Chapter 11: Fundamentals of the Nervous System and Nervous Tissue

Organization of the Nervous System (pp. 388–389; Figs. 11.1–11.2)

The central nervous system consists of the brain and spinal cord, and is the integrating and command center of the nervous system (p. 388; Figs. 11.1–11.2).

The peripheral nervous system is outside the central nervous system (pp. 388–389; Fig. 11.2).

The sensory, or afferent, division of the peripheral nervous system carries impulses toward the central nervous system from sensory receptors located throughout the body.

The motor, or efferent, division of the peripheral nervous system carries impulses from the central nervous system to effector organs, which are muscles and glands.

The somatic nervous system consists of somatic nerve fibers that conduct impulses from the CNS to skeletal muscles, and allow conscious control of motor activities.

The autonomic nervous system is an involuntary system consisting of visceral motor nerve fibers that regulate the activity of smooth muscle, cardiac muscle, and glands.

Histology of Nervous Tissue (pp. 389–397; Figs. 11.3–11.5; Table 11.1)

Neuroglia, or glial cells, are closely associated with neurons, providing a protective and supportive network (pp. 389–391; Fig. 11.3).

Astrocytes are glial cells of the CNS that regulate the chemical environment around neurons and exchange between neurons and  capillaries.

• Take up K⁺ ions that lead from neurons
• Recycle NTs
• Guide neuronal development and synapse formation
• Connected to each other by gap junctions, can signal each other
• Can also have influence on neuronal processing

Microglia are glial cells of the CNS that monitor health and perform defense functions for neurons.

• Differentiate to become a type of macrophage to phagocytize debris and/or microorganisms

Ependymal cells are glial cells of the CNS that line the central cavities of the brain and spinal cord and help circulate cerebrospinal fluid.

Oligodendrocytes are glial cells of the CNS that wrap around neuron fibers, forming myelin sheaths.

Satellite cells are glial cells of the PNS whose function is largely unknown. They are found surrounding neuron cell bodies within ganglia.

Schwann cells, or neurolemmocytes, are glial cells of the PNS that surround nerve fibers, forming the myelin sheath.

Neurons are specialized cells that conduct messages in the form of electrical impulses throughout the body (pp. 391–397; Figs. 11.4–11.5; Table 11.1).

Characteristics

• Function optimally for a lifetime
• Mostly amitotic
• Exceptionally high metabolic rate requiring oxygen and glucose

Neuron cell body

• Perikaryon (around nucleus) or soma
• Major biosynthetic center has usual organelles except for centrioles
  • Nissl bodies - RER
  • Neurofibrils
  • Pigment inclusions
    • Melanin (black)
    • Iron-containing pigment (red)
    • Lipofuscin (brown) - by-product of lysosomal activity
• Nuclei - clusters of cell bodies in the CNS
• Ganglia - clusters of cell bodies in the PNS

Neuron Processes

Bundles of neuron processes are called tracts in the CNS

Bundles of neuron processes are called nerves in the PNS

Dendrites

• Short branching processes
• Have most organelles found in cell body
• Receive input
• Conduct graded impulses toward the cell body
• Dendritic spines – structures on highly branched dendrites that form synapses with other neurons in CNS

The Axon

• Axon Hillock
• Axoplasm
  • Contains the same organelles as the cell body except Nissl bodies
  • Membrane elements and proteins must be made in the cell body and transported down the axon
• Axon Collaterals
• Telodendria – end branches (may have 10,000 per axon)
• Axonal Terminals, synaptic knobs, or boutons
• Axonal Transport
  • Kinesin, ATP, cytoskeletal elements
  • Anterograde – substances move toward the axon terminal
  • Retrograde – substance move back to the perikaryon (usually organelles coming in for degradation and/or recycling)
  • Some viruses and bacterial toxins travel via the retrograde axonal transport mechanism
• Unmyelinated Fibers - part of gray matter
• Myelinated Fibers - make up the white matter
  • Myelin Sheath and Neurilemma
  • Schwann Cells wrap axons in PNS
  • Neurilemma - outer portion containing cytoplasm and nucleus of Schwann cell
  • Myelin sheath is the inner portion consisting of layers of Schwann cell plasma membraned with the cytoplasm squeezed out
  • Nodes of Ranvier - junctions between Schwann cells

 

There are three structural classes of neurons.

Multipolar neurons have three or more processes.

Bipolar neurons have a single axon and dendrite.

Unipolar neurons have a single process extending from the cell body that is associated with receptors at the distal end.

There are three functional classes of neurons.

Sensory, or afferent, neurons conduct impulses toward the CNS from receptors.

Motor, or efferent, neurons conduct impulses from the CNS to effectors.

Interneurons, or association neurons, conduct impulses between sensory and motor neurons, or in CNS integration pathways.

Neurophysiology (pp. 397–421; Figs. 11.6–11.22; Tables 11.2–11.3)

Basic Principles of Electricity (pp. 397–398)

Voltage is a measure of the amount of difference in electrical charge between two points, called the potential difference.

The flow of electrical charge from point to point is called current, and is dependent on voltage and resistance (hindrance to current flow).

Ohm's law: Current (I) =	voltage (V)
	resistance(R)

 

In the body, electrical currents are due to the movement of ions across cellular membranes.

The Role of Membrane Ion Channels (pp. 393–399; Fig. 11.6)

Passive or leakage

• Only allows specific ion
• Na⁺ attracts lots of water, makes it too big a package to pass through the K⁺ channel
• K⁺ is too large to pass through a Na⁺ channel (Na⁺ channel proteins can strip off water molecules)

Active or gated

Chemically-gated – NT binding causes channel to open

Voltage-gated - open in response to ion flow (changes in membrane polarization)

• Have activation and inactivation gates that open and close differentially
• At rest the cytoplasmic sided inactivation gate is open and the extracellular sided activation gate is closed

Mechanically gated channels open when a membrane receptor is physically deformed.

When ion channels are open, ions diffuse across the membrane, creating electrical currents.

The Resting Membrane Potential (pp. 399–400; Figs. 11.7–11.8)

The neuron cell membrane is polarized, being more negatively charged inside than outside. The degree of this difference in electrical charge is the resting membrane potential.

The resting membrane potential is generated by differences in ionic makeup of intracellular and extracellular fluids, and differential membrane permeability to solutes.

Na⁺ diffuses in faster than K⁺ diffuses out, positive charges accumulate on outside of membrane

The Na⁺/K⁺ pump actively transports 3 Na⁺ ions back out of the cell while transporting 2 K⁺ ions back into the cell, maintains the charge difference

Membrane Potentials That Act as Signals (graded potentials or action potentials) (pp. 400–409; Figs. 11.9–11.17)

Neurons use changes in membrane potential as communication signals. These can be brought on by changes in membrane permeability to any ion, or alteration of ion concentrations on the two sides of the membrane.

Changes in membrane potential relative to resting membrane potential can either be depolarizations, in which the interior of the cell becomes less negative, or hyperpolarizations, in which the interior of the cell becomes more negatively charged.

Graded potentials are short-lived, local changes in membrane potentials. They can either be depolarizations or hyperpolarizations, and are critical to the generation of action potentials.

Receptor potentials are graded potentials that occur at the receptor of sensory neurons.

Post-synaptic potentials are graded potentials generated by NT binding to a post-synaptic receptor on a dendrite, neuronal cell body, or even an axon.

Changes in membane potential produced by a depolarizing graded potential

Action potentials, or nerve impulses, occur on axons and are the principle way neurons communicate.

When a wave of depolarization (a graded local potential) reaches a voltage-regulated Na⁺ channel at sufficient strength (above threshold) the voltage-regulated Na⁺ channel opens and allows a large amount of Na⁺ to flood into the cell.

This depolarizes the cell to the point that the polarization of the plasma membrane actually reverses briefly, and the depolarization wave generated (an action potential) will flow across the membrane until it reaches the next voltage-regulated Na⁺ channel and be above threshold, thus causing it to open.

Action potentials are self-reinforcing because they always are above threshold when they reach the next voltage-regulated Na⁺ channel.

Repolarization, which restores resting membrane potential, follows depolarization along the membrane.

As voltage-regulated Na⁺ channels close voltage-regulated K⁺ channels open, allowing K⁺ to exit the cell and reestablish the resting membrane potential by creating an accumulation of positive charges outside the cell.

Action potentials are an all-or-none phenomena: they either happen completely, in the case of a threshold stimulus, or not at all, in the event of a subthreshold stimulus.

Stimulus intensity is coded in the frequency of action potentials.

This means the membrane must repolarize quickly enough to be restimulated in a reasonable amount of time.

The refractory period of an axon is related to the period of time required so that a neuron can generate another action potential.

Absolute and Relative Refractory Periods
• Absolute – from opening of voltage gated Na⁺ channels until sodium inactivation gates close
  • Cannot be stimulated at all during this time
• Relative – period of repolarization (including hyperpolarization)
  • Takes a stronger stimulus to depolarize the membrane enough to generate another action potential
  • Na⁺ channels have to reset the activation gates
  • K⁺channels have to close - they close more slowly than Na⁺ channels

Influence of Axon Diameter and the Myelin Sheath on Conduction Velocity (p. 407; Fig. 11.16)

Influence of Axon Diameter – larger diameter has lower resistance (greater cross-sectional area)

Influence of Myelin Sheath

Unmyelinated sheaths have voltage regulated channels relatively close to each other to account for ion leakage across the membrane, conduct impulses relatively slowly

Myelin insulates, the only place ion leakage occurs is at Nodes of Ranvier

This is also where voltage-gated Na⁺ channels occur, so action potentials are generated only at nodes and travel from node to node (saltatory conduction) – much faster than in unmyelinated sheaths

 

The Synapse (pp. 408–411; Figs. 11.17–11.18)

A synapse is a junction that mediates information transfer between neurons or between a neuron and an effector cell.

Neurons conducting impulses toward the synapse are presynaptic cells, and neurons carrying impulses away from the synapse are postsynaptic cells.

Electrical synapses have neurons that are electrically coupled via protein channels and allow direct exchange of ions from cell to cell.

Chemical synapses are specialized for release and reception of chemical neurotransmitters.

Neurotransmitter effects are terminated in three ways: degradation by enzymes from the postsynaptic cell or within the synaptic cleft; reuptake by astrocytes or the presynaptic cell; or diffusion away from the synapse.

Synaptic delay is related to the period of time required for release and binding of neurotransmitters.

Postsynaptic Potentials and Synaptic Integration (pp. 411–413; Figs. 11.19– 11.20; Table 11.2)

Neurotransmitters mediate graded potentials on the postsynaptic cell that may be excitatory or inhibitory.

Summation by the postsynaptic neuron is accomplished in two ways:

Temporal summation, which occurs in response to several successive releases of neurotransmitter

Spatial summation, which occurs when the post­synaptic cell is stimulated at the same time by multiple terminals.

Synaptic potentiation results when a presynaptic cell is stimulated repeatedly or continuously, resulting in an enhanced release of neurotransmitter.

Presynaptic inhibition results when another neuron inhibits the release of excitatory neurotransmitter from a presynaptic cell.

Neuromodulation occurs when a neurotransmitter acts via slow changes in target cell metabolism, or when chemicals other than neurotransmitter modify neuronal activity.

Neurotransmitters and Their Receptors (pp. 413–421; Figs. 11.21–11.22; Table 11.3)

Neurotransmitters are one of the ways neurons communicate, and they have several chemical classes.

Functional classifications of neurotransmitters consider whether the effects are excitatory or inhibitory, and whether the effects are direct or indirect.

There are two main types of neurotransmitter receptors: channel-linked receptors mediate direct transmitter action and result in brief, localized changes; and G protein-linked receptors mediate indirect transmitter action resulting in slow, persistent, and often diffuse changes.

 

Neurotransmitter Receptor Mechanism

Basic Concepts of Neural Integration (pp. 421–423; Figs. 11.23–11.24)

Organization of Neurons: Neuronal Pools (p. 421; Fig. 11.23)

Neuronal pools are functional groups of neurons that integrate incoming information from receptors or other neuronal pools and relay the information to other areas.

Types of Circuits (pp. 421–422; Fig. 11.24)

Diverging, or amplifying, circuits are common in sensory and motor pathways. They are characterized by an incoming fiber that triggers responses in ever-increasing numbers of fibers along the circuit.

Converging circuits are common in sensory and motor pathways. They are characterized by reception of input from many sources, and a funneling to a given circuit, resulting in strong stimulation or inhibition.

Reverberating, or oscillating, circuits are characterized by feedback by axon collaterals to previous points in the pathway, resulting in ongoing stimulation of the pathway.

Parallel after-discharge circuits may be involved in complex activities, and are characterized by stimulation of several neurons arranged in parallel arrays by the stimulating neuron.

Patterns of Neural Processing (pp. 422–423; Fig. 11.25)

Serial processing is exemplified by spinal reflexes, and involves sequential stimulation of the neurons in a circuit.

Parallel processing results in inputs stimulating many pathways simultaneously, and is vital to higher level mental functioning.

Developmental Aspects of Neurons (pp. 423–426)

The nervous system originates from a dorsal neural tube and neural crest, which begin as a layer of neuroepithelial cells that ultimately become the CNS.

Differentiation of neuroepithelial cells occurs largely in the second month of development.

Growth of an axon toward its target appears to be guided by older “pathfinding” neurons and glial cells, nerve growth factor and cholesterol from astrocytes, and tropic chemicals from target cells.

The growth cone is a growing tip of an axon. It takes up chemicals from the environment that are used by the cell to evaluate the pathway taken for further growth and synapse formation.

Unsuccessful synapse formation results in cell death, and a certain amount of apoptosis occurs before the final population of neurons is complete.