The Nervous System: Neurons and Synapses
Neurons and Supporting Cells
I. The nervous system is divided into the central nervous system (CNS) and the peripheral
nervous system (PNS).
A. The central nervous system includes the brain and spinal cord, which contain nuclei
and tracts.
B. The peripheral nervous system consists of nerves and ganglia.
II. A neuron consists of dendrites, a cell body, and an axon.
A. The cell body contains the nucleus, Nissl bodies, neurofibrils and other organelles.
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.
A. A sensory, or afferent, neuron is pseudounipolar and conducts impulses from
sensory receptors into the CNS.
B. A motor, or efferent, neuron is multipolar and conducts impulses from the CNS to
effector organs.
C. Interneurons, or association neurons, are located entirely within the CNS.
D. Somatic motor nerves innervate skeletal muscle; autonomic nerves innervate
smooth muscle, cardiac muscle, and glands.
IV. Supporting cells include Schwann cells and satellite cells in the PNS; in the CNS they
include the various types of glial cells; oligodendrocytes, microglia, astrocytes, and ependymal
cells.
A. Schwann cells form a sheath of Schwann around axons of the PNS.
B. Some neurons are surrounded by successive wrappings of supporting cell
membranes called a myelin sheath. This sheath is formed by Schwann cells in the
PNS and oligodendrocytes in the CNS.
C. Astrocytes in the CNS may contribute to the blood-brain barrier.
Electrical Activity in Axons
I. The permeability of the axon membrane to Na+ and K+ is regulated by gates at the openings of
the ion channels.
A. At the resting membrane potential of -70mV, the membrane is relatively
impermeable to Na+ and only slightly permeable to K+.
B. The voltage-regulated Na+ and K+ gates open in response to the stimulus of
depolarization.
C. When the membrane is depolarized to a threshold level, the Na+ gates open first,
+
followed quickly by opening of the K gates.
II. The opening of voltage-regulated gates produces an action potential.
A. The opening of Na+ gates 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
-70mV to +30 mV.
C. The opening of K+ gates 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.
B. In myelinated axons, action potentials are produced only at the nodes of Ranvier;
this saltatory conduction is faster than conduction in an unmyelinated nerve fiber.
The Synapse
I. Gap junctions are electrical synapses, found in cardiac muscle, smooth muscle, and some
regions of the brain.
II. In chemical synapses, neurotransmitters are packaged in synaptic vesicles and released by
exocytosis into the synaptic cleft.
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
I. There are two different subtypes of ACh receptors: nicotinic and muscarinic.
A. Nicotinic receptors enclose membrane channels and open when ACh bonds to the
receptor. This causes a depolarization called an excitatory postsynaptic potential
(EPSP) in skeletal muscle cells.
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 with distance as
they are conducted.
III. ACh is used in the PNS as the neurotransmitter of somatic motor neurons, which stimulate
skeletal muscles to contract, and by some autonomic neurons.
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 gates, and generate action
potentials in the axon.
Monoamines as Neurotransmitters
I. Monoamines include serotonin, dopamine, norepinephrine, and epinephrine. The last three
are also included in the subcategory known as catecholamines.
A. These neurotransmitters are inactivated after being released, primarily by reuptake
into the presynaptic nerve endings.
B. Catecholamines may activate adenylate cyclase in the postsynaptic cell, which
catalyzes the formation of cyclic AMP.
II. Dopaminergic neurons (those that use dopamine as a neurotransmitter) are implicated in the
development of Parkinson 抯 disease and schizophrenia. Norepinephrine is used as a
neurotransmitter by sympathetic neurons in the PNS and by some neurons in the CNS.
Other Neurotransmitters
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.
II. There are a large number of polypeptides that function as neurotransmitters, including the
endogenous opioids.
III. Nitric oxide functions as both a local tissue regulator and a neurotransmitter in the PNS and
CNS. It promotes smooth muscle relaxation and is implicated in memory.
Synaptic Integration
I. Spatial and temporal summation of EPSPs allows a sufficient depolarization to be produced 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.
II. Long-term potentiation is a process that improves synaptic transmission as a result of the use
of the synaptic pathway. This process thus may be a mechanism for learning.
(From: http://www.mhhe.com/biosci/ap/foxhumphys/student/olc/chap07summary.html)