Vander's Human Physiology Chapter 6

central nervous system (CNS)

the brain and spinal cord

peripheral nervous system (PNS)

the nerves that connect the brain and spinal cord with the body's muscles, glands, sense organs, and other tissues

neuron

The functional unit of the nervous system is the individual cell. They generate electrical signals that move from one part of the cell to another part of the same cell or to neighboring cells

neurotransmitters

chemical messengers that are triggered by the electrical signals that neurons send out. They communicate with other cells.

cell body (soma)

contains the nucleus and ribosomes and thus has the genetic information and machinery necessary for protein synthesis

dendrites

a series of highly branched outgrowths of the cell that receive incoming information from other neurons. These increase the cell's surface area

dendritic spines

knoblike outgrowths that increase the surface area of dendrites still further and receive signals from axons at the synapse

the structure of dendrites in the CNS

increases a cell's capacity to receive signals from many other neurons

axon

a long process that extends from the cell body and carries outgoing signals to its target cells. They range in length from a few microns to over a meter

axon hillock (initial segment)

The region of the axon that arises from the cell body and where propagated electrical signals are generated

collaterals

branches of the axon

The greater the degree of branching of the axon and axon collaterals

the greater the cell's sphere of influence

axon terminal

endings of collaterals that are responsible for releasing neurotransmitters from the axon

varicosities

A series of bulging areas along the axon where some neurons release their chemical messengers.

myelin sheath

covers around the axon that consist of 20 to 200 layers of highly modified plasma membrane wrapped around the axon by a nearby supporting cell. They speed up conduction of the electrical signals along the axon and conserve energy

oligodendrocyte

a type of glial cell in the CNS that wraps axons in a myelin sheath. Each may branch to form myelin on as many as 40 axons

Schwann cells

glial cells in the PNS that form individual myelin sheaths surrounding the axon

nodes of Ranvier

the spaces between adjacent sections of myelin where the axon's plasma membrane is exposed to extracellular fluid

axonal transport

the movement of various organelles and other materials between the cell body and the axon terminals to maintain the structure and function of the axon

axonal transport depends on

a scaffolding of microtubule "rails" running the length of the axon, kinesins, and dyneins

kinesins and dyneins

specialized types of motor proteins

Kinesin transport

moves away from the cell body (anterograde) and is important in moving nutrient molecules, enzymes, mitochondria, neurotransmitter-filled vesicles, and other organelles

Dynein transport

moves toward the cell body (retrograde) and carries recycled membrane vesicles, growth factors, and other chemical signals that can affect the neuron's morphology, biochemistry, and connectivity.

anterograde

movement from the cell body toward the axon terminals

retrograde

movement toward the cell body and is the route by which some harmful agents invade the CNS, including tetanus toxin and the herpes simplex, rabies, and polio viruses

three functional classes of neurons

afferent, efferent, and interneurons

Characteristics of Three Classes of Neurons (table 6.1)

Afferent neurons

Transmit information into the CNS from receptors at their peripheral endings

Single process from the cell body splits into a long peripheral process (axon) that is in the PNS and a short central process (axon) that enters the CNS

Efferent neurons

Transmit information out of the CNS to effector cells, particularly muscles, glands, neurons, and other cells

Cell body with multiple dendrites and a small segment of the axon are in the CNS; most of the axon is in the PNS

Interneurons

Function as integrators and signal changers

Integrate groups of afferent and efferent neurons into reflex circuits

Lie entirely within the CNS

Account for > 99% of all neurons

afferent neurons

convey information from the tissues and organs of the body toward the CNS. They propagate electrical signals from their receptors into the brain or spinal cord.

Both the cell body and the long axon are outside the CNS and only a part of the central process enters the brain or spinal cord

efferent neurons

convey information from the tissues and organs of the body away the CNS.

Their cell bodies and dendrites are within the CNS, and the axons extend out to the periphery.

interneurons

convey information within the CNS. They account for over 99% of all neurons and have a wide range of physiological properties, shapes, and functions

for each afferent neuron entering the CNS

there are 10 efferent neurons and 200,000 interneurons. Thus, the great majority of neurons are interneurons.

sensory receptors

receptors at afferent neurons' peripheral ends (the ends farthest from the CNS) that respond to various physical or chemical changes in their environment by generating electrical signals in the neuron

receptor region on afferent neurons

a specialized portion of the plasma membrane or a separate cell closely associated with the neuron ending.

nerves

groups of afferent and efferent neuron axons, together with myelin, connective tissue, and blood vessels in the PNS

synapse

the anatomically specialized junction between two neurons where one neuron alters the electrical and chemical activity of another. This is where the signal is transmitted from one neuron to another by neurotransmitters

presynaptic neuron

a neuron that conducts a signal toward a synapse

postsynaptic neuron

a neuron conducting signals away from a synapse. These may have thousands of synaptic junctions on the surface of its dendrites and cell body, so that signals from many presynaptic neurons can affect it

neurons in the nervous system exemplify the general principle of physiology that

information flow between cells, tissues, and organs is an essential feature of homeostasis and allows for complex integration of physiological processes.

glial cells

non-neuronal cells in the CNS that surround the axon and dendrites of neurons, and provide them with physical and metabolic support. They retain the capacity to divide throughout life.

Do not participate directly in electrical communication from cell to cell as do neurons.

oligodendrocyte, astrocyte, microglia, and ependymal

types of CNS glial cells

astrocyte

a second type of CNS glial cell that helps regulate the composition of the extracellular fluid in the CNS by removing potassium ions and neurotransmitters around synapses AND stimulates the formation of tight junctions between the cells that make up the blood-brain barrier

blood-brain barrier

the walls of capillaries found in the CNS which is a much more selective filter for exchanged substances than is present between the blood and most other tissues

microglia

specialized, macrophage-like glial cells that perform immune functions in the CNS, and may also contribute to synapse remodeling and plasticity

ependymal cells

line the fluid-filled cavities within the brain and spinal cord and regulate the production and flow of cerebrospinal fluid

The elaborate networks of neuronal processes that characterize the nervous system depend upon

the outgrowth of specific axons to specific targets.

growth cone

forms the tip of each extending axon and is involved in finding the correct route and final target for the process.

Zika virus

a virus that, when women are infected during pregnancy, causes babies to be born with severely underdeveloped brains (microcephaly)

plasticity

the ability to modify its structure and function in response to stimulation or injury

After degeneration of a severed axon

damaged peripheral neurons may regrow the axon to their target organ. Functional regeneration of severed CNS axons does not usually occur.

charges of the same type repel each other

positive charge repels positive charge, and negative charge repels negative charge

electrical potential

separated electrical charges of opposite sign have the potential to do work if they are allowed to come together

potential difference

the difference in the amount of charge between two points

volts

The units of electrical potential

current

The movement of electrical charge

The amount of charge that moves or the magnitude of the current

depends on the potential difference between the charges and on the nature of the material or structure through which they are moving

resistance

The hindrance to electrical charge movement

Ohm's law

expresses the effect of voltage V and resistance R on current I

I = V/R

Membrane potential

The voltage difference between the inside and outside of a cell

resting membrane potential

neurons have a potential difference across their plasma membranes, with the inside of the cell negatively charged with respect to the outside; the steady potential of an unstimulated cell

The magnitude of the resting membrane potential depends mainly on two factors

(1) differences in specific ion concentrations in the intracellular and extracellular fluids

(2) differences in membrane permeabilities to the different ions, which reflect the number of open channels for the different ions in the plasma membrane

equilibrium potential

The membrane potential at which these two fluxes become equal in magnitude but opposite in direction. The voltage difference across a membrane that produces a flux of a given ion species that is equal but opposite to the flux due to the concentration gradient of that same ion

Nernst equation

describes the equilibrium potential for any ion—that is, the electrical potential necessary to balance a given ionic concentration gradient across a membrane so that the net flux of the ion is zero

Eion = (61/Z)log(Cout/Cin)

Eion

equilibrium potential for a particular ion, in mV

Cin

intracellular concentration of the ion

Cout

extracellular concentration of the ion

Z

the valence of the ion

61

a constant value that takes into account the universal gas constant, the temperature (37°C in all our examples), and the Faraday electrical constant

Goldman-Hodgkin-Katz (GHK) equation

Vm=61 logPK [Kout] + PNa [Naout] + PCl [Clin]

an expanded version of the Nernst equation that takes into account individual ion permeabilities

leak (ungated) channels

constitutively open K+ channels

electrogenic pump

a pump moves net charge across the membrane and contributes directly to the membrane potential

excitability

the ability to produce electrical signals that can transmit information between different regions of the membrane

excitable membranes

the membranes with excitability

depolarized

its potential becomes less negative (closer to zero) than the resting level

overshoot

a reversal of the membrane potential polarity; when the inside of a cell becomes positive relative to the outside

repolarized

When a membrane potential that has been depolarized returns to the resting value

hyperpolarized

when the potential is more negative than the resting level

Graded potentials

changes in membrane potential that are confined to a relatively small region of the plasma membrane. They are usually produced when some specific change in the cell's environment acts on a specialized region of the membrane.

Action potential

A brief all-or-none depolarization of the membrane, which reverses polarity in neurons; has a threshold and refractory period and is conducted without decrement

Synaptic potential

A graded potential change produced in the postsynaptic neuron in response to the release of a neurotransmitter by a presynaptic terminal; may be depolarizing (an excitatory postsynaptic potential or EPSP) or hyperpolarizing (an inhibitory postsynaptic potential or IPSP)

Receptor potential

A graded potential produced at the peripheral endings of afferent neurons (or in separate receptor cells) in response to a stimulus

Pacemaker potential

A spontaneously occurring graded potential change that occurs in certain specialized cells

Threshold potential

The membrane potential at which an action potential is initiated

threshold stimuli

Stimuli that are just strong enough to depolarize the membrane to this level

Depending upon the initiating event, graded potentials can occur in either a depolarizing or a hyperpolarizing direction.

their magnitude is related to the magnitude of the initiating event

charge is lost across the membrane because the membrane is permeable to ions through open leak channels

The result is that the change in membrane potential decreases as the distance increases from the initial site of the potential change

Plasma membranes are so leaky to ions that these currents die out almost completely within a few millimeters of their point of origin

this causes local current to be decremental

decremental

the flow of charge decreases as the distance from the site of origin of the graded potential increases

summation

if additional stimuli occur before the graded potential has died away, these can add to the graded potential from the first stimulus

propagation of action potentials down the axon

the mechanism the nervous system uses to communicate from cell to cell over long distances

Ligand-gated ion channels

open in response to the binding of signaling molecules

mechanically gated ion channels

open in response to physical deformation (stretching) of the plasma membranes.

voltage-gated ion channels

give a membrane the ability to undergo action potentials

inactivation gate

sometimes visualized as a "ball and chain," limits the flux of Na+Na+ by blocking the channel shortly after depolarization opens it

When the membrane repolarizes, the channel closes, forcing the inactivation gate back out of the pore and allowing the channel to return to the closed state

Integrating these channel properties with the basic principles governing membrane potentials, we can now explain how action potentials occur.

threshold potential

depolarization becomes a positive feedback loop

afterhyperpolarization

This portion of the action potential

At depolarizations less than threshold

the positive feedback cycle cannot get started

the membrane will return to its resting level as soon as the stimulus is removed and no action potential will be generated. These weak depolarizations are called subthreshold potentials, and the stimuli that cause them are subthreshold stimuli

all-or-none

Action potentials either occur maximally or they do not occur at all

ocal anesthetics such as procaine (Novocaine) and lidocaine (Xylocaine)

these drugs block voltage-gated Na+Na+ channels, preventing them from opening in response to depolarization

absolute refractory period

This occurs during the period when the voltage-gated Na+ channels are either already open or have proceeded to the inactivated state during the first action potential. The inactivation gate that has blocked these channels must be removed by repolarizing the membrane and closing the pore before the channels can reopen to a second stimulus