Download The Nervous System

Chapter 7: The Nervous System
Functions of the Nervous System

Sensory Input- gathering information to monitor changes occurring inside and outside the
body

Integration- to process and interpret sensory input and decide if action is needed

Motor Output- A response to integrated stimuli, the response activates muscles or glands
Organization of the Nervous System

Structural Classification- Two Subdivisions:
o Central Nervous System (CNS) - occupies the dorsal cavity, integration and
commanding centers

includes the brain and spinal cord
o Peripheral Nervous System (PNS) - contains mainly the nerves that extend from the
brain and spinal cord


includes cranial and spinal nerves
Functional Classification- concerned only with the PNS and has two subdivisions
o
Sensory or Afferent division- Nerve fibers that carry information to the central
nervous system
o
2. Motor or Efferent division- Nerve fibers that carry impulses away from the central
nervous system. The two subdivisions are the somatic and autonomic nervous
system.

Somatic nervous system = voluntary

Autonomic nervous system = involuntary. Further divided into the
sympathetic parasympathetic divisions.
 Sympathetic division- the “fight-or-flight” system. Its activity is evident
when we are frightened, in emergency, or threatening situations.
 Parasympathetic division- sometimes called the “resting and digesting”
system. It is most active when the body is at rest and not threatened.
Organization of the Nervous System
Nervous Tissue: Structure and Function

Supporting Cells
o Astrocytes- abundant star-shaped cells, brace neurons, form the boundary between
capillaries and neurons, and control the chemical environment of the brain
o Microglia- spider-like phagocytes that dispose of debris
o Ependymal cells- line cavities of brain and spinal cord, help circulate cerebrospinal fluid
o Oligodendrocytes- produce myelin sheath around nerve fibers in CNS
o Satellite cells- protect neuron cell bodies
o Schwann cells- form myelin sheath in PNS

Neurons= excitable little cells that make use of their potential!
o
Cells specialized to transmit messages
o
Major regions of neurons:

Cell body- nucleus and metabolic center

Processes- fibers that extend from the body
 dendrites- convey messages towards the cell body
 axons- conduct messages away from the cell body, axons usually end
with terminals that contain vesicles (small sacs) that contain
neurotransmitters


Synapse- the space between two neurons
Nerve Fiber Coverings:
o Schwann Cells- produce myelin sheaths- wrapped membranes that enclose the axon
o Nodes of Ranvier- gaps in myelin sheaths along the axon- allows the impulse to travel
quickly down the neuron
o Functional Classification of neurons- grouped according to the direction the impulse
is traveling relative to the CNS

Sensory or Afferent neurons- carry impulses from sensory receptors.
Examples- cutaneous nerve receptors

Motor or Efferent Neurons- carry impulses from the CNS

Interneuron (association neurons) - connect sensory and motor neurons
o Structural neuron classification:
a. Unipolar-have a short process leaving the cell body;
found in sensory neurons of PNS
b. Bipolar-one axon and one dendrite; found in sense organs (eye, ear)
c. Multipolar- many extensions from the cell body; most common type

Functional Properties of Neurons
o Irritability- neurons have the ability to respond to a stimulus
o Conductivity- the ability to transmit an impulse
o The plasma membrane at rest is polarized, this is called the Resting potential (-70 mV);
this means fewer positive ions are inside the cell (K+) than outside (Na+). As long as the
inside remains more negative than the outside the neuron remains inactive.

Action Potentials or Nerve Impulses- an electrical charge traveling down a neuron
1. A nerve impulse begins with a stimulus- usually this is a neurotransmitter released by
other neurons, pain receptors, light excites receptors in the eye, etc.
2. Once the neuron is stimulated the “sodium gates” of the neuron open and sodium ions
begin flowing across the cell membrane. This is called Depolarization: a decrease in
membrane potential (inside less negative) increases the chances of an impulse (action
potential).
3. If the action potential starts, it continues down the entire neuron.
Action potentials are an “all or nothing” response.
4. Potassium ions rush out of the neuron after sodium ions rush in, which repolarizes
the membrane (returns to resting). A neuron cannot conduct another impulse until
repolarization occurs.
5. The propagation of a nerve impulse occurs more rapidly in cells that contain a
myelin sheath. This helps the impulse jump from node to node along the length of a
neuron. This is called saltatory conduction.
Conductivity- How does the impulse travel to the next neuron?
1. Once the impulse reaches the axon endings, vesicles containing neurotransmitters fuse
with the plasma membrane, which ruptures releasing the neurotransmitter (a chemical
messenger).
2. Some neurotransmitters fit receptor sites, some return through reuptake sites and are
reused, and some are destroyed by enzymes.
3. If enough neurotransmitter is released the action potential will continue in the next neuron.
 Reflexes are a predictable, rapid, involuntary response to a stimulus. These responses do not carry
the impulse to the brain, they take a shorter path to allow for quicker response.
1. Reflex Arcs- the direct route from a sensory neuron, to interneuron, to an effector.
2. Autonomic reflexes include regulation of: smooth muscle, blood pressure, digestive
system, and glands.
3. Somatic reflexes involve the activation of skeletal muscles.
4. All reflex arcs have five key elements:
a. sensory receptor- reacts to a stimulus
b. effector organ- gland or muscle stimulated
c. afferent neurons- carry the message to the integration center
d. efferent neurons- carry the message away from the integration center
e. integration center- the connection between the afferent and efferent pathways
Developmental Aspects of the Nervous System
A. The nervous system begins to form in the first month of embryonic development.
Therefore maternal and environmental factors may impair brain development. Mothers that smoke
impair the body’s ability to carry oxygen sufficiently which increased the chances of oxygen
deprivation to the babies brain cells that are forming. Other severe congenital brain disorders include
cerebral palsy which is thought to be caused by a temporary lack of oxygen during delivery.
Furthermore, premature babies have trouble regulating their temperature because the hypothalamus
is one of the last brain areas to mature prenatally.
B. The development of motor control indicates the progressive myelination and maturation of
a child’s nervous system.

Neurons continue to die throughout life and cannot be replaced. However, most healthy
aged people maintain optimal intellectual functioning. Cardiovascular disease is the major
cause of declining mental function with age.