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The Functions of the Nervous System Chapter 3
The Central Nervous System
· The nervous system has
two major divisions, the central nervous system (CNS) and the peripheral nervous system (PNS)
§ In the CNS (brain and spinal cord):
§ A bundle of axons traveling together is called a tract.
§ A group of cell bodies is called a nucleus.
§ In the PNS:
§ A bundle of axons travelling together is called a nerve.
(A neuron is not a nerve.)
§ A group of cell bodies is called a ganglion.
◊
The Central Nervous System
· During development,
three major parts of the brain are formed: the forebrain, midbrain, and hindbrain.
· The forebrain is the
largest part of the brain.
§ It is made up of two cerebral hemispheres separated by the longitudinal
fissure.
§ The cortex covers the cerebral hemispheres and is wrinkled or
convoluted, increasing the amount of cortex.
· A ridge is called a gyrus.
· A groove is called a sulcus or, if large,
a fissure.
§ The cortex is composed of layers and columns.
§ The forebrain also includes the thalamus and hypothalamus.
The Central Nervous System
·
Do
intelligent people have bigger brains?
§ Brain size is mostly related to body size,
because larger bodies require larger brains.
§ Examples: Elephants and sperm whales have brains that are 5-6
times larger than humans.
·
Among
humans, there is a correlation between brain size and intelligence.
However, the relationship is small and highly variable.
·
Two
key features characterize brains of more “intelligent” species.
·
The
cortex has more convolutions (cortalization).
§ The cerebral hemispheres are larger in proportion
to the lower areas of the brain.
§ This illustrates a hierarchy of increasing
complexity from spinal cord to hindbrain to midbrain and finally to the forebrain.
The Central Nervous System
The four lobes of the brain:
§ Frontal lobe
§ In front of (anterior to) the central
sulcus
§ Above (superior to) the lateral fissure
§ Parietal lobe
§ Behind (posterior to) the
central sulcus
§ Occipital lobe
§ At the back (posterior) of the brain
§ Temporal lobe
Located on the sides (laterally) of the brain
These are somewhat arbitrary divisions but useful for locating structures and functions.
Lobes and Functional Areas/The Central Nervous System
· The frontal lobes are
important for movement and complex human capabilities.
§ The primary motor cortex
§ is found on the precentral gyrus;
§ is a “map” of the human body, or homunculus, with
larger areas devoted to parts of the body that make precise movements;
§ works with secondary motor cortex and with subcortical structures,
for example, the basal ganglia.
§ Broca’s area is important for speech
production.
§ Prefrontal cortex:
§ is the largest part of the human brain;
§ plays a role in organizing and planning;
§ is involved in some types of decision making;
§ is important for impulse control;
§ adjusts behavior in response to rewards and punishments.
§ Prefrontal lobe dysfunction:
§ impairs the ability to learn from consequences;
§ decreases the ability to control impulses;
§ is often found in
depression and schizophrenia.
· Frontal lobotomy
§ is a surgical procedure that disconnects
the prefrontal area from the rest of the brain;
§ was performed on 40,000 patients in the
U.S. during the 1940s and 1950s, mostly to calm agitated patients;
§ provided little benefit at high cost to
the patient;
§ has largely been replaced by drug treatment,
along with other forms of psychosurgery;
§ occurred as the result of an accident in
the famous case of Phineas Gage.
Lobotomy Procedure
The Central Nervous System
· The parietal lobes are
important for body sensations and spatial localization.
§ The primary somatosensory cortex
§ is located on the postcentral gyrus;
§ receives information about the skin senses, body position, and
movement;
◊
·
The
parietal lobes also maps these functions as a sensory homunculus (with size
corresponding
to sensitivity in that part of the body).
§ Parietal association areas
§ combine information from body senses and vision;
§ identify objects by touch, determine the location of the limbs,
and locate objects in space.
Damage to the posterior parietal cortex
causes neglect of objects, people, and activity on the
Opposite side.
Usually this is caused by damage on the right side. The patient may deny there is anything
wrong, even when a limb is paralyzed.
◊
The temporal lobes are separated from the frontal and parietal lobes by the
lateral fissure; they:
·
contain
the auditory cortex, which receives information from the ears;
·
include
language and auditory and visual association areas.
The Central Nervous System
§ Wernicke’s area is involved in language comprehension and
production.
Damage results in meaningless speech and poor comprehension of written and spoken communication called
Werenicke’s Aphasia.
§ The inferotemporal cortex is concerned with visual identification.
Damage causes difficulty in recognizing objects and familiar faces – Facial Agnosia
The Central Nervous System
· The occipital lobes
§ are located in the posterior part of the
brain;
§ are the location of the primary visual cortex;
§ contain a map of visual space because adjacent
receptors in the eye send information to adjacent points in the visual cortex;
§ have secondary visual areas that process
individual components of a scene, including
§ color,
§ movement,
§ and form.
The Central Nervous System
· Another structure in
the forebrain is the thalamus, located below the lateral ventricles.
§ It receives information from all the senses
except olfaction.
§ It relays this sensory information to the
cortex.
§ Some portions of the thalamus project more
diffusely and play a role in arousal.
· The hypothalamus is
located beneath the thalamus.
§ It controls emotions and motivated behaviors
such as eating, drinking, and sexual activity.
§ It exerts major control over the autonomic
nervous system and the endocrine system (by way of the pituitary).
The Central Nervous System
· Just posterior to the thalamus is the pineal gland.
·
The pineal gland secretes melatonin, a hormone that induces sleep.
·
It controls seasonal cycles in nonhuman animals .
·
It participates with other structures in controlling daily rhythms in humans.
The Central Nervous System
· The corpus callosum, a dense band of fibers at the bottom of the longitudinal fissure, shares information between
the hemispheres.
·
Epilepsy patients whose corpus callosum has been severed have been helpful in studying the
specializations of the two hemispheres.
The Central Nervous System
· During development, the hollow interior of the nervous system becomes the ventricles of the brain and the central
canal in the spinal cord.
§ The ventricles
are filled with cerebrospinal fluid that
§ carries material from the blood
vessels to the central nervous system
§ transports waste materials in
the other direction.
§ The two
lateral ventricles and the third ventricle are found in the forebrain.
The Central Nervous System
· The midbrain contains structures that have secondary roles in vision, audition and movement.
§ The superior
colliculi help guide eye movements and fixation of gaze.
§ The inferior
colliculi help locate the direction of sounds.
§ The substantia
nigra projects to the basal ganglia to integrate movements.
§ The ventral
tegmental area plays a role in the rewarding effects of food, sex, drugs and so on.
§ The midbrain
is located at the top of the brain stem.
The Central Nervous System
·
The hindbrain is composed of the medulla, the pons, and the cerebellum.
§ The medulla
is involved in the control of essential life processes such as cardiovascular activity and respiration (breathing).
§ The pons
contains centers related to sleep and arousal, which are part of the reticular formation.
§ The reticular
formation contributes to attention, reflexes, and muscle tone.
◊
The Central Nervous System
· The cerebellum, is the most distinctive structure in the brain stem.
§ It is located
on the back of the brain stem.
§ It is wrinkled and divided down the middle like the cerebral hemispheres, and is often referred to as the
“little brain.”
§ The cerebellum
refines movements initiated by the motor cortex by controlling their speed, intensity and direction.
§ It also
plays a role in motor learning, as well as in other cognitive processes and in emotion.
The Central Nervous System
·
The spinal cord is a finger-sized cable of neurons that carries commands from the brain to
the muscles and organs, and sensory information into the brain.
§ Sensory
neurons enter the spinal cord through the dorsal root of each spinal nerve.
§ The axons
of the motor neurons pass out through the ventral root.
§ In some
cases sensory neurons connect, directly or through an interneuron, with motor neurons; this pathway produces a simple, automatic
movement in response to a sensory stimulus, called a reflex.
The Central Nervous System
·
Protecting the Central Nervous System
§ The space
between the meninges and the CNS is filled with cerebrospinal fluid, which cushions the neural tissue from blows and sudden
movement.
§ The blood-brain
barrier limits passage into the brain of toxic substances and neurotransmitters circulating in the blood.
§ Some places
are not protected by this barrier, such as the area postrema, which produces vomiting if toxins are ingested.
The Peripheral Nervous System
·
The peripheral nervous system (PNS)
·
is made up of:
§ the cranial nerves that enter
and leave the underside of the brain:
§ and the spinal nerves that connect to the
sides of the spinal cord at each vertebra;
·
and can be divided into :
§ the somatic nervous system, composed
of the motor and sensory neurons that allow us to sense and react to the environment;
§ the autonomic nervous system
(ANS), which regulates general activity levels in the body and controls smooth muscle, the glands, the heart, and other organs.
The Peripheral Nervous System
·
The autonomic nervous system is composed of two branches.
§ The sympathetic
nervous system
§ activates the body in ways that
help it cope with demands, such as emotional stress and physical emergencies;
§ has most of its ganglia in the
sympathetic ganglion chain.
§ The parasympathetic
nervous system
§ slows the activity of most organs
to conserve energy;
§ activates digestion to renew
energy;
§ has its ganglia near the muscles
and glands they control.
◊
Development and Change in the Nervous System
·
The nervous system begins as a hollow tube that later becomes the brain and spinal cord.
§ The nervous
system begins development when the surface of the embryo forms a groove.
§ The edges
of this groove curl upward until they meet, turning the groove into a tube.
·
Further development occurs in four stages:
§ cell proliferation
§ migration
§ circuit
formation
§ circuit
pruning
Development and Change in the Nervous System
· During proliferation the cells that will become neurons divide and multiply.
§ Occurs
in the ventricular zone, the area surrounding the hollow tube.
§ 250,000
new cells are “born” every minute.
· These newly formed neurons then migrate, moving from the ventricular zone outward to their final location.
§ They do
so with the aid of specialized radial glial cells.
§ Each neuron’s
function is determined by the time and location that it develops.
Development and Change in the Nervous System
· During circuit formation, the axons of developing neurons grow toward their target cells and form functional connections.
§ Growth
cones at the axon’s tip guide the migrating axons.
§ They detect
chemical and molecular signposts that attract or repel the advancing axon.
§ Using these
signposts, the axon is able to navigate to intermediate stations and past inappropriate targets until they reach their final
destinations.
Development and Change in the Nervous System
§ The route
to the destination is not always direct.
§ Changing
genetic controls allow the axon to make course changes along the way; for example:
Development and Change in the Nervous System
During circuit pruning excess neurons and synapses are
eliminated.
§ Neurons
that are unsuccessful in finding a place on a target cell or that arrive late die.
§ Next, the
nervous system eliminates excessive synapses.
§ Synapses
are strengthened or weakened depending on whether the presynaptic neuron and the postsynaptic neuron fire together.
§ Postsynaptic
neurons apparently release neurotrophins that enhance development in the presynaptic neuron.
§ Later the
plasticity (ability to be modified) of these synapses decreases.
Development and Change in the Nervous System
·
Fetal alcohol syndrome (FAS), which often produces mental retardation, is caused by the mother’s
use of alcohol during a critical period of brain development.
§ FAS brains
are often small and malformed, and neurons are dislocated.
§ During
migration many cortical neurons fail to line up in columns as they normally would, because the radial glial cells revert to
their more typical glial form prematurely.
§ Other neurons
continue migrating beyond the usual boundary of the cortex.
·
Exposure to ionizing radiation affects both proliferation and migration.
Development and Change in the Nervous System
· Stimulation continues to shape synaptic construction and reconstruction throughout the individual’s life.
· Much of the change resulting from experience in the mature brain involves reorganization.
§ Reorganization
is a shift in connections that changes the function of an area of the brain.
§ Reorganization
may provide compensation for losses, for example, in syndactyly and blindness.
§ However,
reorganization is not always beneficial.
§ Example: kittens reared in abnormal
visual environments
§ Example: phantom pain
Development and Change in the Nervous System
· Two major sources of
brain injury are:
§ Stroke
§ Stroke is caused by artery blockage (ischemic) or rupture (hemorrhagic).
§ Damage is due to oxygen and glucose deprivation, excitotosis,
and edema (swelling).
§ Stroke is a leading cause of death and disability in the U.S.
§ Traumatic Brain Injury
§ Traumatic brain injury is caused by a blow to the head, penetration,
or sudden acceleration or deceleration.
§ Even trauma that does not produce concussion can result in brain
changes typically seen in Alzheimer’s patients.
Development and Change in the Nervous System
·
Limitations
of Self Repair
§ Regeneration
is the regrowth of severed axons.
§ Myelin provides a guide tube
for the neuron to grow through, and the axon is guided to its destination much as in development.
§ Occurs in the amphibian brain
and in the mammalian PNS.
§ In the mammalian CNS, glia produce
scar tissue and growth inhibitors, and immune cells may also interfere.
◊Neurogenesis is the birth of new neurons.
§ It appears to support learning
(in the hippocampus) and odor discrimination (in the olfactory bulbs).
§ There is no evidence neurogenesis
contributes to self repair.
§ However neurogenesis does increase
in damaged brains, and there is some hope this could be enhanced as a means of recovery.
Development and Change in the Nervous System
·
Compensation
§ Presynaptic
neurons sprout more terminals to form additional synapses.
§ Postsynaptic
neurons add more receptors.
§ Silent
side branches from adjacent neurons become active within minutes of injury.
·
Reorganization
§ Functions
are taken over by other areas.
§ Typically,
compensation is by an adjacent area, but it can involve the other hemisphere.
§ Reorganization
is more likely if the damage occurs early in life.
Development and Change in the Nervous System
·
Possibilities for CNS Repair
§ Neuron
growth enhancers
§ Growth enhancer in brain-damaged
rats induced axons to grow from the undamaged side into the spinal cord pathway on the other side; the rats recovered most
of their mobility.
§ Providing
guide tubes or scaffolding
§ Rats regained use of their legs
when glial cells were provided as scaffolding between the cut ends of the spinal cord.
§ Almost no improvement occurred
in trials with humans.
§ Counteracting
regrowth inhibitors
§ Monkeys recovered 80% of the
use of their paralyzed hands after the effects of Nogo-A (which inhibits axon growth) were blocked.
· Stem cells seem an ideal means of neural repair.
§ Stem cells
are undifferentiated cells that can develop into specialized cells such as neurons, muscle, or blood.
§ Embryonic
stem cells placed in an adult nervous system differentiate into neurons appropriate to that area.
§ Later in
life, stem cells lose most of their flexibility, but those found in the olfactory mucosa show promise for repairing neural
damage.
· Other efforts at CNS
repair utilize computer chips.
§ One sends signals from the brain directly
to a paralyzed limb, bypassing the damaged area in the spinal cord.
§ Monkeys with a blocked nerve learned to flex, extend, and rotate
the paralyzed wrist.
§ Another device simulates a central pattern
generator in the spinal cord.
§ With just 10 simulated neurons and 190 synapses, it allowed a
cat with paralyzed hind legs to walk.
§ A chip implanted in a paraplegic’s
motor cortex enabled him to operate a robotic arm and to access email on a computer.
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