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CHAPTER 48
NERVOUS SYSTEMS
Section A: An Overview Of Nervous Systems
1. Nervous systems perform the three overlapping functions of sensory input,
integration, and motor output
2. Networks of neurons either intricate connections form nervous systems
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Chapter 48
Nervous system
Nervous systems perform the three overlapping functions of
sensory input, integration, and motor output
 A nervous system has three overlapping functions:
(1) Sensory input: conduction of signals from sensory receptors:
such as light detecting cells in the eyes to integration centers.
(2) Integration : is the process by which the signal from sensory
input is interpreted and associated with appropriate responses of the
body. Integration is carried out in the central nervous system (CNS)
which consists of the brain and the spinal cord.
(3) Motor output: is the conduction of signals from the integration
center, the CNS, to effector cells, the muscle cells or the glands.
Fig. 48.1
The signals of the nervous system are conducted by
nerves.
Nerves: ropelike bundles of extensions of neurons
tightly wrapped in connective tissue.
The nerves that communicate motor and sensory
signals between the CNS and the rest of the body are
collectively called peripheral nervous system (PNS).
Networks of neurons with intricate connections form
nervous systems
Neuron structure and synapses
The structural and functional unit of the nervous sytem is
the neuron (nerve cell)
A neuron has a:
1) Cell body: contains the nucleus and
other organelles.
2) Dendrites: short highly branched
processes that receive incoming messages
from other cells and carry this information
toward the cell body.
Fig. 48.2
3) Axon: usually much longer than dendrite.
convey outgoing messages from the neuron to other cells.
 Axon hillock: the region of the axon where it joins the cell body
Some axons are enclosed by an insulated layer called the myelin sheath
Axon endings are called synaptic terminals, that contain chemical
messengers called neurotransmitters (which conduct a signal across a
synapse).
 The site of contact between a synaptic terminal and a target cell is
called a synapse .
 A synapse: is the
junction where one
neuron (presynaptic
neuron) communicate
with another neuron
(postsynaptic neuron) in
a neural pathway or
where a neuron
communicates with a
muscle or gland cell.
Fig. 48.2
 Nerve impulses are conducted along a neuron.
Dendrite  cell body  axon hillock  axon
Fig. 48.2
Functional organization of neurons
Functionally there are three classes of neurons:
1) Sensory neurons: convey information about the external and
internal environment from sensory receptors to CNS
2) Interneurons: located
within the CNS, integrate
sensory input and motor
output
3) Motor neurons: convey
impulses from the CNS to
effector cells
A Simple Nerve Circuit – the Reflex Arc.
The simplest type of nerve circuit regulates a reflex (or
autonomic response) and is called a reflex arc.
The simplest reflex arc require only two kinds of nerve
cells:
1) Sensory neuron: conveys signals from a sensory
receptor to a motor receptor
2) Motor neuron: which sends signals to an effector cell,
a muscle or a gland cell that carries out the response.
 Human knee-jerk reflex (Figure 49.3):
Knee jerk reflex the one that makes your leg jerk forward when
the doctor hits your knee with a small hammer:
 Knee jerk reflex involves more than simple sensory/motor circuit.
 Contraction of the quadriceps (the front thigh muscle) is
accompanied by inhibition of the back thigh muscles (flexor muscles)
that flex the lower leg (pull it toward the body).
 This inhibition involves a second nerve circuit:
 The sensory neurons from the quadriceps form synapses not
only with motor neurons but also with interneurons in the spinal
cord.
 These interneurons inhibit motor neurons to the flexor muscles,
preventing them from contracting.
Fig. 48.3
 The cell body of the sensory neuron is located outside the spinal
cord in a structure called ganglion.
 A ganglion is a cluster of nerve cell bodies within the PNS.
 Cell bodies of motor neurons and interneurons are located in the
gray matter of the spinal cord.
Supporting Cells (Glia)
 These are essential for the structural integrity of the nervous system
and for the normal functioning of the neurons.
 Glia outnumber neurons by tenfold to fiftyfold.
 There are several types of glia in the brain and in the spinal cord.
(a) Astrocytes are found within the CNS.
 Provide structural and metabolic support.
Induce the formation of tight junctions between cells lining the capillaries in
the brain. This results in the formation of the blood-brain barrier, which
restricts the passage of most substances into the brain.
 Like neurons, astrocytes communicate with one another via chemical signals.
(b) Oligodendrocytes are found within the CNS.
 Form a myelin sheath by insulating axons.
(c) Schawnn cells are found within the PNS.
 Form a myelin sheath by insulating axons
Fig. 48.5
CHAPTER 48
NERVOUS SYSTEMS
Section D1: Vertebrate Nervous Systems
1. Vertebrate nervous systems have central and peripheral components
2. The divisions of the peripheral nervous system interact in maintaining
homeostasis
3. Embryonic development of the vertebrate brain reflects its evolution from three
anterior bulges of the neural tube
4. Evolutionarily older structures of the vertebrate brain regulate essential
autonomic and integrative functions
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Vertebrate Nervous Systems
Vertebrate nervous systems have central and peripheral
Figure 48.16 The nervous system of a vertebrate
components
In all vertebrates the nervous
system is divided into the
Central nervous system
(CNS) and the
Peripheral nervous system
(PNS) (Figure 48.16)
Figure 48.16 The nervous system of a vertebrate
The nervous system
Central Nervous System
(CNS)
Brain
Spinal cord
Peripheral Nervous System
(PNS)
Cranial
Nerve
12 pairs
Spinal
Nerves
31 pairs
(viscera)
(skeletal muscle)
 Central nervous system (CNS): consists of the brain and
spinal cord
The brain provides the integrative power that underlies the
complex behavior characteristic of all vertebrates.
The spinal cord runs lengthwise inside the vertebral column,
integrates simple responses (knee-jerk reflex) and conveys
information to and from the brain.
 The central canal of the spinal cord is continuous with the fluid
filled spaces, called ventricles, of the brain. These cavities are filled
with cerebrospinal fluid (CSF), which is formed in the brain by
filtration of the blood.
 It conveys nutrients, hormones and white blood cells across
the blood-brain barrier, to different parts of the brain.
 It acts as a shock absorber, cushioning the brain.
 The brain and the spinal cord are surrounded by layers of
connective tissues called meninges.
 In mammals cerebrospinal fluid circulates between the
meninges providing an addition cushion for the brain.
Axons in the CNS are located in bundles and their myelin sheaths
give them a white appearance (white matter).
White matter : is composed of bundles of myelinated axons
The white matter is distinguishable from the gray matter which
consists of dendrites, unmyelinated axons and nerve cell bodies.
Gray matter : consists of unmyelinated axons, nuclei, and
dendrites.
The divisions of the peripheral nervous system interact
in maintaining homeostasis
 Peripheral nervous system (PNS): everything outside the
CNS. It consists of paired cranial and spinal nerves and associated
ganglia (Fig. 48.17).
The cranial nerves (12 pairs) originate in the brain and
innervate organs of the head and upper body.
The spinal nerves (31 pairs) originate in the spinal and innervate
the entire body.
Most of the cranial nerves and all of the spinal nerves contain
both sensory and motor neurons; a few of the cranial nerves are
sensory only (e.g. the olfactory and optic nerves).
Fig. 48.17
The PNS can also be divided into:
1) The sensory division of the PNS is made up of the sensory or
afferent (incoming) neurons that convey information to the CNS from
sensory receptors that monitor the external and internal environment.
2) The motor division is composed of the motor or efferent (outgoing)
neurons that convey signals from CNS to effector cells.
The motor division is divided into:
a) The somatic nervous system: carries signals to skeletal muscles,
mainly in response to external stimuli. It is often considered voluntary,
(but a substantial proportion of skeletal muscle movement is
determined by reflexes).
b) The autonomic nervous system: conveys signals that regulate
the internal environment by controlling smooth and cardiac muscles
and the organs of the gastrointestinal, cardiovascular, excretory and
endocrine systems. This control is generally involuntary.
The somatic and autonomic nervous systems often cooperate in
maintaining homeostasis.
e.g. in response to a drop in temperature, the hypothalamus of the
brain signals the autonomic nervous system to constrict surface
blood vessels, which reduces heat loss; at the same time the
hypothalamus signals the somatic nervous system and causes
shivering.
The autonomic nervous system consists of two subdivisions that act
on our body organs with opposing effects (Figure 48.18):
i) sympathetic
ii) parasympathetic
When sympathetic and parasympathetic nerves innervate the same
organ, they often have antagonistic (opposite) effects.
Sympathetic
Parasympathetic
Correlates with arousal and energy
generation
A calming and a return to emphasis
on self-maintenance functions.
Increases heart beats
Decreases heart rate
The liver converts glycogen to
glucose
Decreases energy storage
Bronchi of the lungs dilate and
support increased gas exchange
Constricts bronchi in he lungs
Inhibits digestion
Stimulates digestion
Stimulates secretion of adrenaline
Stimulates secretion of acetlycholine
Fig.48.18
The nervous system
Central Nervous System
(CNS)
Brain
Spinal cord
Peripheral Nervous System
(PNS)
Cranial
Nerve
12 pairs
Spinal
Nerves
31 pairs
(viscera)
(skeletal muscle)