Introduction to the ANS

What is the Autonomic Nervous System (ANS)?

• ANS is part of the peripheral nervous system – contains 3 divisions

  • Sympathetic and parasympathetic nervous systems linking central nervous system (CNS) to peripheral organs e.g. heart, lungs, glands

  • Enteric nervous system  - intrinsic control of  GI tract (see later sessions)

What does the Somatic nervous system do

provides voluntary motor information to skeletal muscle

+ transmits sensory information in afferent fibres to CNS, e.g. pain

What does the autonomic nervous system do

Provides ‘involuntary’ information from CNS to peripheral organs

e.g. increasing heart rate during exercise

What systems and organs does the ANS regulate

Cardiovascular system, respiratory system, alimentary system, urinal-genital system, vision

What diseases involve the ANS

Hypertension, heart failure, asthma, IBS, Gluacoma

Describe what cells the ANS works on in the heart

SA node cells – heart rate

AV node cells – speed of conduction

Muscle cells – force of contraction

What does the ANS do to smooth muscle cells

ANS contracts or relaxes smooth muscle cells

e.g. blood vessels, airways, intestine, bladder

What does the ANS do to gland cells

Cause secretions

e.g. saliva, sweat, gastric acid,

regulates insulin + glucagon release

Describe the 2 efferent neurons in the ANS

The autonomic nervous system (ANS) has two main types of efferent neurons:

1. Preganglionic Neurons: These neurons originate in the central nervous system (CNS) and extend to autonomic ganglia. They release acetylcholine (ACh) as a neurotransmitter.

2. Postganglionic Neurons: These neurons extend from the ganglia to target organs. They can release either acetylcholine (in the parasympathetic division) or norepinephrine (in the sympathetic division) as neurotransmitters.

What are the differences between the ANS and the somatic motor system

• ANS – Two efferent neurones arranged “in series” (defined as “in succession”)

• Different to somatic motor system – where a single neurone connects CNS to skeletal muscle (termed “efferent pathways”)

Describe the differences in basic anatomy between the parasympathetic and sympathetic nerves in the ANS

• Origin:

  • Sympathetic: Arises from the thoracolumbar region (T1-L2) of the spinal cord.

  • Parasympathetic: Arises from the craniosacral region (brainstem and S2-S4).

• Ganglia Location:

  • Sympathetic: Ganglia are located close to the spinal cord (sympathetic chain).

  • Parasympathetic: Ganglia are located near or within the target organs.

• Fiber Length:

  • Sympathetic: Short preganglionic fibers and long postganglionic fibers.

  • Parasympathetic: Long preganglionic fibers and short postganglionic fibers.

• Neurotransmitters:

  • Sympathetic: Primarily norepinephrine.

  • Parasympathetic: Primarily acetylcholine.

List the parasympathetic nerves

• Cranial nerves III, VII, IX, X

  • Oculomotor nerve (III) : Eye

  • Facial (VII): Lacrimal glands

  • Glossopharyngeal (IX): Salivary glands

  • Vagus (X): Heart & Bronchi, Upper GI

• Sacral nerves (S2-S4)

Where do the sympathetic nerves arise

T1 – L2-3

Describe parasympathetic pre and post ganglionic fibers

preganglionic: from nuclei in CNS or sacral region of spinal cord

Exits CNS from cranial nerves or pelvic splanchnic nerve

Short postganglionic fibers to target organs - more specific & limited innervation to a particular organ

Describe sympathetic pre and post ganglionic fibers

• Short preganglionic:

  • cell bodies in grey matter of T1 & L2 of spinal cord

  • Axons pass out in ventral ramus & enter paravertebral sympathetic chain

• Long postganglionic fibres

  • travel within walls of blood vessels-axons can spread out & innervate many organs.

Describe chemical transmission in the ANS

• Similar to CNS and neuromuscular junction

• Autonomic nerves communicate at synapses

  • With other autonomic nerves - at ganglia

  • With peripheral organs - post-ganglionic junctions

• Use chemical transmission

What are the eseential processes in chemical transmission of the ANS

1. Synthesis of neurotransmitter

2. Storage of NT in vesicles

3. Arrival of AP at synaptic terminal

4. Terminal depolarises, activation of voltage-dependent Ca²⁺ channels, influx of Ca²⁺ ions

5. Ca²⁺-dependent release of NT

6. NT binds to receptor induces response

7. Uptake/breakdown of NT

What neurotransmitter do all preganglionic fibers release

• All pre-ganglionic sympathetic and parasympathetic nerves release

  • Ach acts at nicotinic receptors (Nic) at ALL autonomic ganglia

What neurotransmitter do post ganglionic parasympathetic nerves release

• Release Ach which acts at  muscarinic receptors (Mus)

• Exception - non-adrenergic-non-cholinergic (NANC) nerves which use other chemicals such as nitric oxide

What neurotransmitter do post ganglionic sympathetic nerves release

• Release NA which acts at a or b adrenoceptors

  •Exception – In sweat glands, sympathetic nerves release Ach which acts at muscarinic receptors (Mus)

What does Stimulation of pre-ganglionic sympathetic fibres release from the adrenal glands

Stimulation of pre-ganglionic sympathetic fibres release adrenaline (80%) and noradrenaline (20%) from adrenal glands: There are NO post-ganglionic fibres involved

What does the sympathetic nervous system do to the body

1. Heart rate and contractility  – ­ cardiac output, more blood to muscle

2. Diversion of blood to skeletal muscle – more energy for movement

3. Blood pressure – consequence of diversion of blood

4. Air into to lungs –  ­ O₂ uptake/CO₂ removal

5. Dilation of pupils – more light into eye, better vision

6. Fuel in muscles – breakdown glycogen/lipids

7. Sweating – temperature regulation

8. ¯ Non-essential functions (¯ GI tract activity) – limit energy use

Happy Dogs Bark At Dogs For Some Nice treats.

What structures does the sympathetic nervous system act on

Heart, blood vessels, kidney, metabolism, lungs, eye, sweat glands, GI tract

What does the parasympathetic nervous system do to the body

1. Accommodation of the eye - Rest (newspaper reading)

2. Bladder : micturition - Rest (time for a pee)

3. Slowing heart rate – Rest

4. GI tract motility/secretions - Time for eating / digestion

5. Metabolism - Insulin release, glucose uptake and storage

6. Bronchoconstriction - Rest, less O₂ intake required

ABS GMB

Give examples of where Sympathetic and Parasympathetic innervate same organ to produce opposite effects

e.g. Heart - sym increase ­ / parasym decrease = heart rate and force of contraction

Intestine- sym decrease / parasym increase -­ motility

Iris - sym dilates / parasym constricts pupil

Give examples of where Sympathetic and Parasympathetic innervate same organ to produce the same effects

both increase secretions from salivary gland

What organs are only innervated by the sympathetic nerve

sweat glands, kidney, blood vessels

What organs are only innervated by the parasympathetic nerves

pancreas, secretory cells of stomach, lungs

(but airways contain β-adrenoceptors, modulated by circulating adrenaline)

What is the ANS important for mediating and how does it achieve this

ANS is of primary importance in mediating homeostatic involuntary mechanisms – achieved through feedback systems

Give an example of an alpha 1 agonist

Phenylephrine

Noradrenaline

(Used in Circulatory collapse, sepsis)

Give an example of an alpha 1 antagonist

Prazosin (anti-hypertensive)

Give an example of an Beta 1 antagonist

Atenolol (angina, chronic HF)

Give an example of an Beta 1 agonist

Dobutamine

(cardiac arrest)

Give an example of an Beta 2 agonist

Salbutamol (asthma)

Give an example of an Muscarinic agonist

Pilocarpine (glaucoma)

Bethanechol (urinary incontinence)

Give an example of an Muscarinic antagonist

Atropine (raise HR)

Hyoscine (IBS)

Give an example of an neuromuscular junction antagonist

Suxamethonium (muscle relaxant during surgery)