CHAPTER 48 NERVOUS SYSTEMS

I. AN OVERVIEW OF NERVOUS SYSTEMS A. NERVOUS SYSTEMS PERFORM THE THREE OVERLAPPING FUNCTIONS OF SENSORY INPUT, INTEGRATION, AND MOTOR OUTPUT THE NERVOUS SYSTEM’S THREE MAIN FUNCTIONS ARE SENSORY INPUT, INTEGRATION, AND MOTOR OUTPUT TO EFFECTOR CELLS THE CENTRAL NERVOUS SYSTEM (CNS) INTEGRATES INFORMATION, WHILE THE INTERCONNECTING NERVES OF THE PERIPHERAL NERVOUS SYSTEM (PNS) COMMUNICATE SENSORY AND MOTOR SIGNALS BETWEEN THE CNAS AND THE REST OF THE BODY

B. THE NERVOUS SYSTEM IS COMPOSED OF NEURONS AND SUPPORTING CELLS CELLS OF THE NERVOUS SYSTEM INCLUDE NEURONS, WHICH TRANSMIT THE SIGNALS, AND SUPPORTING CELLS WHICH SUPPORT, INSULATE AND PROTECT THE NEURONS A NEURON’S FIBERLIKE DENDRITES AND AXONS CONDUCT INFORMATION IN THE FORM OF NERVE IMPULSES AXONS OFTEN ORIGINATE FROM THE ACON HILLOCK AND TERMINCATE IN NUMEROUS BRANCHES SYNAPTIC TERMINALS AT THE ENDS OF AXONS RELEASE NEUROTRANSMITTER MOLECULES INTO THE SYNAPSES, THEREBY RELAYING NEURAL SIGNALS TO THE DENDRITES OR CELL BODIES OF OTHER NEURONS OR EFFECTORS

THE CNS CONSISTS OF THE BRAIN AND APINAL CORD THE PNS CONTAINS SENSORY NEURONS, WHICH TRANSMIT INFORMATION FROM INTERNAL AND EXTERNAL ENVIRONMENTS TO TNE CNS, AND MOTOR NEURONS, WHICH CARRY INFORMATION FROM THE CNS TO TARGET ORGANS THE SPINAL CORD MEDIATES MANY REFLEXES THAT INTERGRATE SENSORY INPUT WITH MOTOR OUTPUT IT ALSO HAS TRACTS OF NEURONS THAT CARRY INFO. TO AND FROM THE BRAIN INTERNEURONS OF THE CNS INTEGRATE SENSORY INPUT AND MOTOR OUTPUT

GROUPS OF NEURONS MAY INTERACT AND CARRY INFORMATION ALONG SPECIFIC PATHWAYS CALLED CIRCUITS NERVE CELL BODIES ARE ARRANGED IN FUNCIONAL GROUPS USUALLY CALLED GANGLIA IN THE PNS AND NUCLEI IN THE BRAIN GLIAL CELLS, SUPPORTING CELLS IN THE CNS, INCLUDE ASTROCYTES, WHICH LINE THE CAPILLARIES IN THE BRAIN AND CONTRIBUTE TO THE BLOOD-BRAIN BARRIER, OLIGONDENDROCYTES, WHICH WRAP AND INSULATE SOME NEURONS IN A MYELIN SHEATH IN THE PNS, MYELIN SHEATHS ARE FORMED BY SUPPORTING CELLS CALLED SCHAWNN CELLS.

II. THE NATURE OF NEURAL SIGNALS A.    MEMBRANE POTENTIALS ARISE FROM DIFFERENCES IN ION CONCENTRATION BETWEEN A CELL'S CONTENTS AND THE EXTRACELLULAR FLUID THE MEMBRANE POTENTIAL OF A NONTRANSMITTING NEURON IS DUE TO THE UNEQUAL DISTRIBUTION OF IONS, PARTICULARLY SODIUM AND POTASSIUM, ACROSS THE PLASMA MEMBRANE THE CYTOPLASM IS MORE NEGATIVELY CHARGED THAN THE EXTRACELLULAR FLUID MEMBRANE POTENTIAL IS MAINTAINED BY DIFFERENTIAL ION PERMEABILITIES AND THE SODIUM-POTASSIUM PUMP

A.    AN ACTION POTENTIAL IS AN ALL-OR-NONE CHANGE IN THE MEMBRANE POTENTIAL A STIMULUS THAT AFFECTS THE MEMBRANE’S PERMEABILITY TO IONS CAN EITHER DEPOLARIZE OR HYPERPOLARIZE THE MEMBRANE RELATIVE TO THE MEMBRANE’S RESTING POTENTIAL THIS LOCAL VOLTAGE CHANGE IS CALLED A GRADED POTENTIAL, AND ITS MAGNITUDE IS PROPORTIONAL TO THE STRENGTH OF THE STIMULUS AN ACTION POTENTIAL, OR NERVE IMPULSE, IS A RAPID, TRANSIENT DEPOLARIZATION OF THE NEURON’S MEMBRANE

A LOCAL DEPOLARIZATION TO THE THRESHOLD POTENTIAL OPENS VOLTAGE-GATED SODIUM CHANNELS, AND RAPID INLUX OF NA+ BRINGS THE MEMBRANE POTENTIAL TO A POSITIVE VALUE THE MEMBRANE POTENTIAL IS RESTORED TO ITS NORMAL RESTING VALUE BY THE CLOSING OF THE NA+ CHANNELS A REFRACTORY PERIOD FOLLOWS AN ACTION POTENTIAL, CORRESPONDING TO THE PERIOD WHEN THE VOLTAGE-GATED NA+ CHANNELS ARE INACTIVATED THE ALL OR NONE GERERATION OF AN ACTION POTENTIAL ALWAYS CREATES THE SAME AMPLITUDE OF VOLTAGE CHANGE FOR A GIVEN NEURON THE FREQUENCY OF ACTION POTENTIALS VARIES WITH THE INTENSITY OF THE STIMULUS

C. ACTION POTENTIALS "TRAVEL" ALONG AN AXON BECAUSE THEY ARE SELF-PROPAGATING ONCE AN ACTION POTENTIAL IS INITIATED IN AN AXON, A WAVE OF DEPOLARIZATION PROPAGATES A SERIES OF ACTION POTENTIALS TO THE END OF AN AXON THE RATE OF TRANSMISSION OF A NERVE IMPIULSE IS DIRECTLY RELATED TO THE DIAMETER OF THE AXON SALTATORY CONDUCTION, A MECHANISM BY WHICH ACTION POTENTIALS JUMP BETWEEN THE NODES OF RANVIER OF MYELINATED AXONS, SPEEDS NERVOUS IMPULSES IN VERTEBRATES.

D. CHEMICAL OR ELECTRICAL COMMUNICATION BETWEEN CELLS OCCURS AT SYNAPSES SYNAPSES B/W NEURONS CONDUCT SIGNALS FROM THE AXON OF A PRESYNAPTIC CELL TO A DENDRITE OR CELL BODY OF A POSTSYNAPTIC CELL. ELECTRICAL SYNAPSES DIRECTLY PASS AN ACTION POTENTIAL B/W THE 2 NEURONS VIA GAP JUNCTIONS IN A CHEMICAL SYNAPSE, A DEPOLARIZATION STIMULATES THE FUSION OF SYNAPTIC VESICLES WIH THE PRESYNAPTIC MEMBRANE AND THE RELEASE OF NEUROTRANSMITTER MOLECULES INTO THE SYNAPTIC CLEFT NEUROTRANSMITTERS BIND TO RECEPTOR PROTEINS ASSOCIATED WITH PARTICULAR ION CHANNELS ON THE POSTSYNAPTIC MEMBRANE THE NEUROTRANSMITTER IS RAPIDLY BROKEN DOWN BY ENZYMES.

ONE OF THE MOST COMMON NEUROTRANSMITTERS IS ACYTOCHOLINE F.    THE SAME NEUROTRANSMITTER CAN PRODUCE DIFFERENT EFFECTS ON DIFFERENT TYPES OF CELLS ONE OF THE MOST COMMON NEUROTRANSMITTERS IS ACYTOCHOLINE OTHER TRANSMITTERS THAT HAVE BEEN IDENTIFIED INCLUDE THE BIOGENIC AMINES (EPINEPHRINE, NOREPINEPHRINE, DOPAMINE, AND SEROTONIN)