Chapter 12 Nervous System
The nervous system coordinates and regulates the functioning of the body’s other systems.
The nervous system consists of two major systems that work together:
• Central nervous system (CNS): brain and spinal cord
• Peripheral nervous system (PNS): nerves that carry sensory messages to the CNS and motor
commands from the CNS to the muscles and glands
The nervous system contains two types of cells:
• Neurons: cells that transmit nerve impulses between parts of the nervous system • Neuroglia: support and nourish neurons, maintain homeostasis, form myelin that
surrounds neurons, and aid in signal transmission
Types of Neurons and Neuron Structure There are three classes of neurons:
• Sensory neurons: take messages to the CNS; have sensory receptors that detect
changes in the environment
• Interneurons: receive input from sensory neurons and other interneurons in the CNS
• Motor neurons: take messages away from the CNS to an effector (an organ, muscle
fibre, or gland);
o Effectors carry out responses to environmental changes
Neurons vary in appearance, but most of them have three parts:
1) Cell body: contains the nucleus and other organelles 2)Dendrites: extensions leading toward the cell body that receive signals from other neurons
and send them to the cell body 3)Axon: conducts nerve impulses away from the cell body toward other neurons or effectors Myelin Sheath
Some axons are covered by a protective myelin sheath.
• In the PNS, a myelin sheath is formed by Schwann cells, a type of neuroglia that
contains myelin in the plasma membranes Schwann cells wrap around an axon and lay down many layers of plasma membrane Each Schwann cell myelinates only part of an axon, leaving gaps called nodes of Ranvier Myelin in the PNS
• The myelin sheath plays an important role in nerve generation in the PNS
• If an axon is severed, the myelin sheath remains and serves as a passageway for new
fibre growth
Myelin in the CNS
• In the CNS, myelin is produced by oligodendrocytes, a type of neuroglia
• Nerve regeneration does not occur to any significant degree in the CNS
Nervous tissue in the CNS
The CNS is composed of two types of nervous tissue:
• Grey matter
o Contains neurons with short, nonmyelinated axons
o Found in the surface layer of the brain and the central part
of the spinal cord
• White matter
o Contains myelinated axons that run together in bundles
called tracts
o Found deep within the grey matter of the brain and
surrounds the grey matter in the spinal cord
Check Your Progress
1. Identify the three classes of neurons, and describe their relationship to each other.
2. Describe the three parts of a neuron.
3. Distinguish the cell types that form the myelin in the PNS versus the CNS.
4. Review the structure of grey matter and white matter, and describe where each is found in
the CNS and the PNS.
12.2 Transmission of nerve impulses
The nervous system uses the nerve impulse to convey information.
The nerve impulse can be studied using excised axons and a voltmeter called an oscilloscope.
Voltage: measured in millivolts (mV); a measure of the electrical potential difference between two
points
In a neuron, the two points are in the inside (axoplasm) and the outside of the axons
On a voltmeter, voltage is displayed as a trace (pattern) over time
Resting Potential
In an axon that is not conducting an impulse, the voltmeter records a potential difference across an
axon membrane equal to -70mV.
• This reading, known as the resting potential, shows that the inside of the axon is negative
compared to the outside (there is polarity across the axonal membrane)
• The resting potential is the potential difference across the membrane in a resting n The
polarity of the resting axonal membrane is due to a difference in ion distribution on each
side.
• The concentration of Na+ is greater outside the axon than inside
• The concentration of K+ is greater inside the axon than outside the neuron
•
This unequal distribution is maintained by carrier proteins called sodium-potassium pumps,
which actively transport Na+ out of the axon and K+ into the axon



The pumps are always working because the membrane is permeable to Na+ and K+
The membrane is more permeable to K+, therefore there are always more positive ions
outside the membrane than inside
Negatively charged organic ions on the inside of the axon also contribute to the polarity
across a resting axonal membrane
An action potential is a rapid change in polarity across an axonal membrane as the nerve impulse
occurs.
• An all-or-none phenomenon: if a stimulus causes the membrane to depolarize to a certain
level (threshold), an action potential occurs
• The strength of an action potential does not change, but an intense stimulus can cause an
axon to fire (start an action potential) more often
• Requires two gated channel proteins in the membrane:
One channel protein allows Na+ to pass into the axon
One channel protein allows K+ to pass out of the axon
Action potential sequence of events:
Sodium Gates Open (Depolarization)
• When an action potential begins, sodium channel gates open, and Na+ flows down its
concentration gradient into the axon
• As Na+ moves inside the axon, the membrane potential changes from -70 mV to +35 mV
This is called depolarization because the charge inside the axon changes from negative to
positive
Potassium Gates Open (Repolarization)
• The potassium channel gates open, and K+ flows down its concentration gradient out of the
axon
• As K+ flows out of the axon, the action potential becomes more negative again
(repolarization)
During this time, it briefly becomes slightly more negative that its original resting potential
(hyperpolarization)
Action potentials in nonmyelinated axons
• The action potential travels down an axon one small section at a time
• When an action potential has moved on, the previous section undergoes a refractory
period, during which the sodium gates are unable to open
• The action potential cannot move backward; it always moves down an axon
• When the refractory period is over, the sodium-potassium pump has restored the ion
distribution by pumping Na+ out of the axon and K+ into the axon
Action potentials in myelinated axons
• The gated ion channels that produce an action potential are concentrated at the nodes of
Ranvier
• Ion exchange only occurs at these nodes, therefore the action potential travels faster than in
nonmyelinated axons
• The action potential “jumps” from node to node (saltatory conduction)
TRANSMISSION ACROSS A SYNAPSE
Every axon branches into endings that have a small swelling called an axon terminal
• Each terminal lies close to the dendrite or cell body of another neuron or a muscle cell
• This region of close proximity is called a synapse or chemical synapse
Membrane of the first neuron: presynaptic membrane
Membrane of the second neuron: postsynaptic membrane
• Two neurons at a synapse do not physically touch each other; they are separated by a tiny
gap called the synaptic cleft
An action potential cannot cross a synapse.
• Communication between two neurons at a chemical synapse is carried out by
neurotransmitters (chemicals stored in the synaptic vesicles in axon terminals)
When an action potential arrives at an axon terminal:
• Gated channels for Ca2+ open, and Ca2+ enters the terminal
• Ca2+ interacts with contractile proteins, which contract and pull the synaptic vesicles to the
presynaptic membrane
• Rise in Ca2+ stimulates synaptic vesicles to merge with the presynaptic membrane, resulting
in exocytosis
•
Neurotransmitter molecules are released into the synaptic cleft and diffuse across the
synapse to the postsynaptic membrane, where they bind to specific receptors
• Depending on the neurotransmitter, the postsynaptic neuron can either be excited (causing
an action potential) or inhibited (stopping an action potential)
Synaptic Integration
A neuron may receive many excitatory and inhibitory signals since its dendrites and cell body can
have synapses with many other neurons.
• Excitatory signals: cause a depolarizing effect
• Inhibitory signals: cause a hyperpolarizing effect
•
Synaptic integration is the summing up of the excitatory and inhibitory signals in a postsynaptic
neuron.
• If the combined signals cause the membrane potential to rise above threshold, an action
potential will occur
NEUROTRANSMITTERS
Once a neurotransmitter has been released into a synaptic cleft and has initiated a response, it is
removed from the cleft.
• This prevents continuous stimulation (or inhibition) of postsynaptic membranes
In some synapses, the postsynaptic membrane contains enzymes that break down the
neurotransmitter
In other synapses, the presynaptic membrane reabsorbs the neurotransmitter for repackaging
in synaptic vesicles
Many drugs that affect the nervous system act by interfering or enhancing the action of
neurotransmitters. Drugs can:
• Enhance or block the release of neurotransmitter
• Mimic the neurotransmitter
• Block the receptor for the neurotransmitter
• Interfere with the removal of the neurotransmitter
Example:
• Sarin gas is a chemical weapon that inhibits acetylcholinesterase (AChE), an enzyme that is
responsible for the breakdown of acetylcholine (ACh)
Leads to prolonged ACh activity (convulsive spasms)
Check your Progress
1)Describe the activity of the sodium-potassium pump present in neurons.
2)Explain how the changes in Na+ and K+ ion concentrations that occur during an action potential
are associated with depolarization and repolarization.
3)Define refractory period, saltatory conduction, and synaptic integration.
12.3 Central Nervous System
The central nervous system is composed of the spinal cord and the brain.
• Brain: controls breathing, heart rate, body temperature, blood pressure, emotions,
reasoning, memory, and creativity
• Spinal cord: a means of communication between the brain and the peripheral nerves that
leave the cord
• The brain and spinal cord are wrapped in protective membranes called meninges
• The spaces between meninges are filled with cerebrospinal fluid, which cushions and
protects the CNS
• This fluid is produced and stored in the brain’s ventricles (hollow cavities) and the
spinal cord’s central canal
• If the fluid accumulates in the brain and does not properly drain, the brain can push
against the skull, causing brain damage
Structure of the Spinal Cord
• Individual vertebra protect the spinal cord
• Spinal nerves project from the cord between the vertebrae in the vertebral column
• Fluid-filled intervertebral disks cushion and separate the vertebrae
• Central canal: contains the cerebrospinal fluid
• Grey matter: centrally located, shaped like the letter H
• Contains parts of sensory neurons, motor neurons, and interneurons
Dorsal root: contains sensory fibres entering grey matter
Ventral root: contains motor fibres exiting grey matter
Spinal nerves: part of PNS
White matter: surrounds grey matter
• Contains ascending tracts taking information to the brain and descending tracts
taking information from the brain
• Tracts cross each other after entering and exiting CNS
• Left side of brain: controls right side of body
• Right side of brain: controls left side of body
Functions of the Spinal Cord
The spinal cord sends sensory information to the brain, receives motor input from the brain, and
carries out reflex actions.
• Example: Sensation
When someone touches your hand, sensory receptors generate nerve impulses that pass
through sensory fibres to the spinal cord and up ascending tracts to the brain
• Example: Voluntary movement
When we move our limbs, motor impulses in the brain pass down descending tracts to the
spinal cord and out to our muscles through motor fibres
The brain has four major parts:
• Cerebrum (two lateral ventricles)
• Diencephalon (third ventricle)
• Cerebellum (fourth ventricle)
• Brain stem (fourth ventricle)
The cerebrum is the largest part of the brain in humans
• Communicates with and coordinates activities of other parts of the brain
Structure and Function of the Cerebrum
The cerebrum has two halves (cerebral hemispheres) that communicate via the corpus callosum, a
bridge of nerve tracts.
• The cerebral cortex is a thin outer layer of grey matter that covers the cerebral
hemispheres
• Grooves called sulci divide the hemisphere into four lobes: frontal, parietal, occipital,
temporal
•
•
•
•
Frontal Lobe
• Primary motor area: involved in voluntary movement
• Premotor area: involved in organizing motor functions
• Prefrontal area: processing centre involved in reasoning and planning
• Broca’s area: involved in speech musculature (lips, tongue, larynx)
Parietal Lobe
• Primary somatosensory area: involved in somatic sensing
• Primary taste area: involved in taste
• Somatosensory association area: processes and analyzes sensory information from skin and
muscles
Temporal Lobe
• Primary auditory area: involved in hearing
• Auditory association area: associates new audio information with previous audio
information
• Wernicke’s area: helps us understand written and spoken words
Occipital Lobe
Primary visual area: involved in vision
Visual association area: associates new visual information with previous visual information
(e.g., facial recognition)
Central White Matter
• Most of the cerebrum beneath the cerebral cortex is composed of white matter
• Tracts within the cerebrum take information between different sensory, motor, and
association areas
Basal Nuclei
• Basal nuclei are masses of grey matter located deep within the white matter of the
cerebrum
• Integrate motor commands to ensure proper muscle groups are activated or inhibited
• The diencephalon is a region that encircles the third ventricle.
Structure and Function of the Diencephalon
Hypothalamus
• Integrating centre that helps maintain homeostasis
• Regulates hunger, sleep, thirst, body temperature, and water balance
• Controls the pituitary gland and serves as a link between the nervous and endocrine systems
Thalamus
• Consists of grey matter that receives all sensory input except smell
• Integrates visual, auditory, taste, and somatosensory information and sends it to the
appropriate area in the cerebrum
• Involved in higher mental functions (memory, emotions)
Pineal gland
• Secretes the hormone melatonin, which is involved in maintaining a normal sleep-wake cycle
• The cerebellum is located under the occipital lobe of the cerebrum.
• Has two portions that are primarily composed of white matter (a thin layer of grey matter
overlays the white matter)
• Involved in maintaining posture and balance
• Receives sensory input from the joints, muscles, and other sensory pathways about
the position of body parts
• Receives motor output from the cerebral cortex about where body parts should be
located
• Involved in producing smooth, coordinated voluntary movements (e.g., playing piano, hitting
a baseball)
• The brain stem contains the midbrain, the pons, and the medulla oblongata.
Midbrain
• A relay station for tracts passing between the cerebrum and spinal cord or cerebellum
• Has reflex centres for visual, auditory, and tactile responses
Pons
• Contains bundles of axons travelling between the cerebellum and the rest of the CNS
• Functions with medulla oblongata to regulate breathing rate
Check your Progress
1. Summarize the functions of the spinal cord.
2. Identify the four major parts of the brain and describe the general functions of each.
•
•
12.4 The peripheral nervous system (PNS) contains the motor and sensory pathways.
The PNS is composed of nerves and ganglia.
• Nerves: bundles of axons; the axons in nerves are called nerve fibres
• Ganglia: swellings associated with nerves that contain collections of cell bodies
Cranial nerves
• Humans have 12 pairs of cranial nerves attached to the brain
• Nerves can contain sensory input fibres, motor output fibres, or a combination of both
• Largely concerned with the head, neck, and facial regions
• Vagus nerve (X) branches to internal organs
Spinal nerves
• Humans have 31 pairs of spinal nerves (mix of motor and sensory fibres)
• Dorsal root: contains sensory fibres that conduct impulses toward the spinal cord
• Dorsal root ganglion: contains cell bodies of sensory neurons
• Ventral root: contains motor fibres that conduct impulses away from the spinal cord to
effectors
• Each spinal nerve serves a region of the body in which it is located
• The somatic system takes sensory information from external sensory receptors to the CNS
and motor commands away from the CNS to skeletal muscles
• Serves the skin, skeletal muscles, and tendons
• Some actions are due to reflex actions (automatic responses to a stimulus)
The PNS is divided into the somatic system and the autonomic system
A reflex arc is a nerve pathway that carries out a reflex.
• Reflexes are built-in circuits that allow for protection and survival
• Reflexes allow the body to react quickly to stimuli that could disrupt homeostasis
• Reflexes are present at birth and do not require conscious thought
• Example: withdrawal reflex when touching a sharp object
The autonomic system of the PNS regulates the activity of cardiac and smooth muscle, and
glands.
The sympathetic division is involved in the “fight or flight” response.
• Inhibits tears
• Dilates pupils
• Inhibits salivation
• Increases heartbeat
• Dilates airways
• Stimulates liver to release glucose
• Inhibits digestive tract and urination
• Uses the neurotransmitters epinephrine (adrenaline) and norepinephrine (NE), which act on
different cells to add to the “fight or flight” response
The parasympathetic division is involved in the “rest and digest” response.
• Stimulates tears
• Constricts pupils
• Stimulates salivation
• Decreases heartbeat and blood pressure
• Constricts airways
• Stimulates gall bladder to release bile
• Stimulates digestive tract and urination
• Uses the neurotransmitter acetylcholine (ACh), which acts on different cells to add to the
“rest and digest” response
Check Your Progress
1. Contrast cranial and spinal nerves.
2.Explain why, if you touch a hot stove, you usually withdraw your hand before feeling the pain.
3.Summarize the features of the autonomic system that are different from the somatic system.