Transcript The Nervous System

The Nervous System
• Neuron
– Cell body; Dendrites; Axon
• Three general groups of neurons
– Sensory neurons (afferent or receptor)
• Receive the initial stimulus
– Motor neurons (efferent)
• Stimulate effectors, target cells that produce some kind of
response.
– Interneurons (connector or association)
• Located in the spinal cord and brain
• Receive impulses from sensory neurons and send impulses to
motor neurons.
The Nervous System
• Transmission of nerve impulse
– Chemical changes across the membrane of neuron.
– Membrane of a unstimulated neuron is polarized.
• Difference in electrical charges between the outside and inside of the
membrane.
• Inside is negative; outside is positive.
– Polarization
• Excess sodium ions (Na+) on the outside
• Excess potassium ions (K+) on the inside
• Leakage of Na+ and K+ ions membranes, but Na+/K+ pumps in the
membrane actively restore the ions to the appropriate side.
• Other ions, such as large negatively charged proteins and nucleic
acids, reside inside the cell, creating the negatively charged interior.
The Nervous System
• Transmission of a nerve impulse
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Resting potential
Threshold potential
Depolarization
Action potential
Repolarization
Refractory period
Hyperpolarization
The Nervous System
• Myelin sheath
– Schwann cells
• Insultors
– Nodes of Ranvier
– Action potentials jump from node to node,
thereby speeding the propagation of the
impulse.
The Nervous System
• Synapse or Synaptic cleft
– The gap that separates adjacent neurons
– Transmission of impulse across the synapse
• Presynaptic cell to postsynaptic cell
• Electrical or Chemical
– Most synaptic clefts are traversed by chemicals
The Nervous System
• Chemical synapse transmision
– Calcium (Ca++) gates open
• When the action potential reaches the end of an
axon, the depolarization of the membrane causes
gated channels to open and allow Ca++ to enter the
cell.
– Synaptic vesicles release neurotransmitter
• Influx of Ca++ into the terminal end of the axon
causes synaptic vesicles to release molecules called
neurotransmitters into the synaptic cleft.
The Nervous System
• Chemical synapse transmision
– Neurotransmitter binds with postsynaptic receptors
• Neurotransmitter diffuses across the synaptic cleft and binds
with proteins on the postsynaptic membrane.
– The postsynaptic membrane is excited or inhibited.
• Depending upon the kind of neurotransmitter and the kind of
membrane receptors, there are two possible outcomes for
postsynaptic membrane
– Excitatory postsynaptic potential
• Na+ gates open; membrane becomes depolarized; action
potential is generated.
– Inhibitory postsynaptic potential
• K+ gates open; membrane becomes more polarized
(hyperpolarized); inhibits action potential
The Nervous System
• Chemical synapse transmision
– The neurotransmitter is degraded and
recycled.
• After the neurotransmitter binds to the postsynaptic
membrane receptors, it is broken down by enzymes
in the synaptic cleft.
• Example
– Acetylcholine is broken down by cholinesterase.
– Degraded neurotransmitters are recycled by the
presynaptic cell.
The Nervous System
Neurotransmitters
• Acetylcholine
– Commonly secreted at neuromuscular
junctions (between motor neurons and muscle
cells).
– Stimulates muscles to contract
– At other junctions, it produces a inhibitory
postsynaptic potential.
The Nervous System
Neurotransmitters
• Epinephrine; norepinephrine; dopamine;
and serotonin.
– Dervived from amino acids
– Secreted between neurons of the CNS
• Gamma aminobutyric acid (GABA)
– Usually an inhibitory neurotransmitter among
brain neurons
The Nervous System of Humans
and other Vertebrates
• Central nervous system
– Brain and spinal cord
• Peripheral nervous system
– Consists of:
• Sensory neurons
– Transmit impulses to the CNS
• Motor neurons
– Transmits from the CNS to effectors.
The Nervous System of Humans
and other Vertebrates
• Motor neuron system (two groups)
– Somatic nervous system
• Directs the contraction of skeletal muscles
– Autonomic nervous system
• Controls the activities of organs and various
involuntary muscles (cardiac and smooth)
The Nervous System of Humans
and other Vertebrates
• Two divisions of the Autonomic N.S
– Sympathetic nervous system
• Stimulates the activities of the body for action
– Increasing heart rate
– Increasing the release of sugar from the liver into the
blood
– “fight or flight” response
– Parasympathetic nervous system
• Activates activities of relaxation
– Stimulating the secretion of saliva or digestive enzymes