Chapter 35 Integration and Control: Nervous Systems

I. Introduction
A. The more complex the life-style of an animal, the more elaborate are its modes of receiving, integrating,
and responding to information in the external and internal worlds.
B. These five points are basic to understanding nervous systems:
1 Reflexes are the simple, stereotyped movements that provide the basic operating machinery of
nervous systems.
2 Cephalization is the evolutionary result of the layering of more and more nervous tissue over reflex
pathways of ancient origin.
3 Brain regions allow more control over reflex actions and are the foundation for flexible, less
stereotyped responses to stimulation.
4 In existing vertebrates, the oldest parts of the brain deal with reflex coordination, other parts deal with
storage of information, and most recent layerings are the basis of memory, learning, and reasoning.
5 The coevolution of nervous, sensory, and motor systems was central to the development of more
intricate behavior.

II. Invertebrate Nervous Systems
A. Nerve Nets
1 A network of sensory cells, nerve cells, and contractile epithelial cells makes up the nerve net in
cnidarians.
2 The arrangement follows the radial symmetry of the cnidarian body.
B. Bilateral, Cephalized Nervous Systems
1 Flatworms are the simplest animals with bilateral symmetry, which is reflected in their arrangement of
muscles and nerves.
a. The nervous system includes two longitudinal nerve cords, associated ganglia, and nerves.
b. Some flatworms have a small brainlike clump of nervous tissue at the head end of the nerve cords.
2 Perhaps this arrangement evolved from the nerve net of cnidarian planula larvae.

III. Vertebrate Nervous Systems
A. How the System Developed
1 The vertebrate nervous system shows a shift from reliance on a notochord to reliance on a vertebral
column and nerve cord.
a. The single, stiffened rod called the notochord was replaced by a series of individual, movable
bones called the vertebral column.
b. The nerve cord underwent a series of expansions leading to the brain (fore-, mid-, and hind-) and
spinal cord.
2 The neural tube in vertebrate embryos undergoes expansion to form the brain and spinal cord along
with their associated nerves.
B. Functional Divisions of the System
1 The central nervous system includes the brain and spinal cord.
2 The peripheral nervous system includes all of the nerves carrying signals to and from the brain and
spinal cord.

IV. Peripheral Nervous System
A. This portion includes two sets of nerves:
1 Spinal nerves (31 pairs) connect with the spinal cord and innervate most areas of the body.
2 Cranial nerves (12 pairs) connect vital organs directly to the brain.
3 Some peripheral nerves carry only sensory information (optic nerve); others carry sensory and motor
(vagus nerve).
B. Somatic and Autonomic Subdivisions
1 The somatic system includes all nerves that relay commands to skeletal muscles (voluntary control). 2 The autonomic system sends signals to smooth muscles, cardiac muscle, and glands (involuntary
C. The Sympathetic and Parasympathetic Nerves
1 Parasympathetic nerves tend to slow down body activity when the body is not under stress.
2 Sympathetic nerves increase overall body activity during times of stress, excitement, or danger; they
also call on the hormone epinephrine to increase the “fight-flight” response.
V. Central Nervous System
A. The Spinal Cord
1 The spinal cord is a pathway for signal travel between the peripheral nervous system and the brain; it
also is the center for controlling some reflex actions.
2 The spinal cord (and also the brain) is covered with tough membranes—the meninges—and resides
within the protection of the stacked vertebrae.
3 Signals move up and down the spinal cord in nerve tracts, which are bundles of sheathed axons. a. The tracts located on the periphery of the spinal cord glisten because of their myelin sheaths and
are called white matter.
b. The central, butterfly-shaped area (in cross-section) consists of unsheathed axons, dendrites,
and cell bodies and is called gray matter.
B. Divisions of the Brain
1 The body’s master control panel, the brain, is a continuation of the anterior end of the spinal cord; it is
protected by meninges and bones.
2 Hindbrain
a. The medulla oblongata has influence over respiration, blood circulation, motor response
coordination, and sleep/wake responses.
b. The cerebellum acts as reflex center for maintaining posture and coordinating limbs.
c. The pons (“bridge”) possesses nerve tracts that pass between brain centers.
3 Midbrain
a. The midbrain originally coordinated reflex responses to visual input; the tectum still integrates
visual and auditory signals in vertebrates such as amphibians and reptiles.
b. In mammals it is now mostly a pathway switching center.
4 Forebrain
a. The forebrain has the most recent layerings of nerve tissues.
b. The large olfactory lobes dominated early vertebrate forebrains.
c. The cerebrum integrates sensory input and selected motor responses.
d. The thalamus (below cerebrum) relays and coordinates sensory signals.
e. The hypothalamus monitors internal organs and influences responses to thirst, hunger, and sex.
f. The cerebral cortex is the information coding/decoding center in mammals.
C. Cerebrospinal Fluid
1 The brain and spinal cord actually float in this fluid, which fills four cavities (ventricles) within the brain. 2 A mechanism called the blood-brain barrier controls which substances will pass to the fluid and
subsequently to the neurons.
a. Tight junctions between endothelial cells of the capillaries force materials to pass through the
cells, not around them.
b. Lipid-soluble substances, such as alcohol, nicotine, and drugs, diffuse quickly through the lipid
bilayer of the plasma membrane.

VI. The Human Brain
A. The Cerebrum
1 The human cerebrum is divided into left and right cerebral hemispheres, which communicate with
each other by means of the corpus callosum.
2 The thin surface (cerebral cortex) is gray matter, divided into lobes by folds and fissures.
3 Its white matter consists of major nerve tracts that permit communication between hemispheres and
the body.
B. Functional Regions of the Cerebral Cortex
1 Motor centers coordinate instructions for motor responses, largely thumb and tongue muscles.
2 Primary receiving centers process sensory input from eyes, ears, and skin.
3 Association centers add information from memory to help in processing the sensory input.
C. Memory
1 “Memory” is the storage and retrieval of information about previous experiences.
2 Information becomes stored in “memory traces”—chemical and structural changes in brain regions. a. Short-term memory lasts from seconds to hours and is limited to seven to eight bits of information.
b. Long-term memory is more permanent and seems to be limitless.
3 Persons suffering from retrograde amnesia lose short-term memory, but long-term memory remains intact. 4 Nerve cells are not replaced, but they may change over the course of a lifetime.
D. States of Consciousness
1 States of consciousness are governed by the central nervous system and altered by psychoactive drugs. 2 The reticular activating system (spinal cord, cerebellum, cerebrum) governs changing levels of
consciousness.
a. The flow of signals can maintain wakefulness, or
b. Other sleep centers can secrete serotonin, which causes drowsiness and sleep.
E. Emotional States
1 Our emotions are governed by the limbic system.
2 The cerebral cortex and thalamus plus the “gatekeeper” hypothalamus are able to regulate reactions
to emotional situations.