Neurons and Supporting Cells 152
A. The central nervous system includes the brain and spinal cord, which contain nuclei and tracts.
II. A neuron consists of dendrites, a cell body, and an axon.
B. Dendrites receive stimuli, and the axon conducts nerve impulses away from the cell body.
III. A nerve is a collection of axons in the PNS.
C. Interneurons, or association neurons, are located entirely within the CNS.
A. Schwann cells form a sheath of Schwann, or neurilemma, around axons of the PNS.
C. Astrocytes in the CNS may contribute to the blood-brain barrier.
I. The permeability of the axon membrane to Na+ and K+ is regulated by gated ion channels.
A. At the resting membrane potential of -70 mV, the membrane is relatively impermeable to Na+ and only slightly permeable to K+.
B. The voltage-regulated Na+ and K+ channels open in response to the stimulus of depolarization.
C. When the membrane is depolarized to a threshold level, the Na+ channels open first, followed quickly by opening of the K+ channels.
II. The opening of voltage-regulated channels produces an action potential.
A. The opening of Na+ channels in response to depolarization allows Na+ to diffuse into the axon, thus further depolarizing the membrane in a positive feedback fashion.
B. The inward diffusion of Na+ causes a reversal of the membrane potential from -70 mV to +30 mV.
C. The opening of K+ channels and outward diffusion of K+ causes the reestablishment of the resting membrane potential. This is called repolarization.
D. Action potentials are all-or-none events.
E. The refractory periods of an axon membrane prevent action potentials from running together.
F. Stronger stimuli produce action potentials with greater frequency.
III. One action potential serves as the depolarization stimulus for production of the next action potential in the axon.
A. In unmyelinated axons, action potentials are produced fractions of a micrometer apart.
The Synapse 167
I. Gap junctions are electrical synapses found in cardiac muscle, smooth muscle, and some regions of the brain.
A. The neurotransmitter can be called the ligand of the receptor.
B. Binding of the neurotransmitter to the receptor causes the opening of chemically regulated gates of ion channels.
Acetylcholine as a Neurotransmitter 170
I. There are two subtypes of ACh receptors: nicotinic and muscarinic.
B. The binding of ACh to muscarinic receptors opens ion channels indirectly, through the action of G-proteins. This can cause a hyperpolarization called an inhibitory postsynaptic potential (IPSP).
C. After ACh acts at the synapse, it is inactivated by the enzyme acetylcholinesterase (AChE).
II. EPSPs are graded and capable of summation. They decrease in amplitude as they are conducted.
IV. ACh in the CNS produces EPSPs at synapses in the dendrites or cell body. These EPSPs travel to the axon hillock, stimulate opening of voltage-regulated channels, and generate action potentials in the axon.
Monoamines as Neurotransmitters 176
I. Monoamines include serotonin, dopamine, norepinephrine, and
A. These neurotransmitters are inactivated after being released, primarily by reuptake into the presynaptic nerve endings.
II. Dopaminergic neurons (those that use dopamine as a neurotransmitter) are implicated in the development of Parkinson's disease and schizophrenia. Norepinephrine is used as a neurotransmitter by sympathetic neurons in the PNS and by some neurons in the CNS.
Other Neurotransmitters 179
I. The amino acids glutamate and aspartate are excitatory in the CNS.
A. The subclass of glutamate receptor designated as NMDA receptors are implicated in learning and memory.
B. The amino acids glycine and GABA are inhibitory. They produce hyperpolarizations, causing IPSPs by opening Cl-channels.
I. Spatial and temporal summation of EPSPs allows a depolarization of sufficient magnitude to cause the stimulation of action potentials in the postsynaptic neuron.
A. IPSPs and EPSPs from different synaptic inputs can summate.
B. The production of IPSPs is called postsynaptic inhibition.
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