Chapter 10: Nervous System I
I. General Functions
of the Nervous System
nervous system is composed predominately of nervous tissue but also includes some blood vessels and connective tissue.
cell types of nervous tissue are neurons and neuroglial cells.
are specialized to react to physical and chemical changes in their surroundings.
are small cellular processes that receive input.
are long cellular processes that carry information away from neurons.
impulses are bioelectric signals produced by neurons.
of axons are called nerves.
spaces between neurons are called synapses.
are biological messengers produced by neurons.
central nervous system contains the brain and spinal cord.
peripheral nervous system contains cranial and spinal nerves.
general functions of the nervous system are sensory, integrative, and motor.
M. Sensory receptors
are located at the ends of peripheral neurons and provide the sensory function of the nervous system.
convert their information into nerve impulses, which are then transmitted over peripheral nerves to the central nervous system.
the central nervous system, the signals are integrated.
integration, decisions are made and acted upon by means of motor functions.
motor functions of the nervous system use neurons to carry impulses from the central nervous system to effectors.
of effectors are muscles and glands.
two divisions of the motor division are somatic and autonomic.
nervous system is involved in conscious activities.
autonomic nervous system is involved in unconscious activities.
W. The nervous system can detect changes in the body, make decisions, and stimulate muscles
or glands to respond.
three parts all neurons have are cell body, axon, and dendrites.
neurons cell body contains granular cytoplasm, mitochondria, lysosomes, a Golgi apparatus, and many microtubules. It also
contains a large nucleus, chromatophilic substance, and cytoplasmic inclusions.
are fine threads that extend into axons.
substance is membranous sacs that contain rough endoplasmic reticulum.
neurons generally do not divide but neural stem cells do.
are usually highly branched to provide receptive surfaces to which processes from other neurons communicate.
spines are tiny, thornlike spines on the surface of dendrites.
neuron may have many dendrites but will have only one axon.
axonal hillock is the initial portion of an axon closest to the cell body.
axon is specialized to carry nerve impulses away from the cell body.
cytoplasm of an axon includes mitochondria, microtubules, and neruofibrils.
are branches of axons.
synaptic knob is a specialized ending of an axon.
synaptic cleft is the space between a synaptic knob and the receptive surface of another cell.
transport is the process an axon uses to convey biochemicals that are produced in the neuron cell body.
cells produce myelin.
is a lipid-rich substance.
myelin sheath is a coating produced by Schwann cells that is wrapped around an axon.
neurilemma is a portion of a Schwann cell outside of the myelin sheath.
node of Ranvier is a narrow gap between myelin sheaths.
axons have myelin sheaths.
axons have no myelin sheaths.
matter is composed of myelinated axons.
matter is composed of unmyelinated axons, dendrites, and cell bodies of neurons.
of Neurons and Neuroglia
1. The three major
classifications of neurons based on structural differences are bipolar, multipolar, and unipolar.
neurons have two processes; one process is a dendrite and the other an axon.
neurons are found within the eyes, ears, and nose.
neurons have one process which functions as an axon.
peripheral process of a unipolar neuron is associated with dendrites near a peripheral body part and the central process enters
the brain or spinal cord.
neurons are located in ganglia.
neurons have multiple dendrites and one axon.
neurons are located in the brain and spinal cord.
three classes of neurons based on functional differences are sensory, motor, and interneurons.
neurons carry impulses from peripheral body parts to the brain and spinal cord.
neurons have specialized receptors ends at the tips of their dendrites.
sensory neurons are unipolar but some are bipolar.
are located in the brain and spinal cord.
are multipolar and form links between other neurons.
neurons carry nerve impulses from the brain and spinal cord to effectors.
specialized groups of motor neurons are accelerator neurons and inhibitory neurons.
neurons that control skeletal muscle are under conscious control.
neurons that control glands, smooth muscle and cardiac muscle are under involuntary control.
of Neuroglial Cells
1. In the
embryo, neuroglial cells guide neurons to their positions and may stimulate them to grow.
cells also produce growth factors that nourish neurons.
cells are the neuroglia of peripheral nervous system.
four neuroglial cells of the central nervous system are astrocytes, oligodendrocytes, microglial cells, and ependymal cells.
are star shaped and are commonly found between neurons and blood vessels.
provide support and hold structures together.
aid metabolism of glucose.
respond to injury of brain tissue and form a special type of scar tissue.
play a role in the blood-brain barrier, which restricts movement of substances between the blood and CNS.
occur in rows along myelinated axons and form myelin in the brain and spinal cord.
Schwann cells, oligodendrocytes do not form neurilemmal sheaths.
function to support neurons, and phagocytize bacterial cells and cellular debris.
form the inner lining of the central canal of the spinal cord and ventricles of the brain.
junctions join ependymal cells together.
cells form a porous layer through which substances diffuse freely between the interstitial fluid of the brain tissues and
the fluid within the ventricles.
16. Covering the choroids plexus, ependymal cells also regulate the composition of the
of Nerve Axons
to a neuron cell body usually kills the neuron but damaged peripheral axons usually regenerate.
2. If a
peripheral axon is separated from its cell body, the distal portion of the axon deteriorate but the proximal end of the axon
develops sprouts shortly after injury.
of a regenerating axon is slow but eventually the new axon may reestablish the former connection.
within the central nervous system do not regenerate because there is no tube of sheath cells to guide it.
III. Cell Membrane Potential
means electrically charged.
a cell membrane is polarized, the inside is negatively charged with respect to the outside.
polarization of a cell membrane is due to an unequal distribution of positive and negative ions on either side of the membrane.
1. Potassium ions
are the major intracellular positive ion and sodium ions are the major extracellular cation.
distribution of potassium and sodium is largely created by the sodium-potassium pump.
passage of potassium and sodium ions through the cell membrane depends on the presence of channels.
1. A resting
nerve cell is one that is not being stimulated to send a nerve impulse.
2. At rest,
a cell membrane gets a slight surplus of positive charges outside, and inside reflects a slight negative surplus of impermeable
negatively charged ions because the cell membrane is more permeable to potassium ions than sodium ions. Also the cell may contain anions and proteins that are negatively charged that cannot diffuse out
of the cell.
cell uses ATP to actively transport sodium and potassium ions in opposite directions.
are the electrical differences between two points
5. A volt
is called a potential difference because it represents stored electrical energy that can be used to do work
membrane potential is the potential difference across the cell membrane and is measured in millivolts.
potential is the membrane potential of a resting neuron and has a value of 70 millivolts.
negative sign of a resting membrane potential is relative to the inside of the cell and is due to the excess negative charges
on the inside of the cell membrane.
are described as excitable because they can respond to the changes in their surroundings.
on neurons usually affect the membrane potential in the region of the membranes exposed to the stimulus.
stimulus affects the membrane potential of a neuron by opening a gated ion channel.
4. A membrane
is hyperpolarized if the membrane potential becomes more negative than the resting potential.
5. A membrane is depolarized if the membrane becomes less negative that the resting potential.
6. Local potential changes are graded meaning that the amount of change in the membrane
potential is directly proportional to the intensity of the stimulation.
7. A threshold potential is sufficient depolarization that triggers an action potential.
8. Summation is an additive phenomenon in which two or more depolarizing stimuli are
9. At threshold, an axon potential is produced in an axon.
trigger zone of an axon is the initial portion of an axon.
trigger zone contains a great number of voltage-gated sodium channels.
3. At the
resting membrane potential, sodium channels are closed but when threshold is reached, sodium channels open.
4. As sodium
ions rush into the cell, the membrane potential changes and temporarily becomes positive on the inside.
sodium channels close and potassium channels open, potassium diffuses out across the membrane and the inside of the membrane
becomes negatively charged again.
means the membrane polar again or returned to its original resting state.
are capable of action potentials but the cell body and dendrites are not.
8. A nerve
impulse is the propagation of action potentials along an axon.
1. A nerve
impulse is an all-or-nothing response, meaning if a neuron responds at all to a nerve impulse, it responds completely.
2. A greater
intensity of stimulation on the neuron produces more impulses per second but not a stronger impulse.
refractory period is the period in which a threshold stimulus will not trigger another impulse on an axon.
2. An absolute
refractory period is the period when an axons membrane cannot be stimulated and is the first part of the refractory period.
3. A relative
refractory period is the period in which a stronger stimulus can trigger an impulse.
refractory period limits how many action potentials may be generated in a neuron in a given amount of time.
H. Impulse Conduction
serves as an insulator.
conduction is the type of nerve impulse conduction that occurs only at nodes.
axons exhibit salutatory conduction.
axons send nerve impulses faster than unmyelinated axons.
diameter of an axon also affects the speed of a nerve impulse.
IV. The Synapse
are the places where impulses are passed from one neuron to another.
2. A presynaptic
neuron is the neuron that brings the impulse to the synapse.
3. A postsynaptic
neuron is the neuron that is stimulated by the presynaptic neuron.
transmission is the process by which the impulse in the presynaptic neuron signals the postsynaptic neuron.
1. A nerve
impulse travels along an axon to the axon terminals.
synaptic knobs of axons contain sacs called synaptic vesicles.
vesicles contain neurotransmitters.
a nerve impulse reaches a synaptic knob, calcium diffuses inward from the extracellular fluid.
calcium inside the synaptic knob initiates a series of events that causes the synaptic vesicles to fuse with the cell membrane,
releasing the neurotransmitter by exocytosis.
neurotransmitters diffuse across the synaptic cleft and react with specific receptors on the postsynaptic neuron.
neurotransmitters cause ion channels to open, some cause ion channels to close.
potentials are local potentials created by changes in chemically gated ion channels.
potentials are graded and can depolarize or hyperpolarize the receiving cell membrane.
2. An excitatory
postsynaptic potential is a type of membrane change in which the receiving cell membrane is depolarized.
3. An inhibitory postsynaptic potential is a type of membrane change in which the receiving
cell membrane is hyperpolarized.
the brain and spinal cord, each neuron may receive the synaptic knobs of a thousand or more axons on its cell body and dendrites.
integrated sum of EPSPs and IPSPs determines whether an action potential results.
1. The nervous system
produces at least thirty different kinds of neurotransmitters.
stimulates skeletal muscle contractions.
of monoamines are epinephrine, norepinephrine, dopamine, and serotonin.
of unmodified amino acids that act as neurotransmitters are glycine, glutamic acid, aspartic acid, and GABA.
5. Examples of peptides are enkephalins and substance P.
neurotransmitters are synthesized in the cytoplasm of synaptic knobs and stored in synaptic vesicles.
more calcium that enters the synaptic knob, the more neurotransmitters that are released.
a vesicle releases its neurotransmitter, it becomes part of the cell membrane.
enzyme acetlycholinesterase functions to break down acetylcholine.
process of reuptake is when neurotransmitters are transported back into the synaptic knobs of the presynaptic neurons.
oxidase functions to inactivate epinephrine and norepinephrine after reuptake.
1. Neuropeptides are
substances that alter a neurons response to a neurotransmitter or block the release of a neurotransmitter.
examples of neuropeptides are enkephalins, beta endorphin, and substance P.
function to relieve pain sensations.
function to relieve pain.
function of substance P is to transmit pain impulses into the spinal cord and on to the brain.
V. Impulse Processing
pools are groups of neurons that make synaptic connections with each other and work together to perform a common function.
pools may have excitatory or inhibitory effects on other pools or on peripheral effectors.
is a condition created in which a neuron is brought closer to threshold.
originating from different parts of the nervous system leading to the same neuron exhibit convergence.
allows the nervous system to collect, process, and respond to information.
may branch many times.
leaving a neuron of a neuronal pool may exhibit divergence by reaching several other neurons.
axons can amplify and impulse.