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Chapter 11

Chapter 11 Study Guide


Department
Biological Sciences
Course Code
BIOD27H3
Professor
A.Elia
Chapter
11

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CHAPTER 11
EFFERENT DIVISION: AUTONOMIC AND SOMATIC MOTOR CONTROL
Pages 377 ± 391
- efferent division of peripheral nervous system can be subdivided into somatic motor neurons, which control
skeletal muscles, and autonomic neurons, which control smooth muscle, cardiac muscle, many glands, and
some adipose tissue
- somatic and autonomic divisions are sometimes called the voluntary and involuntary divisions of the nervous
system, respectively
THE AUTONOMIC DIVISION
- its functions not under voluntary control
- control over internal organs
- divided into sympathetic and parasympathetic branches
- if you are resting quietly after a meal, the parasympathetic branch is dominant, taking command of the
routine, quiet activities of day-to-day living, such as digestion. Its QHXURQVDUHVRPHWLPHVVDLGWRFRQWURO³UHVW
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- in contrast, sympathetic branch is dominant in stressful situations, such as the potential threat from the snake
- fight-or-flight responseÆ brain triggers massive simultaneous sympathetic discharge throughout the body; as
the body prepares to fight or flee, the heart speeds up; blood vessels to muscles of the arms, legs, and heart
dilate; and the liver starts to produce glucose to provide energy for muscle contraction. The discharge during
fight-or-flight situations is mediated through the hypothalamus and is a total-body response to a crisis
Autonomic reflexes are important for homeostasis
- works closely with endocrine and behavioural state system to maintain homeostasis in the body
- sensory info from somatosensory and visceral receptors goes to homeostatic control centres in hypothalamus,
pons, and medulla. These centres monitor and regulate important functions such as BP, temp regulation, and
water balance
- hypothalamus Æ has neurons that act as sensors, such as osmoreceptors, which monitor osmolarity, and
thermoreceptors, which monitor body temp
- motor output from hypothalamus and brain stem creates autonomic responses, endocrine responses, and
behavioural responses such as drinking, food-seeking, and temp regulation
- sensory info integrated in cerebral cortex and limbic system can create emotions that influence autonomic
output
- some can place without input from brain. These spinal reflexes include urination, defecation, and penile
erection ± bodily functions that can be influenced by descending pathways from the brain but do not require
this input
Antagonistic control is a hallmark of the autonomic division
- sympa and parasympa display all 4 properties of homeostasis:
o preservation of fitness of internal environment
o up-down regulation by tonic control,
o antagonistic control
o chemical signals with diff. effects in diff. tissues
- most internal organs are under antagonistic control, Æ one autonomic branch is excitatory and one the other
is inhibitory. Exception: sweat glands and smooth muscel in most blood vessels Æ innervated only by sympa
branch and rely strictly on tonic (up-down) control
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- para and sympa work cooperatively on diff. tissues to achieve a common goal
- in some autonomic pathways, neurotransmitter receptor determines the response of the target tissue. Most
blood vessels contain one type of adrenergic receptor that causes smooth muscle contraction
(vasoconstriction). However, some blood vessels also contain a second type of adrenergic receptor that causes
smooth muscle to relax (vasodilation)
Autonomic pathways have 2 efferent pathways in series
- preganglionic neuron Æ originates in CNS and projects to an autonomic ganglion outside the CNS. There,
the preganglionic neuron synapses with the second neuron in the pathway, the postganglionic neuron Æ has
its cell body in ganglion and projects its axon to the target tissue. ( A ganglion is a cluster of nerve cell bodies
that lie outside the CNS. The equivalent in the CNS is a nucleus
- divergence Æ single signal from the CNS can affect a large number of target cells simultaneously
- ganglia contain interneurons that lie completely within them. These interneurons enable the autonomic
ganglia to act as miniintegrating centres, receiving sensory input from periphery of body and modulating
outgoing autonomic signals to target tissues
Sympathetic and parasympathetic branches exit the spinal cord in different regions
- anatomical differences:
o where the pathways originate in CNS and
o the location of autonomic ganglia
- most sympathetic pathways originate in thoracic and lumbar regions of the spinal cord. Sympathetic ganglia
found primarily in 2 chains that run along either side of spinal column, with additional ganlia along the
descending aorta
- long nerves (axons of postganglion neurons) project from ganglia to the target tissues. Since ganglia lie close
to the spinal cord, sympa pathways generally have short preganglionic neurons and long postganglionic
neurons
- parasympa pathways originate in brain stem, and their axons leave brain in several cranial nerves
- other parasympathetic pathways originate in sacral region (near low end of spinal cord) and control pelvic
organs
- ganglia are located on either on or near their target organs. Hence, parasympa preganglionic neurons have
long axons, and parasympa postganglionic neurons have short axons
- parasympa innervation goes primarily to the head, neck, and internal organs
- major parasymp tract is the vagus nerve (cranial nerve X), which contains about 75% of all parasympa fibers.
This nerve carries both sensory info from internal organs to brain, and parasympa output from brain to organs
- vagotomy Æ procedure in which vagus nerve is surgically cut to effects of ANS on various organs
The autonomic nervous system uses a variety of neurotransmitters and modulators
- chemically, sympa and parasympa branches can be distinguished by their neurotransmitters and receptors,
using following rules:
(1) both sympa and parasympa preganglionic neurons release acetylcholine (Ach) onto nicotinic cholinergic
receptors on postganglionic cell
(2) most postganglionic sympa neurons secrete norepinephrine onto adrenergic receptors on the target cell
(3) most postganglionic parasympa neurons secrete acetycholine onto muscarinic cholinergic receptors on the
target cell
- exceptions: a few sympathetic postganglionic neurons, such as those that terminate on sweat glands, secrete
ACh rather than norepinephrine. These neurons are therefore called sympathetic cholinergic neurons
- small number of neurons secrete neither norepinephrine nor acetylcholine and are known as non-adrenergic,
non-cholinergic neurons
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