Download ANS

Motor (efferent) system
 Divided into:
 Somatic motor division that controls skeletal
muscle
 Visceral/autonomic – controls smooth and cardiac
muscles and the glands (exocrine and endocrine)
 These 2 system differ not only in the effector but also
in their descending pathway and the response
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ANS Versus Somatic Nervous System (SNS)
 The ANS differs from the SNS in the following three areas
 1. Effectors
 The effectors of the SNS are skeletal muscles
 The effectors of the ANS are cardiac muscle, smooth
muscle, and glands
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ANS Versus Somatic Nervous System (SNS)
 2. Efferent pathways
 In the SNS heavily myelinated axons of the somatic motor
neurons extend from the CNS to the effector (one neuron)
 Efferent pathways in the ANS are a two-neuron chain
 The preganglionic (first) neuron has a lightly myelinated
axon
 The post-ganglionic (second) neuron extends to an effector
organ
Pre-ganglionic
Post-ganglionic
Ganglion
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ANS Versus Somatic Nervous System (SNS)
 3. Target organ responses
 All somatic motor neurons release Acetylcholine
(ACh), which has an excitatory effect
 In the ANS:
 Preganglionic fibers release ACh
 Postganglionic fibers release norepinephrine or
ACh and the effect is either stimulatory or
inhibitory
 ANS effect on the target organ is dependent
upon the neurotransmitter released and the
receptor type of the effector
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Characteristic
Somatic Nervous System
Autonomic Nervous System
Voluntary (skeletal) muscle
Smooth muscle, Cardiac
Muscle, Glands
Adjustment to external
environment.
Adjustment within the
internal environment
(homeostasis)
Number of
neurons from CNS
to effectors
1
2
Ganglia outside
the CNS
0
Chain ganglia, collateral
ganglia or terminal ganglia
Acetylcholine
Acetylcholine, adrenaline,
noradrenalin
Effectors
General function
Neurotransmitter
On NMJ - nicotinic receptors – ACh always excitatory
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Receptors for neurotransmitters in the motor division

Receptors are divided into 2 groups:

Cholinergic – receive and respond to acetylcholine (ACh):

Two subgroups that are names of chemicals that mimic some of the
actions of Ach:

Muscarinic (chemical found in the mushroom Amanita
muscarina)

Nicotinic (chemical found in the tobacco plant – nicotina
tabacum)

Adrenergic – receive and respond to norepinephrin (NE) /
epinephrine (E)

Divided into alpha and beta
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Cholinergic Nicotinic Receptors
 Found on
 Motor end plates of skeletal muscle cells
 All
ganglionic
neurons
(sympathetic
and
parasympathetic)
 Hormone-producing cells of the adrenal medulla
 Effect of ACh at nicotinic receptors is always direct
and stimulatory
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Direct effect – receptors are part of the ion channel
Nicotinic receptors
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Cholinergic Muscarinic Receptors
 Found on
 All effector cells stimulated by postganglionic
cholinergic fibers
 The effect of ACh at muscarinic receptors
 Can be either inhibitory or excitatory
 Depends on the receptor type of the target organ
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Indirect effect – through G-protein and 2nd messenger
Muscarinic (cholinergic) and all adrenergic receptors
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ACh (cholinergic) receptors
Nicotinic receptors (nACh)
Pre-ganglionic
Direct mechanism – open
Na+ channels
(depolarization)
Fast excitatory effect
Muscarinic receptors (mACh)
Post-ganglionic
Indirect mechanism – use of G-protein
and 2nd messenger system
Increase
intracellular
Ca2+ release
Slow excitatory
M1, M3, M5
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Inhibition of
adenylate
cyclase
Slow inhibitory
M2, M4
Adrenergic Receptors
 Two types
 Alpha () (subtypes 1, 2) – excited
 Beta () (subtypes 1, 2 , 3)
 Effects of NT depend on which subclass of receptor
predominates on the target organ
 Norepinephrine excites mainly Alpha () receptors
 Epinephrine excites both alpha and beta equaly
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NE (adrenergic) receptors - all indirect through G-protein
1
2
Increase
intracellular
Ca2+ release
Inhibition of
adenylate
cyclase
Slow excitation
Slow inhibition
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1
3
2
2nd messenger – cAMP
Inhibition or activation of
adenylate cyclase
slow
excitation
Slow
inhibition
Lypolysis
(excitation)
http://dvm5.blogspot.com/2010/10/neurotransmitters-and-receptorspharma.html
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Autonomic Nervous System (ANS) function
 The ANS major function is to maintain homeostasis
 The ANS :
 functions via visceral (subconscious) reflexes
 The autonomic system is coordinated primarily
by the hypothalamus and the medulla (higher
centers)
 The brain stem and the spinal cord seem to have
direct influence over autonomic functions
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Autonomic reflexes
 Autonomic reflexes are mediated by neural circuits in the
spinal cord and brain stem
 The afferent pathways include both visceral and somatic
fibers
 The reflex includes interneurons that receive convergent
input from visceral and somatic fibers
 Efferent pathway can be sympathetic or parasympathetic
 Main difference: visceral reflex arc has two neurons in the
motor pathway
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Hypothalamic Function
 Function as the main visceral control in the body
 Regulates blood pressure, rate and force of heartbeat,
digestive tract motility, rate and depth of breathing, eye
pupil size etc.
 Perception of pleasure, fear, and rage
 Maintains normal body temperature (contains body’s
“thermostat”) by initiating cooling or heat-generating
processes
 Regulates food intake (hunger)
 Regulation of water intake and thirst – produce ADH,
contain thirst center
 Regulates sleep and the sleep cycle
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The Hypothalamus other functions
 Controls the the pituitary gland via the infundibulum
(will be discussed in APII)
 Contains the Mammillary bodies
 Relay station for olfactory pathways
 The supraoptic and paraventricular nuclei produce
ADH and oxytocin
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Autonomic control – brain stem and spinal cord
 The brain stem and the spinal cord have direct
influence over autonomic functions
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Brain Stem
 Consists of three regions –
 midbrain,
 Pons
 medulla oblongata
 Similar to spinal cord – deep gray matter surrounded by
white tracts but contains embedded nuclei in the white matter
 Controls automatic behaviors necessary for
(breathing, digestion, heart rate, blood pressure)
survival
 Provides the pathway for tracts between higher and lower
brain centers
 Associated with 10 of the 12 pairs of cranial nerves
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Midbrain
 Located between the diencephalon and the pons
 Midbrain structures include:
 Cerebral peduncles of the midbrain – two
structures that contain descending pyramidal motor
tracts
 Cerebral aqueduct – hollow tube that connects the
third and fourth ventricles
 Various nuclei (what are nuclei)
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Midbrain Nuclei
 Nuclei that control cranial nerves III (oculomotor) and IV
(trochlear)
 Corpora quadrigemina
 2 Superior colliculi – visual reflex centers; coordinate
head and eye movement when we follow a moving
object (does not have to be a conscious decision)
 2 Inferior colliculi – auditory relay centers between
hearing receptors to sensory cortex.
 Reflexive responses to sound – turn head toward
sound
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Midbrain Nuclei
 Substantia nigra – functionally linked to basal nuclei,
contains melanin pigment (precursor of dopamine NT)
 Red nucleus – largest nucleus (rich blood supply) of
the reticular formation; relay nuclei for some
descending motor pathways
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Pons
 between the midbrain and the medulla oblongata
 Forms part of the anterior wall of the fourth ventricle
 Fibers of the pons:
 Connect higher brain centers and the spinal cord
 Relay impulses between the motor cortex and the
cerebellum
 Origin of cranial nerves V (trigeminal), VI (abducens),
and VII (facial)
 Contains nuclei of the reticular formation
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Medulla Nuclei related to autonomic system
 The medulla has an important role in the autonomic
reflex center that maintain homeostasis:
 Cardiovascular control center – adjusts force and
rate of heart contraction
 Respiratory centers – control rate and depth of
breathing
 Additional centers – regulate vomiting, hiccuping,
swallowing, coughing, and sneezing
 There is an overlap between medulla and
hypothalamus that uses medullary centers to carry out
instructions
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Other Medulla nuclei and functions
 Pyramids – two longitudinal ridges formed by corticospinal tracts
descending from the motor cortex (will be discussed with the
somatomotor pathways)
 Inferior olivary nuclei – gray matter that relays sensory information
regarding the stretch of muscle and joints
 Cranial nerves IX (glosopharyngeal, X (vagus), XI (accessory; neck
muscle), and XII (hypoglossal; tongue) are associated with the
medulla
 Vestibular nuclear complex – synapses that mediate and maintain
equilibrium
 Ascending sensory tract nuclei, including nucleus cuneatus and
nucleus gracilis
 Serve as relay station for general somatic sensation
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