic they cannot replace themselves if they die. A few exceptions include olfactory (smell) and hippocampal (memory)
Very high metabolic rate they require almost constant supply of nutrients and can only survive a few minutes without oxygen
Cell Body (Soma) The main focal point for outgrowth of neuronal processes; contains the nucleus, most are located within the CNS under the protection of the skull and vertebrae
Dendrites Function as the main receptive sites or input regions and direct incoming signals toward the cell body. All the branching provides much surface area to receive signals from other neurons
Axon Can be 3 to 4 feet long, conduction component of the neuron. They generate nerve impulses and transmit them away from the cell body, each neuron has only one axon, but rarely may branch.
Synaptic Knobs at the end of an axon, release neurotransmitters which excite or inhibit other neurons or effector cells
Myelin Sheath A white, fatty covering associated with axons, protects and insulates fibers from one another, vastly increases the speed of transmission or nerve impulses (150x!), formed by schwann cells in the PNS and oligodendrocytes in the CNS
Neurilemma The nucleus and most of the cytoplasm that ends up getting squeezed into a buldge
Nodes of Ranvier Gaps that form in the sheath between adjacent schwann cells at regular intervals (1mm apart)
Saltatory Conduction Situation where speed of an impulse is greatly increased by the message, "jumping" the gaps in an axon: only occurs in axons that have myelin
Sensory (afferent) neurons Neurons located near receptor organs (skin, eyes, ears), receive incoming stimuli from the environment , transmit impulses from sensory receptors in the skin or internal organs toward or into the CNS
Motor (efferent) neurons Neurons located near the effector organs (muscles and glands), carry impulses away from CNS to effector organs to initiate a response
Interneurons Lie between sensory and motor neurons and conduct impulses within the CNS, neurons that relay messages between other neurons such as sensory and motor (brain and spinal cord), 99% of neurons in body
The Action Potential The mechanism by which neurons send impulses
Resting Membrane Potential Occurs where neuron is at rest, a condition where outside is positive and inside is negative, polarized, neuron has a voltage difference of -70 mV, maintained by sodium gates are closed, potassium ions leak out, membrane has an abundance of positives
Na/K pump The "sodium Potassium" pump pulls 2 potassium ions in for 3 Sodium ions sent out, this further creates a charge difference
Depolarization When the neuron is stimulated, the following events occur: Sodium ions rush into the axon, this neutralizes the negatives inside, inside becomes temporarily positive and outside negative, nearby sodium channels open to continue the depolarization
Repolarization This is the restoring of the positive charge on the outside and negative on the inside, potassium gates open and potassium floods out, this generates positive charge on the outside of membrane, sodium channels close
Refractory Period Brief period of time between triggering an impulse and when its available for another, no new action potentials can be created during this time
Threshold Stimulus If an axon is stimulated above its threshold it will trigger an impulse down its length, the strength of the response is not dependant upon the stimulus, an axon cannot send a mild or strong response, it either responds or does not
Graded Potentials Incoming signals, vary in strength, lose strength over distance, slower than action potentials, travels to trigger zone
Subthreshold too weak, no generation of action potential
Corresponds to the period of repolarization of the neuron Relative refractory period
Process by which the resting potential is decreased as sodium ions move into the axon Depolarization
State of an unstimulated neuron's membrane Polarized
Period during which potassium ions move out of the axon repolarization
also called the nerve impulse action potential
period when a neuron cannot be restimulated because its sodium gates are open absolute refractory period
Mechanism by which ATP is used to move sodium ions out of the cell and potassium ions into the cell; completely restores and maintains the resting conditions of the neuron Sodium-Potassium Pump
Point at which an axon "fires" Threshold
Term for a weak stimulus Subthreshold
Self-propogated depolarization action potential
Codes for intensity of the stimulus Frequency of impulses
Membrane potential at which the outward current carried by Potassium is exactly equal to the inward current carried by sodium Threshold
A voltage change that reduces the ability of a neuron to conduct an impulse; the membrane potential becomes more negative Hyperpolarization
A local change in membrane potential in which current flow is quickly dissipated, that is, decremental Graded potential
An all-or-none electrical event Action Potential
A voltage change that brings a neuron closer to its threshold for firing; the membrane potential becomes less negative and moves toward 0 Depolarization
Results from the opening of voltage-regulated ionic gates Action Potential
Results from the opening of chemically regulated gates or energetic stimuli graded potential
Characterized by a rapid polarity reversal Action Potential
Created by: knuepril