CENTRAL NERVOUS SYSTEM

I. DEVELOPMENT OF THE BRAIN

A. Prosencephalon (forebrain)

1. Telencephalon = cerebrum.
2. Diencephalon

a. Roof = epithalamus.
b. Walls = thalamus (largest subdivision of diencephalon).
c. Floor = hypothalamus.

B. Mesencephalon (midbrain)

1. Roof = tectum, which contains optic lobes (superior colliculi in mammals).
2. Floor = tegmentum.

C. Rhombencephalon (hindbrain)

1. Metencephalon

a. Roof = cerebellum.
b. Floor = pons (in mammals)

2. Myelencephalon = medulla oblongata.

D. Layers of neural tube:

1. Inner mantle layer = cell bodies (gray matter).
2. Outer marginal layer = nerve fibers or processes (white matter).

E. Nerve centers = areas where large numbers of neurons synapse.

1. Relay centers - receive information primarily from single source and pass it on to one or more other centers.
2. Association centers - receive information from several sources and integrate the information before passing it on to other centers.
3. Largest parts of brain are primarily complex association centers:

a. Cerebrum.
b. Optic lobes (reduced in mammals).
c. Cerebellum.

II. SPINAL CORD

A. Lower vertebrates:

1. Relatively independent of brain.
2. Coordinates locomotor movements via spinal reflexes.

B. Higher vertebrates:

1. Becomes progressively more dependent on brain.
2. Mammals and birds:

a. Very little independent activity.
b. Basically a pathway to and from brain.

III. THE BRAIN AND ITS EVOLUTION

A. Hypothetical ancestor:

1. 3 regions, each associated with a sense organ.

a. Prosencephalon ->
b. Mesencephalon ->
c. Rhombencephalon ->

2. Functions:

a. Coordinate feeding, breathing, and visceral activities.
b. Coordinate behavior patterns.
c. Modify locomotor activity initiated by spinal cord.

B. Medulla oblongata:

1. Relay and association centers for regulation of heartbeat, breathing, and vasoconstriction and dilation.
2. Thin roof forms posterior choroid plexus.

C. Cerebellum:

1. Coordination of muscular movements and equilibrium in all vertebrates.
2. Size is influenced by activity level more than phylogeny. eg. Cerebellum is larger in many fishes than in amphibians.
3. Non-mammalian vertebrates:

a. Median body.
b. Auricles (flocculonodular lobes in amniotes):

(1) Equilibration or reception of impulses from lateral line system.
(2) Reduced in terrestrial forms due to loss of lateral line system.

4. Mammals:

a. Body divided into vermis and hemispheres; correlated with greater involvement in control of limb movement.
b. Pons = fibers from cerebrum to cerebellum.

D. Mesencephalon:

1. Non-mammalian vertebrates - optic lobes contain important visual centers.
2. Mammals - optic lobes reduced to superior colliculi.
3. Tegmentum:

a. Relay center.
b. Well developed in mammals as cerebral peduncles.

E. Diencephalon:

1. Epithalamus:

a. Anterior choroid plexus - secretes cerebrospinal fluid.
b. Pineal body - photoreceptor in lampreys, endocrine organ (stimulated, in part, by light) in gnathostomes.

2. Thalamus = relay center to and from cerebrum.
3. Hypothalamus - regulation of homeostasis.

a. Main control of autonomic nervous system.
b. Birds and mammals - thermoregulation.
c. Secretes hormones involved in water balance.
d. Controls pituitary secretions.

F. Telencephalon - most complex and variable region.

1. Anterior end = olfactory bulbs.
2. Cerebrum is laterally divided into hemispheres.

a. Lower vertebrates - olfaction is important.
b. Amniotes - also includes visual and auditory centers.
c. Mammals - neocortex functions in vision, hearing, memory, learning, and speech (humans).
d. Trend: cerebrum becomes increasingly important as visual center.