the functions of the nervous system chapter 3 t he central nervous system the peripheral nervous...

THE FUNCTIONS OF THE NERVOUS SYSTEM CHAPTER 3

The Central nervous system

The peripheral nervous system

Development and change in the nervous system

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.

CNS Development

Figure 3.3

Layers and Columns of the CortexFigure 3.6

The layered structure of the cortex. Columnar arrangement.


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. 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 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 AreasFigure 3.8


• 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.

◊

The Central Nervous System– 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 ProcedureFigure 3.11


• 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;• 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 Central Nervous System• 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.

• Wernicke’s area is involved in language comprehension and production.

– Damage results in meaningless speech and poor comprehension of written and spoken communication.

• The inferotemporal cortex is concerned with visual identification. Damage causes difficulty in recognizing objects and familiar faces.


• 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).

Interior Features of the BrainFigure 3.13

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 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.


• 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.

◊

Ventricles of the BrainFigure 3.15


• 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 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 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 Brain StemFigure 3.16

The Central Nervous SystemThe 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.

◊

Spinal Cord ReflexFigure 3.17


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.

Divisions of the Nervous SystemFigure 3.19

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


• 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.


• 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.

◊

Neuron with Growth ConeFigure 3.24


– The route to the destination is not always direct. – Changing genetic controls allow the axon to make course

changes along the way; for example:• While the gene Robo1 is active, a neuron is repelled

from the brain’s midline by a chemical located there.• If the axon is to cross the midline, Robo3 becomes

active at the appropriate place, and the axon is attracted to the midline.

• After crossing, control reverts to Robo1 and the midline is avoided.

◊


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.


• 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.


• 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


• 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.


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.

◊


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.


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.

◊

Embryonic Stem CellsFigure 3.28


• 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.

the functions of the nervous system chapter 3 t he central nervous system the peripheral nervous...

Documents

central nervous system

nervous system chapter

cns brain

brain size

central sulcusoccipital

larger brains

larger bodies

larger areas