Most invertebrates have nervous systems derived from the ladder-like
arrangement in flatworms. Vertebrate brains develop in three portions
(forebrain, midbrain, hindbrain).
In mammals, the cerebral hemispheres
enlarge, and their size and complexity become a crude measure of intelligence.
Invertebrate nervous systems:
Cnidaria have a nerve net of interconnected neurons with no center.
Flatworms have two long chains of ganglia in a ladder-like arrangement;
the largest ganglia, near the eyes, form the beginnings of a "brain."
Most other invertebrates have modifications of this ladder-like pattern;
a major nerve cord runs along the ventral midline, splits to form an
esophageal ring, and reunites above the mouth to form a cerebral
ganglion or brain.
Illustrations: Nervous and Endocrine systems
Embryonic vertebrate brains form as three major divisions:
- Forebrain (prosencephalon), primitively devoted to smell
- Midbrain (mesencephalon), primitively concerned with vision
- Hindbrain, primitively dealing with sound and vibrations, developing
into metencephalon and myelencephalon.
Adult vertebrate brains: Organized into five regions:
- Telencephalon: paired parts of the forebrain, including
olfactory bulbs, olfactory lobes, and cerebral hemispheres,
which enlarge greatly in mammals and take over many added functions.
- Diencephalon: unpaired, second portion of the forebrain,
including the pineal body (epiphysis); tela choroidea (thin roof);
thalamus (controls many emotions); hypothalamus (controls
appetite and body temperature); and part of the pituitary gland.
- Mesencephalon: midbrain, including corpora quadrigemini
- Metencephalon: includes cerebellum and pons
- Myelencephalon: medulla, continuing into the spinal cord
Brain ventricles: cavities containing cerebrospinal fluid
Spinal cord:
White matter: myelinated tracts
Gray matter: unmyelinated motor and sensory columns
Spinal reflex pathway:
- Sensory neuron runs from a receptor cell
in skin to cell body in dorsal root ganglion, then into
somatic sensory column of spinal cord.
- Association neuron connects somatic sensory
column to somatic motor column in spinal cord.
- Motor neuron runs from somatic motor column
out ventral root to a voluntary muscle or other effector cell.
Autonomic ("self-governing") nervous system, over which we usually have little or no conscious control:
- Sympathetic division, which prepares the body for "fight and flight"
Nerve endings secrete norepinephrine, which increases heartbeat, breathing,
sweating, muscle activity, and blood flow to muscles (but inhibits peristalsis).
- QUICK! Think of what happens when the fire bell rings! Or when you hear a loud explosion! Or when you realize that you
are under attack by wild animals (or terrorists)! In all these situations, the sympathetic division
prepares the body to either fight back or run away fast: more muscle activity, more rapid heatbeat and
breathing (to supply more blood to the muscles), faster blood clotting (in case you get injured), more
sweating, and also dilation of the pupils to let in more light so you can see better and avoid danger as you flee or fight back.
All these actions use Norepinephrine (similar to adrenaline = epinephrine) as the neurotransmitter.
- Parasympathetic division, which prepares the body to "rest and ruminate"
Nerve endings secrete acetylcholine, which slows heartbeat and breathing
and increases peristalsis and digestive secretions.
- Imagine yourself enjoying a quiet, comfortably relaxed, candlelight dinner with fine food and good companions:
just thinking about such a situation prepares the body to "rest and ruminate",
slowing and relaxing the heartbeat, slowing your breathing, bringing less blood flow to the muscles
and more to the digestive (and sexual) organs, and less visual focus or interest in visual stimulation.
All the nerves that stimulate these responses secrete acetylcholine as a neurotransmitter.
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Endocrine glands secrete chemicals called hormones that travel through the circulatory system and
control the activities of other organs of the body.
Some hormones are proteins or small peptides; others are steroids; a few are amines.
Hormones are chemical messages, secreted in one place, and carried by the bloodstream to a distant "target" (or to multiple targets).
Hormone secretion can be either neuroendocrine or glandular.
Hormone reception: steroid hormones penetrate to the nucleus of the target cells;
other hormones have surface receptors and require "second messengers".
Examples:
- Pituitary hormones (many):
- Anterior pituitary hormones:
- Growth hormone (somatostatin) stimulates growth
- Gonadotrophic Hormone stimulates sexual development and sex hormone secretion
- LuteoTrophic Hormone (LTH) and Lactogenic Hormone (LH) control ovulation, menstrual cycles, and milk secretion
- Adrenal CorticoTrophic Hormone (ACTH) stimulates the adrenal gland
- (several others)
- Posterior pituitary hormones:
- Vasopressin maintains blood pressure
- Oxytocin induces labor and promotes emotional bonding
- Adrenal hormones:
- Adrenal cortex hormones (dozens of them) control carbohydrate metabolism, ion balance,
body defenses (incl. reaction to stress), inflammation, and secondary sexual characteristics
(e.g. development of beards, deer antlers, etc.)
- Adrenal medulla secretes epinephrine, whose effects resemble those of sympathetic nervous system but are often stronger
- Thyroid hormones: Thyroxin stimulates metabolic rate; Calcitonin controls calcium metabolism (lowers blood levels of calcium).
- Parathyroid hormone: Raises blood levels of calcium.
- Pancreatic hormones (insulin and glucagon), which regulate sugar metabolism
- Sex hormones (testosterone, estrogen, progesterone): control sexual development and also menstrual cycles
- Insect hormones:
- Ecdysone: controls molting
- Juvenile hormone: allows molt to continue as a larva (absence promotes metamorphosis into an adult)
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