Neuroanatomy

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Neuroanatomy

• Autonomic Nervous System

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Autonomic Nervous System

• Auto self nomic law
• The autonomic nervous system (ANS) includes
  autonomic sensory neurons, integrating centers in
  the central nervous system (CNS), autonomic motor
  neurons, and the enteric division.
• The ANS is also referred to as the visceral
  nervous system or involuntary nervous system.
• It acts as a control system.
• It operates largely without conscious control
  however, centers in the hypothalamus and brain
  stem do regulate ANS reflexes.

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Flow of nerve impulses in the ANS

• (1) Autonomic sensory neurons in visceral organs
  and blood vessels propagate into (2) integrating
  centers in the CNS. Then, impulses in (3)
  autonomic motor neurons propagate to various
  effector tissues, thereby regulating the activity
  of smooth muscle, cardiac muscle, and many
  glands. (4) The enteric division is a
  specialized network of nerves and ganglia forming
  an independent nerve network within the wall of
  the gastrointestinal tract.

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Functions of ANS

• The ANS affects heart rate, digestion,
  respiratory rate, salivation, perspiration,
  pupillary dilation, micturition (urination), and
  sexual arousal.
• Most autonomous functions are involuntary.
• Some ANS actions can work with some degree of
  conscious control
• Breathing
• Swallowing
• Sexual arousal
• Heart rate

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Divisions of the ANS

• The ANS is classically divided into two
  subsystems the parasympathetic nervous system
  (PSNS) and sympathetic nervous system (SNS).
• For some functions these systems operate
  independently and for others they operate
  co-operatively.
• In many cases the PSNS and SNS have opposite
  actions with one activating a physiological
  response and the other inhibiting it.
• The enteric nervous system is also considered to
  be a part of the ANS.

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Afferent and Efferent

• ANS functions can be divided into sensory
  (afferent) and motor (efferent) subsystems.

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Somatic Nervous System

• The somatic nervous system includes both sensory
  and motor neurons.
• Sensory neurons convey input from receptors for
  somatic senses (tactile, thermal, pain,
  proprioceptive) and special senses (sight,
  hearing, taste, smell, equilibrium).
• These sensations are normally consciously
  perceived.
• Somatic motor neurons innervate skeletal muscles
  to produce both voluntary and involuntary
  movements.
• When a somatic motor neuron stimulates a muscle,
  it contracts the effect is excitation.
• If it fails to stimulate a muscle it becomes
  paralyzed.
• A few skeletal muscles, such as those in the
  middle ear, are controlled by reflexes and cannot
  be contracted voluntarily.

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Autonomic Sensory Neurons

• The main input to the ANS comes from autonomic
  (visceral) sensory neurons. These neurons are
  primarily associated with interoceptors (monitor
  the internal environment).
• Most of the time, these signals are not
  consciously perceived although, intense
  activation of interoceptors may produce conscious
  sensation.
• Pain sensations from damaged viscera and angina
  pectoris (chest pain) form inadequate blood flow
  to the heart can produce conscious sensation.

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Autonomic Motor Neurons

• Autonomic motor neurons regulate visceral
  activities by either increasing (exciting) or
  decreasing (inhibiting) activities in the
  effector tissues (cardiac muscle, smooth muscle,
  and glands).
• Examples of autonomic motor responses include
  changes in the diameter of the pupils, dilation
  and constriction of blood vessels, adjustment of
  the rate and force of the heartbeat.
• Unlike skeletal muscle, tissue innervated by the
  ANS often continue to function to some extent
  even if their nerve supply is damaged.
• The heart continues to beat when it is removed
  for transplantation, smooth muscle lining the GI
  tract contracts rhythmically on its own, and
  glands produce some secretions in the absence of
  ANS control.

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Two Divisions of ANS Output

• Unlike the somatic output (motor), the output
  portion of the ANS has two divisions
  sympathetic division and parasympathetic
  division.
• Most organs have dual innervation (both PSNS and
  SNS innervation).
• In some organs, nerve impulses from one division
  stimulate the organ to increase its activity
  (excitation) and impulses from the other division
  decrease its activity (inhibition).

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Sympathetic Nervous System

• The sympathetic division is often called the
  fight-or-flight division.
• Sympathetic activities result in increased
  alertness and metabolic activities in order to
  prepare the body for an emergency situation.
• Physical activity and emotional stress can
  trigger sympathetic activities.
• Effects of sympathetic stimulation rapid heart
  rate, faster breathing rate, dilation of pupils,
  dry mouth, sweaty but cool skin, dilation to
  blood vessels to organs involved in combating
  stress (heart and skeletal muscles), constriction
  of blood vessels to organs not involved in
  combating stress (GI tract and kidneys), and
  release of glucose from the liver).

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Parasympathetic Division

• The parasympathetic division is often referred to
  as the rest-and-digest division because its
  activities conserve and restore body energy
  during times of rest or digesting a meal.
• The majority of the nerve output is directed
  towards smooth muscle and glandular tissue of the
  gastrointestinal and respiratory tracts.
• The parasympathetic division conserves energy and
  replenishes nutrients.

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Comparison of Somatic and Autonomic Nervous
Systems
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Comparison of Somatic and Autonomic Nervous
Systems
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Two-neuron Efferent Pathway

• The ANS differs from the somatic nervous system
  in that it requires a sequential two-neuron
  efferent pathway.
• The preganglionic neuron will synapse with a
  postganglionic neuron before innervating the
  target organ.
• The first of the two motor neurons is called the
  preganglionic neuron. The cell body is located
  in the brain or spinal cord. It exits the CNS as
  part of a cranial or spinal nerve.
• It synapses with a postganglionic neuron in an
  autonomic ganglion, which is the second neuron in
  the autonomic pathway. The postganglionic neuron
  is located entirely in the PNS.

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Preganglionic Neuron

• Sympathetic Division
• Thoracolumbar division cell bodies in the
  lateral horns of the 12 thoracic segments and the
  first two (sometimes three) lumbar segments
• Thoracolumbar outflow axons
• Parasympathetic Division
• Craniosacral division cell bodies in the nuclei
  of four cranial nerves (III, VII, IX, X) and
  the lateral gray matter of the second through
  fourth sacral segments
• Craniosacral outflow axons

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Structure of the Sympathetic Division
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Structure of the Parasympathetic Division
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Adrenergic and Cholinergic Receptors
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Autonomic Ganglia

• Sympathetic ganglia
• Parasympathetic ganglia

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Sympathetic Ganglia

• Sympathetic ganglia sites of synapse between
  sympathetic preganglionic and postganglionic
  neurons.
• Sympathetic trunk ganglia (also called vertebral
  chain ganglia or paravertebral ganglia). Lie in
  a vertical row on either side of the vertebral
  column. Extend from the base of the skull to the
  coccyx. Innervate organs above the diaphragm
  (head, neck and heart).
• Superior, middle, and inferior cervical ganglia.
• Prevertebral (collateral) ganglia. Lie anterior
  to the vertebral column. Innervate organs below
  the diaphragm.
• Celiac ganglion, superior mesenteric ganglion,
  inferior mesenteric ganglion, aorticorenal
  ganglion, and renal ganglion.

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Parasympathetic Ganglia

• Parasympathetic ganglia sites of synapse
  between preganglionic parasympathetic neurons and
  postganglionic neurons in the terminal
  (intramural) ganglia.
• Terminal ganglia in the head have specific names
  ciliary ganglion, pterygopalatine ganglion,
  submandibular ganglion, and otic ganglion.
• The remaining terminal ganglia do not have
  specific names.

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Postganglionic Neuron

• Axons of preganglionic neurons pass to
  sympathetic trunk ganglia and synapse with
  postganglionic neurons.
• A single sympathetic preganglionic fiber has many
  axon collaterals and may synapse with 20 or more
  postganglionic neurons. This divergence explains
  why many sympathetic responses affect almost the
  entire body simultaneously.
• After exiting their ganglia, the postganglionic
  axons typically terminate in several visceral
  effectors.
• Axons of preganglionic neurons of the
  parasympathetic division pass to terminal ganglia
  near or within a visceral effector.

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Comparison of Sympathetic And Parasympathetic
Divisions of ANS
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Effects of Sympathetic And Parasympathetic
Divisions of ANS
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Effects of Sympathetic And Parasympathetic
Divisions of ANS
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Autonomic Plexuses

• Thorax
• Cardiac plexus innervates the heart
• Pulmonary plexus innervates the bronchial tree
• Abdomen and Pelvis
• Celiac (solar) plexus largest plexus
  innervates the stomach, spleen, pancreas, liver,
  gallbladder, kidneys, adrenal medulla, testes,
  ovaries
• Superior mesenteric plexus innervates the small
  and large intestines
• Inferior mesenteric plexus innervates the large
  intestine
• Hypogastric plexus innervates the pelvic
  viscera
• Renal plexus innervates the renal arteries
  within the kidneys and ureters

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Postganglionic Neurons in the Sympathetic Division
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Autonomic Plexuses in the Thorax, Abdomen, and
Pelvis
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Sympathetic Trunk Ganglia

• Cervical Portion
• Thoracic Portion
• Lumbar Portion
• Sacral Portion

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Pathways from Sympathetic Ganglia to Visceral
Effectors

• Spinal nerves (31 pairs) innervate visceral
  effectors of the skin of the neck, trunk, and
  limbs including sweat glands, smooth muscle in
  blood vessels, arrector pili muscles.
• Gray rami communicantes structures containing
  sympathetic postganglionic axons that connect the
  ganglia of the sympathetic trunk to the spinal
  nerves.
• Cephalic periarterial nerves innervate visceral
  effectors of the skin of the face (sweat glands,
  smooth muscle of blood vessels, arrector pili
  muscles. Innervates visceral effectors of the
  head (smooth muscle of the eye, lacrimal glands,
  pineal gland, nasal mucosa, and salivary glands.
• Sympathetic nerves innervates visceral
  effectors in the thoracic cavity including the
  heart and lungs.
• Splanchnic nerves innervates the organs of the
  abdominopelvic cavity.

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Sacral Parasympathetic Outflow
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Parasympathetic Division of the ANS

• Preganglionic neurons originate from the cranial
  nerves III, VII, IX, X as well as the sacral
  spinal nerves S2-4.
• The presynaptic neuron usually synapses with 4-5
  postsynaptic neurons, all of which supple a
  single visceral effector.

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Enteric Division

• Plexuses
• Myenteric plexus
• Submucosal plexus
• This system controls motility and secretory
  functions of the gastrointestinal tract.

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Cholinergic Neurons

• Cholinergic neurons acetylcholine Ach
• Cholinergic neurons include
• All sympathetic and parasympathetic preganglionic
  neurons
• Sympathetic postganglionic neurons that innervate
  most sweat glands
• All parasympathetic postganglionic neurons

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Cholinergic Receptors

• Cholinergic receptors bind with acetylcholine
• Two types
• Nicotinic receptors
• Muscarinic receptors

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Adrenergic Neurons and Receptors

• Release noripinephrine (noradrenalin)
• Most sympathetic postganglionic neurons are
  adrenergic.
• Two types
• Alpha receptors
• Beta receptors

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Cholinergic and Adrenergic Neurons in the
Autonomic Nervous System
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Autonomic Tone

• Autonomic tone is a balance between sympathetic
  and parasympathetic activity.
• Autonomic tone is regulated by the hypothalamus.

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Sympathetic Responses

• Stress ?sympathetic system ?fight-or-flight
  response.
• ? production of ATP
• Dilation of pupils
• ?heart rate and blood pressure
• Dilation of airways
• Constriction of blood vessels that supply the
  kidneys and gastrointestinal tract
• ?blood supply to the skeletal muscles, cardiac
  muscle, liver and adipose tissue
• ?glycogenolysis ?blood glucose
• ?lipolysis

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Parasympathetic Responses

• Rest-and-digest response
• Conserve and restore body energy
• ?digestive and urinary function
• ?Body functions that support physical activity

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Integration and Control of Autonomic Functions

• Direct innervation brain stem and spinal cord
• Hypothalamus is the major control and integration
  center of the ANS
• It receives input from the limbic system