Simple Nerve Nets and Ladders	Massed Ganglia to Organized Brains	Vertebrates	Human Central Nervous System	Spinal Cord	Human Brain
Cerebrum	Chemicals in the Brain	Electrical Activity in the Brain	Language Centers in the Human Brain	Peripheral Nervous System	Autonomic Nervous System

INVERTEBRATE NERVOUS SYSTEMS
Simple Nerve Nets and Ladders

• Coelenterates show no evidence of cephalization.  They are radially symmetrical and contain a nerve net whose cells concentrate at the mouth and tentacles.
• The flatworm nervous system resembles a ladder with two ganglionic masses (clusters of nerve cell bodies) at the anterior end and the ladder arrangement proceeding posteriorally.

Massed Ganglia to Organized Brains

• The annelids have large ganglia above and below the esophagus and a ventral nerve cord with segmental ganglia.  Giant axons in the cord support fast escape movements.
• While the bivalve nervous system is fairly simple, that of the cephalopod is complex and features a functionally differentiated brain and complex behavior.  Giant axons are found in the squid mantle.
• Arthropod specializations include reduction in the number of segments and ganglia and much greater cephalization than is found in the annelids.  The insect brain includes an enlarged ganglionic mass above the esophagus, which, in those with compound eyes, includes greatly enlarged visual integrating centers.

VERTEBRATE NERVOUS SYSTEMS

• Vertebrate nervous systems include a pronounced trend toward cephalization and a dorsal, hollow nerve cord.
• The vertebrate brain follows a three part plan of forebrain, midbrain and hindbrain.  The cerebrum and surrounding cortex of the forebrain reach its greatest development in mammals.

The Human Central Nervous System

• The human nervous system is divided into the central nervous system (CNS) - the brain and the spinal cord - and the peripheral nervous system (PNS) - the somatic and autonomic systems.
• The somatic system is largely composed of sensory receptors and sensory neurons and the voluntary motor nerves that move skeletal muscle, while the autonomic system is motor and involuntary, dedicated mainly to homeostasis.  The central nervous system arises in the embryo from the neural tube.

The Spinal Cord

• The spinal cord includes pathways between the brain and much of the peripheral nervous system. The white regions are myelinated neurons, while the gray are nerve cell bodies. Many reflexive acts occur at the cord level.
• The cord emerges from the brain through the foramen magnum and lies within the vertebral canal. It is the bathed in the cerebrospinal fluid and surrounded by the meninges, three layers of supporting connective tissue.
• Motor neurons arise from cell bodies in the cord to pass out through the ventral motor root of the spinal nerves. Sensory neurons enter the the cord via the dorsal sensory root from rows of ganglia outside, where their cell bodies cluster.

The Human Brain

• The human brain weighs about 1.4 kg, has a volume of 1300 to 1500 cc  contains some 100 billion neurons and 10 times that number of glial cells. Cell bodies form its gray outer regions, while myelinated fibers make up its white inner mass. Cerebrospinal fluid and the meninges cushion and protect the brain, respectively.
• The hindbrain consist of
• the medulla oblongata (or medulla), which controls breathing and heart rate and contains many pathways, including some form cranial nerves;
• the cerebellum, concerned with balance, equilibrium and voluntary muscle coordination;
• the pons, which contains tracts traveling between forebrain and cord and to and from the cerebellum.
• The midbrain forms connections between the hindbrain and forebrain and receives sensory input from auditory and visual receptors.
• The forebrain is involved in conscious thought, sensory reception, voluntary movement, and other voluntary acts.  It consists of the following:
• The thalamus, which is a relay structure, connects the various parts of the brain and includes the reticular system, which taps incoming and outgoing communications.  It also acts as an alarm system and suppresses irrelevant stimuli, thus permitting sleep.
• The hypothalamus, which monitors many functions, acts as a homeostatic regulator (heart rate, blood pressure, body temperature, thirst, hunger, sex drive).  It also stimulates hormonal activity in the pituitary and is subject to negative feedback.
• The limbic system, containing a number of specific nuclei, includes the hypothalamus, thalamus and some cortical pathways.  It links the fore brain and midbrain, and is involved in emotion (for example, when areas of the amygdala are stimulated, we may experience rage).

The Cerebrum

• The cerebrum, the largest region of the forebrain, is divided into left and right cerebral hemispheres, which are covered by the wrinkled cerebral cortex.  The cortex contains some 15 billion cell bodies and dendrites.  The white matter below includes the myelinated fibers.
• The importance of the cerebrum to common voluntary acts is minimal in frogs but increasingly important in rats, cats, dogs, monkeys, and humans.
• The cerebrum is made up of a number of lobes.  The occipital lobe, receives and analyzes visual information, while the temporal lobe (bounded by the fissure of Sylvius, or lateral sulcus) processes auditory input and some visual information.  The parietal lobe processes sensory information, including body position.  It is separated from the frontal lobe by the fissure of Rolando or central sulcus.
• The processing of sensory input and the instigation of voluntary muscle action occurs in specific regions of the cortex.
• The cerebral hemispheres are functionally distinct and many learned patterns take place in just one hemisphere.  The left hemisphere predominates in the right handed persons and in the more common type of left handedness, but in the rare form of left handedness, dominance is in the right.  Speech is primarily a left hemisphere function, as is analytical thought.  Damage to this hemisphere can provide a loss of language abilities.
• Connections between the hemispheres occur through the corpus callosum.

Chemicals in the Brain

• The brain has at least 50 different neurotransmitter.  The monoamines, modified amino acids, include norepinephrine, dopamine, histamine, and seratonin.  Neuropeptides, short chains of amino acids, include angiotension II, which plays a role in thirst related functions.
• Substances entering the brain must cross the blood-brain barrier, an area of tightly interwoven capillary endothelial cells.  Astrocytes act as intermediaries in selection and transporting materials into the brain tissue.
• Other neuropeptides include enkephalins and endorphins, mood elevators and pain modifiers.  The are called opioids because their effects can be mimicked by opiates.  Amphetamines accelerate neurotransmitter release, while cocaine blocks their normal enzymatic degradation, and lSD, mescaline, and psilocybin mimic neurotransmitters.

Electrical Activity in the Brain

• Electrical activity in the brain can be detected with an electroencephalograph and recorded as electroencephalogram, or EEG.  Such recordings are useful for detecting gross abnormal patterns.
• EEGs reveal that a considerable amount of electrical activity accompanies sleep.  Electrical activity during paradoxical sleep or REM sleep (rapid eye movement), a time of dreaming, is similar to that of wakefulness.

Language Centers in the Human Brain

• Knowledge of language centers comes mainly from studies of stroke victims.  Three left hemisphere regions, Broca's area, Wernicke's area, and the angular gyrus, are involved.
• When heard, words are processed by Wernicke's area.  Words to be spoken begin in this area but are transmitted to Broca's area, which activates the motor regions involved in voice.
• Reading aloud involves all three areas.  Visual images of the words go to the angular gyrus, which associates the words with auditory patterns in Wernicke's area.  Wernicke's area produces the basic sentence format, while the Broca's area refines sentence structure and coordinates vocalization.

THE PERIPHERAL NERVOUS SYSTEM

• The peripheral system includes the somatic and autonomic systems.  The autonomic system is made up of the sympathetic and parasympathetic divisions.

The Autonomic Nervous System

• The ANS is essentially homeostatic in function.  Its motor neurons carry impulses from the brain and spinal cord to the viscera, blood vessels, irises, secretory glands, and other involuntary structures, bringing about fine adjustments as necessary.
• The sympathetic and parasympathetic divisions have opposing functions, with the former generally increasing in action and the latter decreasing it.  For example, the sympathetic cardioaccelerator nerve releases neurotransmitters that increase heart rate, while the parasympathetic vagus nerve releases those that slow the heart rate.
• In fight or flight events, the sympathetic division increases heart and breathing rate and blood pressure, and sends more blood to the muscles and brain.  Afterward, these actions are reversed by the parasympathetic division.
• Most parasympathetic nerves originate in the brain and a few in the lower spinal cord.  Some go to external ganglia, where they synapse with neurons that innervate the target organs.  Sympathetic nerves all emerge from the cord and all pass through, or synapse within , the sympathetic ganglia along the cord.  Secondary ganglia also occur, including those of the celiac plexus or solar plexus.