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Chapter 17: Nervous System
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Nervous Tissue
The nervous system is divided into a central nervous system (CNS), consisting of the brain and spinal cord, and a peripheral nervous system (PNS), consisting of nerves carrying sensory and motor information between the CNS and muscles and glands.
Both systems have two types of cells: neurons that transmit impulses and neuroglial cells that service neurons.
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Organization of the nervous system
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Neuron StructureNeurons are composed of dendrites that
receive signals, a cell body with a nucleus, and an axon that conducts a nerve impulse away.
Sensory neurons take information from sensory receptors to the CNS.
Interneurons occur within the CNS and integrate input.
Motor neurons take information from the CNS to muscles or glands.
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Types of neurons
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Myelin SheathLong axons are covered by a protective
myelin sheath formed by neuroglial cells called Schwann cells.
The sheath contains lipid myelin which gives nerve fibers their white, glistening appearance.
The sheath is interrupted by gaps called nodes of Ranvier.
Multiple sclerosis is a disease of the myelin sheath.
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Myelin sheath
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The Nerve Impulse
The nervous system uses the nerve impulse to convey information.
The nature of a nerve impulse has been studied by using excised axons and a voltmeter called an oscilloscope.
Voltage (in millivolts, mV) measures the electrical potential difference between the inside and outside of the axon.
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Resting PotentialWhen an axon is not conducting a nerve
impulse, the inside of an axon is negative (-65mV) compared to the outside; this is the resting potential.
A sodium-potassium pump in the membrane actively transports Na+ out of the axon and K+ into the axon to establish resting potential.
The membrane is more permeable to K+ and much of the resting potential is due to the excess of K+ outside of the neuron.
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Resting potential
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Action PotentialAn action potential is a rapid change in
polarity as the nerve impulse occurs. The action potential occurs if a stimulus
causes the membrane to depolarize past threshold.
An intense stimulus causes many firings (reaching action potential) in an axon; a weak stimulus may cause only a few.
The action potential requires two types of gated channel proteins: one each for Na+ and K+.
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Sodium Gates OpenThe gates of sodium channels open first
and Na+ flows into the axon. The membrane potential depolarizes to
+40 MV.
Potassium Gates OpenThe gates of potassium channels open
next and K+ flows to the outside of the axon.
The membrane potential repolarizes to –65 MV.
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Action potential
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Propagation of an Action Potential
The action potential travels the length of an axon, with each portion of the axon undergoing depolarization then repolarization.
A refractory period ensures that the action potential will not move backwards.
In myelinated fibers, the action potential only occurs at the nodes of Ranvier.
This “jumping” from node-to-node is called saltatory conduction.
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Transmission Across a SynapseThe tip of an axon forms an axon bulb that
is close to a dendrite or cell body of another neuron; this region of close proximity is called the synapse.
Transmission of a nerve impulse takes place when a neurotransmitter molecule stored in synaptic vesicles in the axon bulb is released into a synaptic cleft between the axon and the receiving neuron.
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When a nerve impulse reaches an axon bulb, gated channels for calcium open and Ca2+ flow into the bulb.
This sudden rise in Ca2+ causes synaptic vesicles to move and merge with the presynaptic membrane, releasing their neurotransmitter molecules into the cleft.
The binding of the neurotransmitter to receptors in the postsynaptic membrane causes either excitation or inhibition.
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Synapse structure and function
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Synaptic Integration
Many synapses per single neuron is not uncommon.
Excitatory signals have a depolarizing effect, and inhibitory signals have a hyperpolarizing effect on the post- synaptic membrane.
Integration is the summing up of these excitatory and inhibitory signals.
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Integration
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Neurotransmitter MoleculesOut of 25, two well-known
neurotransmitters are acetylcholine (ACh) and norepinephrine (NE).
Neurotranmitters that have done their job are removed from the cleft; the enzyme acetylcholinesterase (AChE) breaks down acetylcholine.
Neurotransmitter molecules are removed from the cleft by enzymatic breakdown or by reabsorption, thus preventing continuous stimulation or inhibition.
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The Central Nervous System
The central nervous system (CNS) consists of the spinal cord and brain.
Both are protected by bone, wrapped in protective membranes called meninges, and surrounded and cushioned with cerebrospinal fluid that is produced in the ventricles of the brain.
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The ventricles are interconnecting cavities that produce and serve as a reservoir for cerebrospinal fluid.
The CNS receives and integrates sensory input and formulates motor output.
Gray matter contains cell bodies and short, nonmyelinated fibers; white matter contains myelinated axons that run in tracts.
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Organization of the nervous system
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The Spinal CordThe spinal cord extends from the base of
the brain through the vertebral canal.
Structure of the Spinal CordA central canal holds cerebrospinal fluid. Gray matter of the spinal cord forms an
“H” and contains interneurons and portions of sensory and motor neurons.
White matter consists of ascending tracts taking sensory information to the brain and descending tracts carrying motor information from the brain.
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Spinal cord
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Functions of the Spinal CordThe spinal cord is the center for many
reflex arcs. It also sends sensory information to the
brain and receives motor output from the brain, extending communication from the brain to the peripheral nerves for both control of voluntary skeletal muscles and involuntary internal organs.
Severing the spinal cord produces paralysis.
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The Brain
The brain has four cavities called ventricles.
The cerebrum has two lateral ventricles, the diencephalon has the third ventricle, and the brain stem and cerebellum have the fourth ventricle.
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The human brain
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The CerebrumThe cerebrum or telencephalon has two
cerebral hemispheres connected by the corpus callosum.
Learning, memory, language and speech take place in the cerebrum.
Sulci divide each hemisphere into lobes including the frontal, parietal, occipital, and temporal lobes.
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Cerebral hemispheres
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The Cerebral CortexThe cerebral cortex is a thin, highly
convoluted outer layer of gray matter covering both hemispheres.
The primary motor area is in the frontal lobe; this commands skeletal muscle.
The primary somatosensory area is dorsal to the central sulcus or groove.
The primary visual area is at the back occipital lobe.
The temporal lobe has the primary auditory area.
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The parietal lobe provides taste sensation. All have adjacent association areas that
integrate signals; the prefrontal area is an important association area for appropriate behavior.
White matter consists mostly of long myelinated axons forming tracts; these cross over so the left side of the brain handles right side information.
Basal nuclei are masses of gray matter deep within the white matter integrate motor commands.
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The lobes of a cerebral hemisphere
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The DiencephalonThe hypothalamus and thalamus are in the
diencephalon that encircles the third ventricle.
The hypothalamus controls homeostasis and the pituitary gland, and the thalamus receives all sensory input except smell and integrates it and sends it to the cerebrum.
The pineal gland is also located here and secretes melatonin that may regulate our daily rhythms.
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The CerebellumThe cerebellum receives sensory input
from eyes, ears, joints and muscles and receives motor input from the cerebral cortex.
It integrates this information to maintain posture and balance.
The cerebellum is involved in learning of new motor skills, such as playing the piano.
A thin layer of gray matter covers the white matter.
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The Brain StemThe brain stem contains the medulla
oblongata, pons, and midbrain. The medulla oblongata and pons have
centers for vital functions such as breathing, heartbeat, and vasoconstriction.
The medulla also coordinates swallowing and some other automatic reactions.
The midbrain acts as a relay station between the cerebrum and spinal cord or cerebellum.
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The Reticular Formation
The reticular formation is a complex network of nuclei and fibers that extend the length of the brain stem.
One portion of the reticular formation, called the reticular activating system, arouses the cerebrum via the thalamus causing alertness.
An inactive reticular activating system results in sleep.
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The reticular activating system
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The Limbic System and Higher Mental Functions
Limbic System The limbic system is involved in our
emotions and higher mental functions.The limbic system is a complex network
of tracts and nuclei involving cerebral lobes, basal nuclei and the diencephalon.
Two structures, the hippocampus and amygdala are essential for learning and memory.
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The limbic system
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Higher Mental Functions
Animal research, MRI, and PET scans allow researchers to study the functioning of the brain.
Memory and Learning
Memory is the ability to hold a thought in mind or recall events from the past.
Learning takes place when we retain and utilize past memories.
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Short-term memory involves activity in the prefrontal area.
Long-term memory includes semantic memory (numbers, words, etc.) and episodic memory (persons, events, etc.).
Skill memory involves ability to ride a bike, for example, and involves all motor areas of the cerebrum below the level of consciousness.
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Long-term Memory Storage and Retrieval
Our long-term memories are stored in bits and pieces throughout the sensory association areas of the cerebral cortex.
The hippocampus is a bridge between sensory association areas and the prefrontal area where memories are utilized.
The amygdala associates danger with sensory stimuli.
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Long-term memory circuits
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Long-Term PotentiationLong-term potentiation is increased
response at synapses within the hippocampus and is essential to long-term memory.
However, a postsynaptic neuron in the hippocampus can become too excited and then die.
Excitotoxicity, a form of cell death, is due to the neurotransmitter glutamate rushing in too quickly.
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Language and Speech
Language and speech are dependent upon Broca’s area (a motor speech area) and Wernicke’s area (a sensory speech area) that are involved in communication.
These two areas are located only in the left hemisphere; the left hemisphere functions in language in general and not just in speech.
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Language and speech
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The Peripheral Nervous System
The peripheral nervous system (PNS) contains nerves (bundles of axons) and ganglia (cell bodies).
Sensory nerves carry information to the CNS, motor nerves carry information away, and mixed nerves have both types of fibers.
Humans have 12 pairs of cranial nerves and 31 pairs of spinal nerves.
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Nerve structure
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Cranial nerves
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The dorsal root of a spinal nerve contains sensory fibers that conduct sensory impulses from sensory receptors toward the spinal cord.
Dorsal root ganglia near the spinal cord contain the cell bodies of sensory neurons.
The ventral root of a spinal nerve contains motor fibers that conduct impulses away from the spinal cord to effectors.
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Spinal nerves
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Somatic System
The somatic system serves the skin, skeletal muscles, and tendons.
The brain is always involved in voluntary muscle actions but somatic system reflexes are automatic and may not require involvement of the brain.
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The Reflex ArcInvoluntary reflexes allow us to respond
rapidly to external stimuli.In reflexes, sensory receptors generate
nerve impulses carried to interneurons in the spinal cord.
Next, interneurons signal motor neurons which conduct nerve impulses to a skeletal muscle that contracts, giving the response to the stimulus.
Pain is not felt until the brain receives nerve impulses.
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A spinal nerve reflex arc
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Autonomic SystemThe autonomic system of the PNS
regulates the activity of cardiac and smooth muscle and glands.
The system is divided into sympathetic and parasympathetic divisions that:
1) Function automatically and involuntarily;
2) Innervate all internal organs; and3) Use two neurons and one ganglion.
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Sympathetic Division
The sympathetic division is associated with responses that occur during times of stress, including “fight or flight” reactions.
The postganglionic axon releases mainly norepinephrine which acts similar to adrenaline, the hormone from the adrenal medulla.
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Parasympathetic Division
The parasympathetic system is associated with responses that occur during times of relaxation and promotes “housekeeper” activities.
The postganglionic neurotransmitter used by the parasympathetic division is acetylcholine.
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Autonomic nervous system
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Drug AbuseStimulants increase excitation, and
depressants decrease excitation; either can lead to physical dependence.
Each type of drug has been found to either promote or prevent the action of a particular neurotransmitter.
Medications that counter drug effects work by affecting the release, reception, or breakdown of dopamine, a neurotransmitter responsible for mood.
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Drug actions at a synapse
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Drug use
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Alcohol Alcohol may affect the inhibiting
transmitter GABA or glutamate, an excitatory neurotransmitter.
Alcohol is primarily metabolized in liver and heavy doses can cause liver scar tissue and cirrhosis.
Alcohol is an energy source but it lacks nutrients needed for health.
Cirrhosis of the liver and fetal alcohol syndrome are serious conditions associated with alcohol intake.
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Nicotine
Nicotine is an alkaloid derived from tobacco.
In the CNS, nicotine causes neurons to release dopamine; in the PNS, nicotine mimics the activity of acetylcholine and increases heart rate, blood pressure, and digestive tract mobility.
Nicotine induces both physiological and psychological dependence.
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CocaineCocaine is an alkaloid derived from the
shrub Erythroxylum cocoa, often sold as potent extract termed “crack.”
Cocaine prevents uptake of dopamine by the presynaptic membrane, is highly likely to cause physical dependence, and requires higher doses to overcome tolerance.
This makes overdosing is a real possibility; overdosing can cause seizures and cardiac arrest.
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HeroinDerived from morphine, heroin is an
alkaloid of opium.
Use of heroin causes euphoria.
Heroin alleviates pain by binding to receptors meant for the body’s own pain killers which are the endorphins.
Tolerance rapidly develops and withdrawal symptoms are severe.
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Marijuana
Marijuana is obtained from the plant Cannabis sativa that contains a resin rich in THC (tetrahydrocannabinol).
Effects include psychosis and delirium and regular use can lead to dependence.
Long-term marijuana use may lead to brain impairment, and a fetal cannabis syndrome has been reported.
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Chapter SummaryThe nervous system consists of two
types of cells: neurons and mesoglia.Neurons are specialized to carry nerve
impulses.A nerve impulse is an electrochemical
change that travels along the length of a neuron fiber.
Transmission of signals between neurons is dependent on neurotransmitter molecules.
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The central nervous system is made up of the spinal cord and the brain.
The parts of the brain are specialized for particular functions.
The cerebral cortex contains motor areas, sensory areas, and association areas that are in communication with each other.
The cerebellum is responsible for maintaining posture; the brainstem houses reflexes for homeostasis.
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The reticular formation contains fibers that arouse the brain when active and account for sleep when they are inactive.
The limbic system contains specialized areas that are involved in higher mental functions and emotional responses.
Long-term memory depends upon association areas that are in contact with the limbic system.
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There are particular areas in the left hemisphere that are involved in language and speech.
The peripheral nervous system contains nerves that conduct nerve impulses toward and away from the central nervous system.
The autonomic nervous system has sympathetic and parasympathetic divisions with counteracting activities.
Use of psychoactive drugs such as alcohol, nicotine, marijuana, cocaine, and heroin is detrimental to the body.
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