Autumn.wmf (12088 bytes)Introduction to Organismal Biology (BIOL221) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321;;

Structure & Organization of the Nervous System

I. Organization
    There are two major subdivisions; central nervous system and peripheral nervous. Each of these can be further subdivided.

II. Central Nervous System (CNS)
    The CNS is comprised of brain and spinal cord. Note these are encased in bone (cranium, vertebrae) for protection. The major components of the CNS include:

A. Grey Matter
    Found in the peripheral regions of the brain, makes up the outer 1/4 inch or so; and in the central butterfly shaped region of the spinal cord. The gray matter consists of the bodies of motor neurons and interneurons and glial ("glue") cells. Recall that the latter serve a supporting role and don't transmit electrical signals.

B. White Matter
    This includes the central region of the brain; and the periphery of the spinal cord. This region is white largely because of the myelin sheaths found on many nerve fibers which may occur in bundles (tracts). There are also glial cells present.

C. Fluid-Filled cavities
    For support/cushioning. There are cavities in the brain (called ventricles) and a central canal in the spinal cord - these are filled with cerebrospinal fluid.  Yum.

D. Connective Tissue Covering
    The meninges; three layers - to protect the spinal cord and brain.

III. Peripheral Nervous System
    This includes all parts outside of the CNS such as nerves, etc., that transmit signals to/from the CNS. The PNS includes:

A. Nerves

  • Bundles of axons
  • There are 12 pairs or cranial nerves (a mnemonic to remember them - On Old Olympus Towering Tops A Finn And German Viewed Some Hops) and 31 pairs of spinal nerves.

B. Sensory (or afferent) and Motor (efferent) neurons

  • Motor neurons (efferent) – connected to effectors; conduct signal from CNS to effector
  • Snsory neurons (afferent) – connected to receptors; transmit information to the CNS
  • Interneurons (associative) – connect neurons to one another
  • Effectors can be voluntary (i.e., skeletal muscle) or involuntary (i.e., smooth or cardiac muscle; glands).

C. Reflex arc, or path of a nerve impulse
 stimulus → receptor → sensory (afferent) neuron → CNS → interneuron (may be absent in some cases) → motor (efferent) neuron → effector → response

D. Ganglia
    Aggregates of nerve cell bodies; spinal ganglia are primarily nerve cell bodies of sensory neurons; autonomic ganglia are nerve cell bodies involved in responses not under conscious control.

E.  Controls
    The PNS is involved with the control of both voluntary (somatic nervous system, skeletal muscle, "head, trunk, limbs") and involuntary actions (autonomic nervous system; visceral organs, internal).

IV. Autonomic Nervous System
    Controls involuntary (unconscious) actions. There are two components to the ANS:

A. Sympathetic
    Prepares the body for "flight-or-fight". In other words, prepares the body for "action". These nerves originate from the cervical (neck), thoracic (chest) and lumbar (back) regions of the spinal cord. Acetylcholine (cholinergic) and noradrenaline (noradrenergic) are the neurotransmitters.

B. Parasympathetic
    Prepares the body for rest; or in other words, to gain or conserve energy. The nerves originate from the cranial (head) and sacral (tail) regions of the spinal cord. Acetylcholine is the neurotransmitter.

C. Anatagonists
    The sympathetic and parasympathetic systems typically have antagonistic (opposite) actions. Some examples:

Table 1. Some actions of the sympathetic and parasympathetic nervous systems
Action Sympathetic Parasympathetic
heart rate increase decrease
pupils dilate contract
bronchial secretion inhibit stimulate
stomach/intestine inhibit stimulate
sphincters contract relax

D. Diagram of sympathetic/parasympathetic
    Note that most actions usually involve two nerve pathways. Some differences: in parasympathetic, both synapses rely on acetylcholine and the nerve leading from the CNS is long and synapses near the effector. In the sympathetic, the first nerve is short, and synapses in ganglia close to the spine. Acetylcholine is the transmitter in these ganglia. The second neuron to the effector is long and noradrenaline is the transmitter at this synapse.

V. A look at the brain - top banana of the CNS

A. Videos
    Check out some of the videos in the library. Some excellent videos include Within the Human Brain - QM 455.D53 that features a dissection of the human brain - we will watch a clip from this video (click here for study questions).  Another good video is the series from The Body Atlas. Vol 13. The Brain (QP 34.5 .B45).

B. Evolutionary Trends
    The following evolutionary trends have occurred: (a) increase in size; (b) increase in compartmentalization of functions; (c) increase in complexity of the forebrain (increased folding)

C. Regions of the brain

1.  Forebrain

  • Cerebrum (center for coordinating activities, thinking, memory)
        The cerebrum is made up of two halves (hemispheres) interconnected by the corpus callosum. The thin outer layer is the cerebral cortex, is comprised of gray matter. It is another example of the importance of surface/volume ratios - it has an area of about 2.5 feet (size of a pillowcase) so it must be folded to fit inside the cranium - think of crumpling up a piece of paper - viola, a brain model.  The hemispheres are responsible for different activities: (a) Left hemisphere - responds to signals from right side of body. Contains region for language skills (speech) and hearing; (b) Right hemisphere - responds to signals from left side; nonverbal skills like music, math, and other abstract abilities reside here.  Frontal lobe - "planning ahead functions", sequence things; motor cortex (accounts for 1/3 of hemisphere). Temporal lobe - named for temples, aging first appears as gray hair and thought had to do with time, controls hearing, smell.  Parietal lobe - speech, taste, reading. Occipital lobe - vision
  • Other regions of the forebrain include:  Thalamus (routes neural information to the gray matter); Hypothalamus (hormone production; regulates temperature; senses hunger); Pineal gland (circadian rhythm control); Pituitary gland (important gland that controls many activities); Hippocampus (involved in short/long term memory)

2. Midbrain - receives info; conducts sensory information to the forebrain

3. Hindbrain - comprised of (a) cerebellum (movement, balance, motor coordination - can you touch your nose?); and (b) medulla and pons (breathing/digestion and transfer info from spinal cord to brain).

4. Brain stem = midbrain + medulla + pons (hindbrain)

5. Protective coverings - the brain has a series of protective layers (dura mater, arachnoid, pia mater) and fluid for cushioning (cerebrospinal fluid)

VI. Some Study Tips.

  1. Go over the study questions at the end of the chapter.
  2. Make detailed concept maps to ideas in the chapter.
  3. Make a list of all the terms in the chapter and define them.
  4. View the videos in the Alcuin and Clemens libraries about the brain.
  5. Read other texts and books about the nervous system.
  6. Draw a picture of the nervous system including all the major components and how they interrelate.
  7. Tell your friends about the intelligence of your brain.
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Last updated: April 04, 2007        � Copyright by SG Saupe