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 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).
- 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.
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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 breast tissue, beards, etc.)
- Adrenal medulla secretes epinephrine, whose effects resemble those of sympathetic nervous system but are often stronger
- Thyroid hormones, which stimulate metabolic rate and control calcium metabolism
- 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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