CHAPTER 5 SUMMARY
Evolutionary Considerations and Invertebrate Nervous Systems
•The nervous system is one of the two control systems of the body, the other being the endocrine
system. In general, the nervous system coordinates rapid responses, whereas the endocrine system
regulates activities that require duration rather than speed.
•Nervous systems have become progressively more complex during evolution, evolving from
simple reflex arcs to centralized brains with distributed, hierarchical regulation. Reflex arcs
(found in most animals) consist of sensors, neurons and effectors that quickly detect common
disturbances and activate corrective responses. Higher levels of control include anticipation
activation of reflexes and memory-enhanced responses.
•Sponges have no nerves, but some respond to stimuli using electrical signals. True nervous
systems probably first arose as nerve nets in cnidarians. Nerve nets have no central integrator and
control relatively simple behaviors.
•Simple ganglia (clusters of neurons) and nerve rings evolved for more complex behavior,
appearing first in swimming cnidaria.
•A true central nervous system or CNS first evolved with bilateral symmetry, as found in
flatworms.
•Complex nervous systems with distributed ganglia are characteristic of higher invertebrates,
such as molluscs and arthropods. True brains evolved at the anterior end of advanced animals
such as arthropods, cephalopods and vertebrates. Cephalopod brains are the most advanced
among invertebrates, having complex structures supporting complex behaviors including
learning.
•Vertebrate brain size varies up to 30-fold for a given body size. Relative brain size is
hypothesized to result from several factors, including a trade-off between expensive tissue
(brains, digestive systems).
•Some nervous systems can exhibit plasticity, changing in structure with experience.
The Vertebrate Nervous System
The nervous system consists of the central nervous system (CNS), which includes the brain and
spinal cord, and the peripheral nervous system, which includes the nerve fibers carrying
information to (afferent division) and from (efferent division) the CNS.
Three classes of neurons—afferent neurons, efferent neurons, and interneurons—compose the
excitable cells of the nervous system. Afferent neurons apprise the CNS of conditions in both the
external and internal environment. Efferent neurons carry instructions from the CNS to effector
organs, mainly muscles and glands. Interneurons are responsible for integrating afferent
information and formulating an efferent response, as well as for all higher mental functions
associated with the “mind.”
Autonomic Nervous System
The autonomic nervous system consists of two subdivisions—the sympathetic and
parasympathetic nervous systems.
An autonomic nerve pathway consists of a two-neuron chain. The preganglionic fiber originates
in the CNS and synapses with the cell body of the postganglionic fiber in a ganglion outside the
CNS. The postganglionic fiber terminates on the effector organ.
All preganglionic fibers and parasympathetic postganglionic fibers release acetylcholine.
Sympathetic postganglionic fibers release norepinephrine. The same neurotransmitter elicits
various responses from different tissues. Thus, the response depends on specialization of the
tissue cells, not on the properties of the messenger.
Tissues innervated by the autonomic nervous system possess one or more of several different
receptor types for the postganglionic chemical messengers.
A given autonomic fiber either excites or inhibits activity in the organ it innervates. Most
visceral organs are innervated by both sympathetic and parasympathetic nerve fibers, which in
general produce opposite effects in a particular organ.
Dual innervation of visceral organs by both branches of the autonomic nervous system permits
precise control over an organ’s activity. The sympathetic system dominates in emergency or
stressful situations and promotes responses that prepare the body for strenuous physical activity
(for “fight” or “flight”). The parasympathetic system dominates in quiet, relaxed situations and
promotes body maintenance activities such as digestion.
Somatic Nervous System
The somatic nervous system consists of the axons of motor neurons, which originate in the spinal
cord and terminate on skeletal muscle.
Acetylcholine, the neurotransmitter released from a motor neuron, stimulates muscle contraction.
Motor neurons are the final common pathway by which various regions of the CNS exert control
over skeletal muscle activity.
Central Nervous System: Protection and Nourishment of the Brain
Glial cells form the connective tissue within the CNS and physically and metabolically support
the neurons. The brain is provided with several protective mechanisms, which is important
because neurons cannot divide to replace damaged cells. The brain is wrapped in three layers of
protective membranes—the meninges—and is further surrounded by a hard bony covering.
Cerebrospinal fluid flows within and around the brain to cushion it against physical jarring.
Protection against chemical injury is conferred by a blood–brain barrier that limits access of
blood-borne substances to the brain.
The brain depends on a constant blood supply for delivery of O2 and glucose because it is
unable to generate sufficient ATP in the absence of either of these substances.
Cerebral Cortex
The cerebral cortex is the outer shell of gray matter that caps an underlying core of white
matter; the white matter consists of bundles of nerve fibers that interconnect various cortical
regions with other areas. The cortex itself consists primarily of neuronal cell bodies and dendrites.
Ultimate responsibility for many discrete functions is known to be localized in particular
regions of the cortex as follows: (1) the occipital lobes house the visual cortex; (2) the auditory
cortex is found in the temporal lobes; (3) the parietal lobes are responsible for reception and
perceptual processing of somatosensory input; and (4) voluntary motor movement is set into
motion by frontal lobe activity.
The association areas are areas of the cortex not specifically assigned to processing sensory
input or commanding motor output or language ability. These areas provide an integrative link
between diverse sensory information and purposeful action; they also play a key role in higher
brain functions such as memory and decision making.
Subcortical Structures and Their Relationship with the Cortex in Higher Brain Functions
The subcortical brain structures, which include the basal nuclei, thalamus, and hypothalamus,
interact extensively with the cortex in the performance of their functions.
The basal nuclei inhibit muscle tone; coordinate slow, sustained postural contractions; and
suppress useless patterns of movement.
The thalamus serves as a relay station for preliminary processing of sensory input on its way to
the cortex. It also accomplishes a crude awareness of sensation and some degree of
consciousness.
The hypothalamus regulates many homeostatic functions, in part through its extensive control
of the autonomic nervous system and endocrine system.
The limbic system, which includes portions of the hypothalamus and other forebrain structures,
is responsible for emotion as well as basic, inborn behavior patterns related to survival. It also
plays an important role in motivation and learning.
Cerebellum and Brain Stem
The cerebellum helps to maintain balance, enhances muscle tone, and helps coordinate
voluntary movement. It is especially important in smoothing out fast, phasic motor activities.
The brain stem is an important link between the spinal cord and higher brain levels. It is the
origin of the cranial nerves; contains centers that control cardiovascular, respiratory, and digestive
function; regulates postural muscle reflexes; controls the overall degree of cortical alertness; and
establishes the sleep–wake cycle.
Spinal Cord
The spinal cord has two vital functions:
1. It serves as the neuronal link between the brain and the peripheral nervous system. All
communication up and down the spinal cord is located in well-defined, independent
ascending and descending tracts in the cord’s outer white matter.
2. It is the integrating center for spinal reflexes, including some of the basic protective and
postural reflexes and those involved with the emptying of the pelvic organs. The
components of a basic reflex arc include a receptor, an afferent pathway, an integrating
center, an efferent pathway, and an effector.
•The centrally located gray matter of the spinal cord contains the interneurons interposed
between the afferent input and efferent output as well as the cell bodies of efferent neurons. The
afferent and efferent fibers, which carry signals to and from the spinal cord, respectively, are
bundled together into spinal nerves. These nerves are attached to the spinal cord in paired fashion
throughout its length. They supply specific regions of the body.
Memory and Learning
There are two types of memory: (1) a short-term memory with limited capacity and brief
retention, coded by enhancement of activity at preexisting synapses, such as increased
neurotransmitter release and increased responsiveness of the postsynaptic cell to the
neurotransmitter; and (2) a long-term memory with large storage capacity and enduring memory
traces, involving relatively permanent structural or functional changes, such as the formation of
new synapses, between existing neurons. Enhanced protein synthesis underlies these long-term
changes.