Autonomic Nervous System

PNS (Sensory Divison)

Somatic Sensory:

• General: touch, pain, pressure, vibration, temperature, and proprioception in skin, body wall, and limbs

• Special: hearing, equilibrium, vision, smell

Visceral Sensory:

• General: stretch, pain, temperature, chemical changes, and irritation in viscera; causes nausea and hunger

• Special: taste

PNS (Motor Division)

Somatic Motor:

• General: motor innervation of all skeletal muscles

Visceral Motor:

• General: motor innervation of smooth muscle, cardiac muscle, and glands; equivalent to autonomic nervous system (ANS)

  • Parasympathetic division and sympathetic division

Autonomic Nervous System - Interoception

• Automatic nervous system control of functions that are critical for survival

• Sometimes referred to as the “lizard brain”

Homeostasis

• Maintenance of a constant internal environment in the face of a changing external environment

• Some homeostatic actions are under our conscious control, for example, putting on a sweater when cold. Voluntary: somatic nervous system.

• Most homeostatic mechanisms operate without our conscious awareness or control, for example heart rate and digestion. Involuntary: autonomic nervous system

Hypothalamus

Temperature, thirst, and hunger

Midbrain

Pupillary reflex

Pons

• Respiration

• Urinary bladder

Medulla

• Respiration

• Cardiac

• Vomiting

• Swallowing

Spinal Cord

Visceral reflexes

Autonomic Control Centres in the Brain

These centers provide bidirectional control of peripheral organs via the sympathetic and parasympathetic systems.

• Hypothalamus

• Midbrain

• Pons

• Medulla

• Spinal cord

Hypothalamic circuits for motivated behaviours:

• Fear

• Social

• Eating

• Drinking

Controlling the Sensation of Hunger

• Arcuate nucleus projections to PVH regulate feeding behaviour

• PVH projection suppression evokes feeding from motivational centres but not satiety centres or ANS output

PAG

• motivational center

• endorphins

DR

• motivational center

• serotonin

PBN

nausea/satiety center

LC

arousal center

NTS

visceral-sensory

DVC

visceral-motor

Sympathetic and parasympathetic activity mediate opposing states:

Example:

• As gut motility increases, heart rate decreases

• As heart rate increases, gut motility decreases

Antagonistic Control of the Autonomic Function

• The sympathetic and parasympathetic divisions are tonically active.

• Most internal organs are under antagonistic control of both divisions.

  • One autonomic branch is excitatory and the other branch is inhibitory

  • Example: blood pressure

    • Sympathetic response: increases pressure; Parasympathetic response: decreases pressure

Role of the Sympathetic Division:

Mobilizes the body resources for activity, the “fight-or-flight” system:

• Readies the body for exercise and/or threats

  • Heart rate, blood pressure, breathing rate elevated

  • Blood flow is shunted to skeletal muscles and heart

  • Bronchioles dilate

  • Liver releases glucose

  • Pupils dilate

  • GI tract inhibited

Sympathetic nervous system (SNS) activation:

• Involuntary

◦ Special case: Can a trained spy can hide their autonomic responses to beat a polygraph test?

The common anatomy of autonomic output:

• CNS relays information through autonomic ganglion via ACh

• Autonomic pathways consist of two neurons that synapse in an autonomic ganglion

• This applies to both the sympathetic and parasympathetic nerves

Autonomic output at the target organ:

• Sympathetic nerves = norepinephrine

• Parasympathetic nerves = acetylcholine

The adrenal medulla is a modified sympathetic output:

• The adrenal medulla chromaffin cells serve as postsynaptic sympathetic neurons.

• Instead of axons directed at their targets, these “neurons” release transmitters into the blood which circulate throughout the body.

• Both epinephrine (80%) and norepinephrine (20%) are released (aka adrenaline and noradrenaline).

Sympathetic nervous system anatomy:

• Sympathetic pathways originate in the lateral horn of the thoracic and lumbar regions of the spinal cord (T1-L2; thoracolumbar).

• This system provides sympathetic innervation to the entire body.

Dorsal Horn

cell bodies of interneurons on which afferent neurons terminate

Lateral Horn

cell bodies of autonomic efferent nerve fibres

Ventral Horn

cell bodies of somatic efferent neurons

Ganglion

a group of neuron cell bodies located outside of the central nervous system

The location of the sympathetic ganglia:

• Sympathetic ganglia are found primarily in two chains that run along either side of the bony vertebral column, with additional ganglia along the descending aorta at the origin of major abdominal arteries (for example, the celiac and mesenteric ganglia)

• The ganglia are close to the spinal cord, and so tend to have short preganglionic axons and long postganglionic axons

Preganglionic fibers from T1-L2 enter the paravertebral sympathetic trunk (a chain of ganglia) and then either:

1. Synapse in the trunk and postsynaptic fibers join somatic nerves to the body (vessels and sweat glands);

2. Move up along the trunk to reach cervical ganglia where they synapse to give rise to postganglionic fibers that innervate the head and the thoracic viscera;

3. Pass through the trunk to form splanchnic nerves which synapse in collateral ganglia (celiac, mesenteric ganglia etc.) to innervate abdominal viscera;

4. Pass through the trunk to reach the adrenal gland directly; or

5. Synapse in the trunk and continue as sacral splanchnic nerve to pelvic organs.

Sympathetic (Thoracolumbar) Outputs

• Eye

• Blood vessels

• Salivary glands

• Heart

• Lung

• Liver and gallbladder

• Stomach

• Spleen

• Adrenal medulla

• Kidney

• Small and large intestines

• Genitalia

Sympathetic (paravertebral) Ganglia

• Preganglionic acetylcholine neurons are in the spinal cord

• Presynaptic fibers enter the ganglionic chain to synapse with adrenergic neurons

• Postganglionic fibers join the corresponding spinal nerves and carry on to targets

Role of the Parasympathetic Division

• Promotes maintenance activities and conserves body energy

• Typified by a person relaxing after a meal (rest and digest)

  • Reduction in blood pressure, heart rate, and respiratory rate

  • Increase in activity of the gastrointestinal tract

  • Increased saliva production

  • Constriction of the pupils

Parasympathetic system is divided into cranial and sacral components:

• The salivary glands and eye smooth muscles are innervated by cranial nerves (III, VII and IX).

• The vagus nerve (CN X) descends through the thoracic and abdominal cavities, innervating all the viscera.

• The sacral cord (S2-4) gives rise to parasympathetic fibers that innervate the pelvic organs.

• Note that parasympathetic nerves do not innervate sweat glands (sympathetic only).

• Parasympathetic ganglia are located either in or near their target organs.

• Long preganglionic axons and short postganglionic axons

Atropine (from Atropa Belladonna)

• Blocks ACh and causes pupillary dilation.

• Can be used to prevent salivation to maintain airways during surgery.

• Also is the active component in the poison derived from Deadly Nightshade.

Vagus nerve (CN X)

• Contains about 75% of all parasympathetic fibers.

• It provides parasympathetic innervation to all visceral organs except for the pelvic organs, which receive parasympathetic innervation from the sacral cord (S2-4).

Neurotransmitter Receptors Subtypes

• How receptor subtypes work to produce diversified responses

• Receptor subtypes are structurally related so that they all bind and respond to the same transmitter.

• However, they are coupled to different cellular proteins and induce different signalling cascades for dramatically different effects.

Adrenergic Receptors

Bind to norepinephrine and epinephrine:

• Alpha (α) receptors (subtypes α1 (present in arterioles and is excitatory), α2)

• Beta (β) receptors (subtypes β1 (present in the heart and is excitatory), β2 (present in bronchioles and skeletal muscles and is inhibitory), β3)

Beta Blockers

• reduce blood pressure by antagonizing beta-adrenergic receptors.

• the β2 receptor agonist, salbutamol, causes smooth muscle relaxation for treatment of bronchospasm or asthma

Muscarinic Receptors

All effector cells stimulated by postganglionic cholinergic fibers (i.e. the parasympathetic fibers to target organs):

• The effect of ACh at muscarinic receptors can be either inhibitory or excitatory, depending on the receptor subtypes of the target organ:

  • For example, the M1 and M3 receptors are excitatory and are present in the exocrine glands and gut smooth muscles; the M2 receptors are inhibitory and are present in the heart and bladder. When parasympathetic activities are increased, the heart slows down, gut motility ramps up.