Human Anatomy & Philosophy Quiz & Test Takers

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Somatic vs. Autonomic

  • Voluntary
  • Skeletal muscle
  • Single efferent neuron
  • Axon terminals release acetylcholine
  • Always excitatory
  • Controlled by the cerebrum
  • Involuntary
  • Smooth, cardiac muscle; glands
  • Multiple efferent neurons
  • Axon terminals release acetylcholine or norepinephrine
  • Can be excitatory or inhibitory
  • Controlled by the homeostatic centers in the brain – pons, hypothalamus, medulla oblongata

ANS Versus Somatic Nervous System (SNS)

  • The ANS differs from the SNS in the following three areas
  • Effectors
  • Efferent pathways
  • Target organ responses

Somatic vs. Autonomic Nervous System

  • Both have motor fibers
  • Differ in:
  • Effectors
  • Efferent pathways
  • ANS
  • Preganglionic neuron
  • Ganglion
  • Postganglionic neuron
  • Target organ responses to neurotransmitters

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Efferent Pathways

  • Heavily myelinated axons of the somatic motor neurons extend from the CNS to the effector
  • Axons of the ANS are a two-neuron chain
  • The preganglionic (first) neuron has a lightly myelinated axon
  • The ganglionic (second) neuron extends to an effector organ

Neurotransmitter Effects

  • 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

Comparison of Somatic and Autonomic Systems

Figure 14.2

ANS Architecture

  • Both ANS divisions share the same general structure.
  • Autonomic pathways always consist of 2 neurons in series.
  • They synapse in an autonomic ganglion – would this be inside or outside the CNS?
  • The 1st neuron in the autonomic pathway is the preganglionic neuron,
  • Cell body in CNS, myelinated, and projects to the autonomic ganglion.
  • While the 2nd neuron is the postganglionic neuron.
  • Cell body in autonomic ganglion, unmyelinated, and projects to the effector.

ANS Divisions & Dual Innervation

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Exceptions To Dual Innervation

  • Adrenal medulla
  • Sweat glands
  • Arrector pili
  • Kidneys
  • Most blood vessels

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AUTONOMIC NERVOUS SYSTEM

Regulates (examples)

  • Glands
  • Blood glucose
  • Body temperature
  • Osmotic balance
  • Cardiac muscle
  • Heart rate
  • Blood pressure
  • Smooth muscle
  • Digestion
  • Waste disposal
  • Breathing

Lecture 4

Organization of the Autonomic Nervous System (ANS)

Central components:

hypothalamus

brain stem

spinal cord

Peripheral components

sympathetic nerves

parasympathetic nerves

Autonomic Nervous System (ANS)

  • The ANS consists of motor neurons that:
  • Innervate smooth and cardiac muscle and glands
  • Make adjustments to ensure optimal support for body activities
  • Operate via subconscious control
  • Have viscera as most of their effectors

ANS Anatomy

Distinctions between divisions

  • Unique origin site
  • Relative lengths of fibers
  • Ganglia location

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Parasympathetic Division

Craniosacral division

  • Cranial outflow
  • Sacral outflow

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Role of the Parasympathetic Division

  • Concerned with keeping body energy use low
  • Involves the D activities – digestion, defecation, and diuresis
  • Its activity is illustrated in a person who relaxes after a meal
  • Blood pressure, heart rate, and respiratory rates are low
  • Gastrointestinal tract activity is high
  • The skin is warm and the pupils are constricted

Sympathetic Nervous System

  • Thoracolumbar
  • More complex

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Role of the Sympathetic Division

  • The sympathetic division is the “fight-or-flight” system
  • Involves E activities – exercise, excitement, emergency, and embarrassment
  • Promotes adjustments during exercise – blood flow to organs is reduced, flow to muscles is increased
  • Its activity is illustrated by a person who is threatened
  • Heart rate increases, and breathing is rapid and deep
  • The skin is cold and sweaty, and the pupils dilate

Lecture 4

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A dynamic balance between sympathetic and parasympathetic activity maintains homeostasis in the body.

The sympathetic division helps us during fight or flight situations, while the parasympathetic division helps us during times of rest and calm

Antagonistic Control

  • Most internal organs are innervated by both branches of the ANS which exhibit antagonistic control.
  • A great example is heart rate. An increase in sympathetic stimulation causes HR to increase whereas an increase in parasympathetic stimulation causes HR to decrease.

Sympathetic Outflow

  • Arises from spinal cord segments T1 through L2
  • Sympathetic neurons produce the lateral horns of the spinal cord
  • Preganglionic fibers pass through the white rami communicantes and synapse in the chain (paravertebral) ganglia
  • Fibers from T5-L2 form splanchnic nerves and synapse with collateral ganglia
  • Postganglionic fibers innervate the numerous organs of the body

2.bin

Sympathetic Trunks and Pathways

  • The paravertebral ganglia form part of the sympathetic trunk or chain
  • Typically there are 23 ganglia – 3 cervical, 11 thoracic, 4 lumbar, 4 sacral, and 1 coccygeal

Sympathetic Chain Ganglia

  • Each paravertebral ganglion is connected to spinal nerves by 2 branches
  • White communicating ramus
  • Grey communicating ramus
  • Flow in 3 different ways

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Sympathetic Trunks and Pathways

  • A preganglionic fiber follows one of three pathways upon entering the paravertebral ganglia
  • Synapse with the ganglionic neuron within the same ganglion
  • Ascend or descend the sympathetic chain to synapse in another chain ganglion
  • Pass through the chain ganglion and emerge without synapsing

Neurotransmitters and Receptors

  • Acetylcholine (ACh) and norepinephrine (NE) are the two major neurotransmitters of the ANS
  • ACh is released by all preganglionic axons and all parasympathetic postganglionic axons
  • Cholinergic fibers – ACh-releasing fibers
  • Adrenergic fibers – sympathetic postganglionic axons that release NE
  • Neurotransmitter effects can be excitatory or inhibitory depending upon the receptor type

Cholinergic Receptors

  • The two types of receptors that bind ACh are nicotinic and muscarinic
  • These are named after drugs that bind to them and mimic ACh effects

Nicotinic Receptors

  • Nicotinic receptors are found on:
  • Motor end plates (somatic targets)
  • All ganglionic neurons of both sympathetic and parasympathetic divisions
  • The hormone-producing cells of the adrenal medulla
  • The effect of ACh binding to nicotinic receptors is always stimulatory

Muscarinic Receptors

  • Muscarinic receptors occur on all effector cells stimulated by postganglionic cholinergic fibers
  • The effect of ACh binding:
  • Can be either inhibitory or excitatory
  • Depends on the receptor type of the target organ

Adrenergic Receptors

  • The two types of adrenergic receptors are alpha and beta
  • Each type has two or three subclasses
    (1, 2, 1, 2 , 3)
  • Effects of NE binding to:
  •  receptors is generally stimulatory
  •  receptors is generally inhibitory
  • A notable exception – NE binding to 1 receptors of the heart is stimulatory whereas binding to B2 receptor on the bronchioles is inhibitory

Sympathetic vs. Parasympathetic
Receptor/NT Differences: Symp . Parasymp.

NT at Target Synapse Norepinephrine (adrenergic neurons) Acetylcholine (cholinergic neurons)
Type of NT Receptors at Target Synapse Alpha and Beta ( and ) Muscarinic
NT at Ganglion Acetylcholine Acetylcholine
Receptor at Ganglion Nicotinic Nicotinic

Local vs. Diffuse Effects

  • The parasympathetic division has more localized effects for two reasons:
  • ACH is quickly destroyed by acetylcholinesterase
  • Parasympathetic preganglionic axons synapse with few ganglionic neurons
  • The sympathetic effects are more diffuse:
  • NE is taken back up by the preganglionic neuron
  • Sympathetic preganglionic neurons synapse with many ganglionic neurons

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Duration/Location of Parasympathetic Effects

  • Parasympathetic preganglionic neurons synapse on only a few postganglionic neurons.

Would you expect parasympathetic activity to be widespread or local?

  • All parasympathetic fibers release ACh.
  • ACh is quickly broken down by what enzyme?

What can you say about the duration of parasympathetic effects?

Why Is Sympathetic Activity Diffuse?

  • Preganglionic fibers have their somata in the lateral horns of the thoracic and lumbar spinal cord.
  • Preganglionic fibers leave the cord via the ventral root and enter a white ramus communicans to enter a chain ganglion – which is part of the sympathetic trunk.
  • Prolonged by stimulation of ADRENAL MEDULLA

How Does the Brain Control the ANS?

  • The hypothalamus is the Boss:
  • Its anterior and medial regions direct parasympathetic function while its posterior and lateral regions direct sympathetic function
  • These centers exert control directly and via nuclei in the reticular formation (e.g., the cardiovascular centers in the MO, respiratory centers in MO and pons, etc.)
  • The connection of the limbic system to the hypothalamus mediates our “flight or flight” response to emotional situations.
  • The relationship btwn the hypothalamus and the amygdala and periaquaductal gray matter allow us to respond to fear (emotion).

REGULATION OF AUTONOMIC NERVOUS SYSTEM

HYPOTHALAMUS

  • Major control and integration center
  • Receives input on
  • Smell, taste
  • Temperature
  • Chemical composition

of blood

  • Visceral changes
  • Emotions
  • Sends commands through
  • Medulla and spinal cord

Lecture 4

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The hypothalamus is a critical site of much of autonomic control*Nuclei in centers here control the activities of the autonomic nervous system, and also the pituitary gland