Chapter 7: The Nervous System

I. Functions of the Nervous System A. Sensory Input- gathering

information to monitor changes occurring inside and outside the body

B. Integration- to process and interpret sensory input and decide if action is needed

C. Motor Output- A response to integrated stimuli, the response activates muscles or glands

II. Organization of the Nervous System A. Structural Classification- Two

Subdivisions:1. Central Nervous System (CNS) - occupies the dorsal cavity,

integration and commanding centers

a. includes the brain and

spinal cord2. Peripheral Nervous System

(PNS) - contains mainly the nerves that extend from the brain and spinal cord

a. includes cranial and spinal

nerves

B. Functional Classification- concerned only with the PNS

and has two subdivisions 1. Sensory or Afferent division- Nerve fibers that carry information to the central nervous system

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.

a. Somatic nervous system = voluntary b. Autonomic nervous system =

involuntary further divided into the sympathetic and parasympathetic divisions.

i. Sympathetic division- the “fight- or-flight” system. Its activity is

evident when we are frightened, in emergency, or threatening situations.

ii. 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

Motor neurons

Sensory neurons

A. Supporting Cells1. Astrocytes- abundant star-shaped cells, brace neurons, form the boundary between capillaries and neurons, and control the chemical environment of

the brain2. Microglia- spider-like phagocytes that dispose of debris

-mutant-type SOD1 genes activate microglial cells; in turn, the activated microglial cells release proteins that produce nitric oxide or others (cytokines) that cause inflammation, thus causing damage to motor neurons and accelerating the progress of ALS.

III. Nervous Tissue: Structure and Function

A. Supporting Cells

3. Ependymal cells- line cavities of brain and spinal cord, help circulate cerebrospinal fluid

4.Oligodendrocytes- produce myelin sheath around nerve fibers in CNS

5. Satellite cells- protect neuron cell bodies

6. Schwann cells- form myelin sheath in PNS

B. Neurons- excitable little cells that make

use of their potential! 1. Cells specialized to transmit messages 2. Major regions of neurons:

a. Cell body- nucleus and metabolic centerb. Processes- fibers that extend from the body

1) dendrites- convey messages towards the cell body

2) axons- conduct messages away from the cell body, axons usually

end with terminals that contain vesicles (small sacs) that contain neurotransmitters

c. Synapse- the space between two neurons

3. Nerve Fiber Coverings:a. Schwann Cells- produce myelin sheaths- wrapped membranes that enclose the axonb. Nodes of Ranvier- gaps in myelin sheaths along the axon- allows the impulse to travel quickly down the neuron

B. Neurons- excitable little cells that make use of their potential!4. Functional Classification of

neurons- grouped according to the direction the impulse is traveling relative to the CNSa. Sensory or Afferent neurons- carry impulses from sensory

receptors. Examples- cutaneous nerve receptorsb. Motor or Efferent Neurons-

carry impulses from the CNSc. Interneuron (association

neurons) - connect sensory and motor neurons Reflex Arc Animation

B. Neurons- excitable little cells that make use of their potential!

5. Structural neuron classification: (determined by the number of processes extending from the cell body)

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

B. Neurons- excitable little cells that make use of their potential!C. Functional Properties of Neurons

1. Irritability- neurons have the ability to respond to a stimulus

2. Conductivity- the ability to transmit an impulse

3. 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.

Resting Membrane Potential Animation

D. 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.

                                                           

Action Potential Animation

D. Action Potentials or Nerve Impulses- an electrical charge traveling down a neuron5. 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.

Action Potential Animation

E. 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.

F. 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.

E. Conductivity- How does the impulse travel to the next neuron?

1. 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

IV. Developmental Aspects of the Nervous SystemA. 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 bodies 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.Cerebral refers to the affected area of the brain, the cerebrum, and palsy refers to disorder of movement. Cerebral palsy is caused by damage to the motor control centers of the young developing brain and can occur during pregnancy, during childbirth, or after birth up to about age three.

IV. Developmental Aspects of the Nervous System

C. 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.