Review of Key Concepts
Overview of the Nervous System (p. 444)
- The nervous and endocrine systems are the body's two main systems of internal communication and physiological coordination. Study of the nervous system, or neuroscience, includes neurophysiology, neuroanatomy, and clinical neurology.
- The nervous system receives information from receptors, integrates information, and issues commands to effectors.
- The nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS). The PNS has sensory and motor divisions, and each of these has somatic and visceral subdivisions.
- The visceral motor division is also called the autonomic nervous system, which has sympathetic and parasympathetic divisions.
Nerve Cells (Neurons) (p. 445)
- Neurons have the properties of excitability, conductivity, and secretion.
- A neuron has a soma where its nucleus and most other organelles are located; usually multiple dendrites that receive signals and conduct them to the soma; and one axon (nerve fiber) that carries nerve signals away from the soma.
- The axon branches at the distal end into a terminal arborization, and each branch ends in a synaptic knob. The synaptic knob contains synaptic vesicles, which contain neurotransmitters.
- Neurons are described as multipolar, bipolar, or unipolar depending on the number of dendrites present, or anaxonic if they have no axon.
- Neurons move material along the axon by axonal transport, which can be fast or slow, anterograde (away from the soma) or retrograde (toward the soma).
Supportive Cells (Neuroglia) (p. 449)
1. Supportive cells called neuroglia greatly outnumber neurons. There are six kinds of neuroglia:
oligodendrocytes, astrocytes, ependymal cells, and microglia in the CNS, and Schwann cells and satellite cells in the PNS.
- Oligodendrocytes produce the myelin sheath around CNS nerve fibers.
- Astrocytes play a wide variety of protective, nutritional, homeostatic, and communicative roles for the neurons, and form scar tissue when CNS tissue is damaged.
- Ependymal cells line the inner cavities of the CNS and secrete and circulate cerebrospinal fluid.
- Microglia are macrophages that destroy microorganisms, foreign matter, and dead tissue in the CNS.
- Schwann cells cover nerve fibers in the PNS and produce myelin around many of them.
- Satellite cells surround somas of the PNS neurons and have an uncertain function.
- Myelin is a multilayered coating of oligodendrocyte or Schwann cell membrane around a nerve fiber, with periodic gaps called nodes of Ranvier between the glial cells.
- Signal transmission is relatively slow in small nerve fibers, unmyelinated fibers, and at nodes of Ranvier. It is much faster in large nerve fibers and myelinated segments (internodes) of a fiber.
- Damaged nerve fibers in the PNS can regenerate if the soma is unharmed. Repair requires a regeneration tube composed of neurilemma and endoneurium, which are present only in the PNS.
Electrophysiology of Neurons (p. 455)
- An electrical potential is a difference in electrical charge between two points. When a cell has a charge difference between the two sides of the plasma membrane, it is polarized. The charge difference is called the resting membrane potential (RMP). For a resting neuron, it is typically —70 mV (negative on the intracellular side).
- A current is a flow of charge particles— especially, in living cells, Na+ and K+. Resting cells have more K+ inside than outside the cell, and more Na+ outside than inside. A current occurs when gates in the plasma membrane open and allow these ions to diffuse across the membrane, down their concentration gradients.
- When a neuron is stimulated on the dendrites or soma, Na+ gates open and allow Na+ to enter the cell. This slightly depolarizes the membrane, creating a local potential. Short-distance diffusion of Na+ inside the cell allows local potentials to spread to nearby areas of membrane.
- Local potentials are graded, decremental, reversible, and can be excitatory or inhibitory.
- The trigger zone and unmyelinated regions of a nerve fiber have voltage-regulated Na+ and K+ gates that open in response to changes in membrane potential and allow these ions through.
- If a local potential reaches threshold, voltage-regulated gates open. The inward movement of Na+ followed by the outward movement of K+ creates a quick voltage change called an action potential. The cell depolarizes as the membrane potential becomes less negative, and repolarizes as it returns toward the RMP.
- Unlike local potentials, action potentials follow an all-or-none law and are nondecremental and irreversible. Following an action potential, a patch of cell membrane has a refractory period in which it cannot respond to another stimulus.
- One action potential triggers another in the plasma membrane just distal to it. By repetition of this process, a chain of action potentials, or nerve signal, travels the entire length of an unmyelinated axon. The refractory period of the recently active membrane prevents this signal from traveling backward toward the soma.
- In myelinated fibers, only the nodes of Ranvier have voltage-regulated
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gates. In the internodes, the signal travels rapidly by Na+ diffusing along the intracellular side of the membrane. At each node, new action potentials occur, slowing the signal somewhat, but restoring signal strength. Myelinated nerve fibers are said to show saltatory conduction because the signal seems to jump from node to node.
Synapses (p. 463)
- At the distal end of a nerve fiber is a synapse where it meets the next cell (usually another neuron or a muscle or gland cell).
- The presynaptic neuron must release chemical signals called neurotransmitters to cross the synaptic cleft and stimulate the next (postsynaptic) cell.
- Neurotransmitters include acetylcholine (ACh), monoamines such as norepinephrine (NE) and serotonin, amino acids such as glutamate and GABA, and neuropeptides such as ^-endorphin and substance P. A single neurotransmitter can affect different cells differently, because of the variety of receptors for it that various cells possess.
- Some synapses are excitatory, as when ACh triggers the opening of Na+-K+ gates and depolarizes the postsynaptic cell, or when NE triggers the synthesis of the second messenger cAMP.
- Some synapses are inhibitory, as when GABA opens a Cl" gate and the inflow of Cl" hyperpolarizes the postsynaptic cell.
- Synaptic transmission ceases when the neurotransmitter diffuses away from the synaptic cleft, is reabsorbed by the presynaptic cell, or is degraded by an enzyme in the cleft such as acetylcholinesterase (AChE).
- Hormones, neuropeptides, nitric oxide (NO), and other chemicals can act as neuromodulators, which alter synaptic function by altering neurotransmitter synthesis, release, reuptake, or breakdown.
Neural Integration (p. 468)
- Synapses slow down communication in the nervous system, but their role in neural integration (information processing and decision making) overrides this drawback.
- Neural integration is based on the relative effects of small depolarizations called excitatory postsynaptic potentials (EPSPs) and small hyperpolarizations called inhibitory postsynaptic potentials (IPSPs) in the postsynaptic membrane. EPSPs make it easier for the postsynaptic neuron to fire, and IPSPs make it harder.
- Some combinations of neurotransmitter and receptor produce EPSPs and some produce IPSPs. The postsynaptic neuron can fire only if EPSPs override IPSPs enough for the membrane voltage to reach threshold.
- One neuron receives input from thousands of others, some producing EPSPs and some producing IPSPs. Summation, the adding up of these potentials, occurs in the trigger zone. Two types of summation are temporal (based on how frequently a presynaptic neuron is stimulating the postsynaptic one) or spatial (based on how many presynaptic neurons are simultaneously stimulating the postsynaptic one).
- One presynaptic neuron can facilitate another, making it easier for the second to stimulate a postsynaptic cell, or it can produce presynaptic inhibition, making it harder for the second one to stimulate the postsynaptic cell.
- Neurons encode qualitative and quantitative information by means of neural coding. Stimulus type (qualitative information) is represented by which nerve cells are firing. Stimulus intensity (quantitative information) is represented both by which nerve cells are firing and by their firing frequency.
- The refractory period sets an upper limit on how frequently a neuron can fire.
- Neurons work in groups called neuronal pools.
- A presynaptic neuron can, by itself, cause postsynaptic neurons in its discharge zone to fire. In its facilitated zone, it can only get a postsynaptic cell to fire by collaborating with other presynaptic neurons (facilitating each other).
- Signals can travel diverging, converging, reverberating, or parallel after-discharge circuits of neurons.
- Memories are formed by neural pathways of modified synapses. The ability of synapses to change with experience is called synaptic plasticity, and changes that make synaptic transmission easier are called synaptic potentiation.
- Immediate memory may be based on reverberating circuits. Short-term memory (STM) may employ these circuits as well as synaptic facilitation, which is thought to involve an accumulation of Ca2+ in the synaptic knob.
- Long-term memory (LTM) involves the remodeling of synapses, or modification of existing synapses so that they release more neurotransmitter or have more receptors for a neurotransmitter. The two forms of LTM are declarative and procedural memory.
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