Types of Neurons
Interneurons (Association Neurons)
aka afferent neurons. Conduct impulses toward the CNS. Sensory receptors respond to various stimuli.
aka association neurons. Entirely within the CNS. Conduct impulses from sensory neurons to motor neurons. Interneurons have an integrative function.
aka efferent neurons. Conduct impulses away from the CNS and to the effectors (muscles or glands).
An example of the arrangement of sensory neurons, interneurons, and motor neurons.
Functions of Neurons
Secretion of Neurotransmitters
Structure of Neurons
Cell body or Perikaryon
Conducts nerve impulses away from the soma; there is never more than on axon per neuron. (Some have none)
Receive impulses from other neurons
Insulating layer (mostly lipid) around nerve fibers, formed by neuroglia cells. Not all nerve fibers are myelinated. Those that are myelinated comprise the white matter, unmyelinated fibers comprise the gray matter.
Classes of Neurons
One axon, multiple dendrites; most common type.
One axon, one dendrite
Single axon branches a short distance from the soma, on branch (peripheral fiber) carries impulse from source of sensation, the other (central fiber) carries impulse into the spinal cord.
The have no axon, multiple dendrites. Examples in retina and brain.
Types of Neuroglia
These cells are not neurons, but perform various supportive roles in the nervous system.
Form myelin sheath in CNS
Cover brain surfaces
Formation of blood-brain barrier
Remove neurotransmitters and potassium ions from intercellular fluid
Regulate composition of cerebrospinal fluid
Form supportive network in CNS
Replace damaged nerve tissue (scar tissue)
Line cavities of brain and spinal cord
Produce and circulate CSF
Form myelin sheath in peripheral nervous sytem
Surround somas of neurons in ganglia
Initiation and Conduction of Nerve Impulses
Resting membrane potential --> Action potential
Resting Membrane Potential
Interior of nerve cell is negatively charged, relative to extracellular fluid. Cell is said to be polarized. Maintence of this state requires expenditure of energy (ATP).
1) Na+ gate opens, allowing influx of sodium ions. K+ gate begins to open. Depolarization begins.
2) Na+ gate closes. K+ gate opens fully. K+ ions leave cell, bringing about beginning of repolarization.
3) Both Na+ gate and K+ gate closed, repolarization complete.
A nerve impulse travels along a nerve impulse as a wave of depolarization.
For a short time after action potential, it is impossible to stimulate that region of a neuron to fire again.
Nodes of Ranvier
In myelinated nerve fibers, ions can be exchanged with the extracellular fluid only at the Nodes of Ranvier. Impulse appears to jump (saltate) from node to node.
All or Nothing Law
Action Potential follows an All or Nothing Law, are nondecremental (do not lose strength over a distance), and are irreversible.
Although nerve impulses are conducted electrically along a nerve fiber, a 20-40 nm gap (synaptic cleft) exists between neurons. This necessitates (in most cases) chemical communication between neurons.
Synaptic Knob - pre synaptic neruon
Synaptic Vesicles - (filled with neurotransmitters) in the pre-synaptic neuron
Neurotransmitter Receptors - in the post-synaptic membrane.
Types of Chemical Synapses
Nerve Impulse reaches synaptic knob and causes Ca2+ gates to open. This triggers exocytosis of synaptic vesicles which allows diffusion of neurotransmitter into the synaptic cleft. These bind receptors in the post-synaptic membrane which open and allow passage of Na+ and K+ ions, producing post-synaptic potential.
Binding of neurotrasmitter by receptors in post-synaptic cell activates cyclic AMP production. This can turn certain metabolic pathways on or off, activate genetic transcription, and open ion gates in membrane.
Classes of Neurotransmitters
Acetylcholine acts at the neuromuscular junction
Cessation of Signal @ the Synapse
The "turning off" of th esignal at the syapse is just as important as the stimulation of the post-synaptic cell. Accomplished by:
1) Reuptake of the neurotransmitter by the pre-synaptic neuron.
2) Diffusion of the neurotransmitter into the extracellular fluid, where astrocytes absorb it.
3) Degradation of the neurotransmitter in the synaptic cleft (the enzyme acetylcholinesterase breaks acetylcholine down into acetate and choline, which have no effect on the post-synaptic cell. Certain pesticides and nerve gases are acetylcholinesterase inhibitors).