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Chapter 48 ~ Nervous System
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Nervous System OverviewNervous System Overview Sensory Input Integration Motor Output-signal conducted from processing
center to effector cells
Signals Conducted by Nerves-extensions of nerve cells
Nervous System Composition:Neurons and Glia (supporting cells)
Neurons communicate information via electrical and chemical signals
Both Divisions of the Nervous System InvolvedBoth Divisions of the Nervous System Involved
1. Central nervous system (CNS)~ brain and spinal cord; Integration
2. Peripheral nervous system (PNS)~ sensory (input) and motor neurons (output)
Effector cells~ muscle or gland cells
Nerves~ bundles of neurons wrapped in connective tissue
Neuron structureNeuron structure Neuron- structural and functional unit
– Cell body- nucelus and organelles
– Dendrites- signals to cell body. Short, numerous
– Axons- away from cell body. Long, Myelin sheath- supporting, insulating layer produced by Schwann Cells Schwann cells-PNS support cells; surround axons Axon hillock-Hillock-axon extends from here Synaptic terminals~ neurotransmitter releaser
Synapse- gap / neuron junction
3 Classes of neurons3 Classes of neurons
1. Sensory neuron: receive & convey from sensory environment information to spinal cord
2.Interneurons: information integration; located in CNS. Synapse only with other neurons.
3. Motor neurons: convey impulses from CNS to effector cell. (muscle or gland)
Neurons Grouped into Nerve CircuitNeurons Grouped into Nerve Circuit
The Reflex Arc– Simplest : – Knee-Jerk Reflex (Patellar
Reflex)– Stretch receptor– simple response; sensory
to spinal cord to motor neurons—knee contracts
Neural SignalingNeural Signaling Signal transduction depends on voltages across neuron plasma
membranes.– Membrane Potential: voltage differences across the plasma membrane).
Net negative charge of about -70mV
Ions Intracellular ( -) ; K+ principal cation Large organic ions- anions Extracellular (less negative) Na+- principal cation Cl- main anion. Ion channels- ungated, gated; all selective
K+ diffuses out (Na+ in); large anions cannot follow….selective permeability of the plasma membrane
Creating & Maintaing the Membrane PotentialCreating & Maintaing the Membrane Potential
Na + - K + Pumps --pump against their conc. gradients
ATP
K+ pumped back in
Na+ pumped back out
Changes in membrane potential key to neural transmission
Changes in membrane potential key to neural transmission
Only neurons and muscle cells can change their membrane potentials in response to stimuli – Excitable Cells– Sensory neurons-environmental stimuli– Interneurons stimuli transmitted via other neurons
– Resting Potential: M.P. of excitable cell at rest.– Change due to flow of ions as gated ion channels open.– stimuli cause ion channels to open
Stimuli that open K+ channels HYPERPOLARIZE the neuron
Stimuli that open NA+ channels DEPOLARIZE the neuron
Graded Potentials –these voltage changes Graded Potentials –these voltage changes
1- Hyperpolarization (outflow of K+); increase in electrical gradient; cell becomes more negative
2- Depolarization (inflow of Na+); reduction in electrical gradient; cell becomes less negative
MylenationMylenation
Electrical insulation—lipid is poor conductor
– Increasing speed of nerve impulse propagation
Multiple Sclerosis: myelin sheaths deteriorated-los of coordination
Normal Membrane PotentialNormal Membrane Potential
Resting Potential: Resting Neuron -70 mV Cytoplasm is negatively charged relative to cell
interior
Resting potentialResting potential~ the membrane potential of the unexcited nerve. – A change in voltage MAY result in an
electrical impulse.
When the Threshold potential is reached, usually sl. More positive (-50 to -55 mV)….
The action potential is triggered….
– The rapid change in membrane potential in an excitable cell
– b/c stimulus triggered the selective opening and closing of voltage-gated ion channels
Action Potential-All Or None change in the Membrane Potential Phases
Action Potential-All Or None change in the Membrane Potential Phases
1. Resting stage •both channels closed
2-Depolarization: •a stimulus opens some Na+ channel gates
Na+ influx reverses membrane polarity.
Threshold reached. (cell interior sl. positive)
Action potential generated .
3-Repolarization •Na+ channels close. K+ channels open; K+ leaves
cell returns to resting potential—then ..
4-Undershoot •K+ channels still open-temporarily HYPERPOLAR.
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The Action PotentialThe Action Potential
Followed by a Refractory period~ insensitive to stimulus.
Amplitude not affected by stimuli Intensity
Action Potentials are self-propagatingAction Potentials are self-propagating
Action Potential regenerated along axon membrane begins at Axon Hillock “Travel” of the action potential is self-propagating One direction only.
Nodes of Ranvier-action potential jumps from one node to the next– Gaps, ion sensitive channels concentrated here, extracellular fluid
contact here
Forward direction only
Action potential speed:Action potential speed:1) Axon diameter (larger = faster; 100m/sec)
2) Saltatory Conduction:
– Mylenation
– Nodes of Ranvier (concentration of ion channels in gaps of the myelin).
– A.P. “jumps” from node to node. 120m/sec
Chemical or Electrical Communication between cells occurs at synapses
Chemical or Electrical Communication between cells occurs at synapses Synapse-tiny gap
Presynaptic cell: transmitting cell Postsynaptic cell: receiving cell
1) Electrical Synapses-via gap junctions; no delay or less in signal strength; less common; fish tail-swim away quickly from predator
2) Chemical Synapses: synaptic cleft separates pre and post-synaptic cells.
Not electrically coupled
Synaptic communicationSynaptic communication Synaptic cleft: separation gap Synaptic vesicles: neurotransmitter
releasers
When an Action Potential arrives at synaptic terminal of presynaptic cell
Causes Ca++ influx; Synaptic vesicles fuse with presynaptic membrane and release…. Neurotransmitter
Neurotransmitters quickly degraded
Neurotransmitter maydo one of the followingNeurotransmitter maydo one of the following
1. Excite the membrane by depolarization
Or
2. Inhibit the postsynaptic cells by hyperpolarization
Types of NeurotransmittersTypes of Neurotransmitters Acetylcholine (most common)
– may be excitatory or inhibitatory– skeletal muscle
Biogenic amines (derived from amino acids)•norepinephrine , epinephrine•dopamine •serotonin (from tryptophan)
Amino acids– GABA—most abundant inhibitory transmitter in brain
Neuropeptides (short chains of amino acids)•endorphin-natural analgesics for the brain
Gaseous Signals of the Nervous SystemGaseous Signals of the Nervous System
NO (nitric oxide)—blood vessel dilation.
– Acetylcholine stimulates blood vessel walls to release NO; neighboring smooth muscles relax & dilate heart’s blood vessels.
– Nitroglycerine is converted to NO—similar response
Nervous system organization tends to corrolate with body symmetry
Nervous system organization tends to corrolate with body symmetry
Vertebrate PNSVertebrate PNS
Cranial nerves (brain origin)
Spinal nerves (spine origin)
Sensory division Motor division
•somatic system voluntary, conscious control •autonomic system √parasympathetic
conservation of energy √sympathetic
increase energy consumption
The Vertebrate BrainThe Vertebrate Brain Forebrain
•cerebrum~memory, learning, emotion•cerebral cortex~sensory and motor nerve cell bodies •corpus callosum~connects left and right hemispheres •thalamus; hypothalamus
Midbrain •inferior (auditory) and superior (visual) colliculi
Hindbrain •cerebellum~coordination of movement •medulla oblongata/ pons~autonomic, homeostatic functions