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Textbook notes-Chapter 11-Efferent Division-Autonomic and Somatic Motor Control

Biological Sciences
Course Code
Ingrid L.Stefanovic

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Chapter 11- Efferent Division: Autonomic and Somatic Motor Control
The Autonomic Division
x It is subdivided into sympathetic and parasympathetic branches
- The sympathetic branch is dominant in stressful situations like threat from a snake and one of the most dramatic
examples of this action is the fight-or-flight response, in which the brain triggers massive simultaneous
sympathetic discharge throughout the body; mediated through the hypothalamus
Autonomic Reflexes Are Important for Homeostasis
x The autonomic nervous system works closely w/ the endocrine system and the behavioural state system
x Sensory info from somatosensory and visceral receptors goes to homeostatic control centres in the hypothalamus, pons,
and medulla
- These centres monitor and regulate important functions like blood pressure, temp regulation, and water balance
- The hypothalamus also contains neurons that act as sensors which monitor osmolarity, and thermoreceptors,
which monitor body temp
- Motor output from the hypothalamus and brain stem creates autonomic responses, endocrine responses, and
behavioural responses
- The behavioural responses are integrated in brain centres responsible for motivated behaviour and control of
x Sensory info integrated in the cerebral cortex and limbic system can create emotions that influence autonomic output
Antagonistic Control Is A Hallmark of the Autonomic Division
properties of homeostasis: (1) preservation of the fitness of the internal environment, (2) up-down regulation by tonic
control, (3) antagonistic control, and (4) chemical signals w/ different effects in different tissues
x Most internal organs are under antagonistic control, on which one autonomic branch is excitatory and the other branch
is inhibitory
x Although the two autonomic branches are usually antagonistic in their control for a given target tissue, they sometimes
work cooperatively on different tissues to achieve a common goal
x In some autonomic pathways, the neurotransmitter receptor determines the response of the target tissue; e.g. most blood
vessels contain one type of adrenergic receptor that causes smooth muscle contraction (vasoconstriction)
- Some blood vessels also contain a 2nd type of adrenergic receptor that causes smooth muscle to relax
Autonomic Pathways Have Two Efferent Neurons in Series
x All autonomic (sympathetic and parasympathetic) consist of two neuron in series
1. Preganglionic neuron originates in the CNS and projects to an autonomic ganglion outside the CNS. There
the preganglionic neurons synapses w/ the 2nd neuron in the pathway
2. Postganglionic neuron has its cell body in the ganglion and projects its axon to the target tissue
x Divergence is an important feature of autonomic pathways as on avg, one preganglionic neuron entering ganglion
synapses w/ 8 or 9 postganglionic neuron.
- Each postganglionic neuron may then innervate a different target, meaning that a single signal from the CNS
can affect a large # of target cells simultaneously
x Ganglia are more than a simple collection of axon terminals and nerve cell bodies: they also contain interneurons that
lie completely within them
- These interneurons enable the autonomic ganglia to act as mini-integrating centres, receiving sensory input
from the periphery of the body and modulating outgoing autonomic signals to target tissues.
Sympathetic and Parasympathetic Branches Exit the Spinal Cord in Different Regions
x The main anatomical differences are (1) where the pathways originate in the CNS and (2) the location of the autonomic
x Fig 11-5 shows, most sympathetic pathways (red) originate in the thoracic and lumbar regions of the spinal cord;
parasympathetic pathways (shown in blue) originate in the brain stem, and their axons leave the brain in several cranial
x Sympathetic ganglia are found primarily in two chains that run along either side of the spinal column, w/ additional
ganglia along the descending aorta
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- Long axons of postganglionic neurons project from the ganglia to the target tissues
- B/c most sympathetic ganglia lie close to the spinal cord, sympathetic pathways generally have short
preganglionic neurons and long postganglionic neurons
x Parasympathetic pathways- originate in the sacral region (near the lower end of the spinal cord) and control pelvic
- Generally, parasympathetic ganglia are located either on or near their target organs and consequently, their
preganglionic neurons have long axons, and their postganglionic neurons have short axons
x Parasympathetic innervation goes primarily to the head, neck, and internal organs
- The major parasympathetic tract is the vagus nerve which contains about 75% of all parasympathetic fibres and
this nerve carries both sensory info from internal organs to the brain, and parasympathetic output from the brain
to organs (Fig 11-6)
- Vagotomy, a procedure in which the vagus nerve is surgically cut and for a time, was the preferred treatment for
stomach ulcers b/c removal of parasympathetic innervation to the stomach decreased the secretion of stomach
The Autonomic Nervous System Uses A Variety of Neurotransmitters and Modulators
x Chemically the sympathetic and parasympathetic branches can be distinguished by their neurotransmitters and
receptors, using the following rules (Fig 11-7):
1. Both sympathetic and parasympathetic preganglionic neurons release acetylcholine (Ach) onto nicotinic
cholinergic receptors on the postganglionic cell
2. Most postganglionic sympathetic neurons secrete norepinephrine onto adrenergic receptors on the target cell.
3. Most postganglionic parasympathetic neurons secrete acetylcholine onto muscarinic cholinergic receptors on
the target cell
x Some exceptions to these rules: (1) a few sympathetic postganglionic neurons, like those that terminate on sweat glands
and smooth muscle, secrete Ach rather than norepinephrine and those neurons are therefore called sympathetic
cholinergic neurons
x A small # of autonomic neurons secrete neither norepinephrine nor acetylcholine and are known as non-adrenergic,
non-cholinergic neurons
x Some of the chemicals they use as neurotransmitters include substance P, somatostatin, casoactive intestinal peptide
(VIP), adenosine, nitric oxide, and ATP
x The non-adrenergic, non-cholinergic neurons are assigned to either the sympathetic or parasympathetic branch
according to the location where their preganglionic fibres leave the nerve cord
Autonomic Pathways Control Smooth and Cardiac Muscle, Glands, and Lymphoid and Adipose Tissues
x The targets of autonomic neurons are smooth muscle, cardiac muscle, many exocrine (sweat) glands, a few endocrine
glands, lymphoid tissues, and some adipose tissue
x The synapse b/w a postganglionic autonomic neuron and its target cell is called the neuroeffector junction
x Autonomic postganglionic axons end w/ a series of swollen areas at their distal ends (Fig 11-8)
x Each of these swellings, known as a varicosity contains vesicles filled w/ neurotransmitters
x Parasympathetic pathways end w/ axon terminals, or terminal boutons, which also release neurotransmitter
x The branched ends of the axon lie across the surface of the target tissue, but the underlying target cell membrane
x Instead, the neurotransmitter is simply released into the interstitial fluid to diffuse to wherever the receptors are located
x The result is a less-directed form of communication than that which occurs b/w a somatic motor neuron and a skeletal
x The diffuse release of autonomic neurotransmitter means that a single postganglionic neuron can affect a large area of
target tissue
x The release of autonomic neurotransmitters is subject to modulation from a variety of sources; e.g. sympathetic
varicosities contain receptors for hormones and for paracrines like histamine so these modulators may either
facilitate or inhibit neurotransmitter release
Autonomic Neurotransmitters Are Synthesized in the Axon
x In the autonomic division, neurotransmitter synthesis takes place in the axon varicosities
x The primary autonomic neurotransmitters are acetylcholine (Ach) and norepinephrine, both small molecules easily
synthesized by cytoplasmic enzymes
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