efferent nervous system

Question 1

OVERVIEW OF EFFERENT PNS

Answer 1

Efferent division of PNS: communication link by which the CNS controls the activities of the effector organs

• Autonomic nervous system: involuntary e.g. cardiac
  and smooth muscles, exocrine and some endocrine
  – Sympathetic nervous system
  – Parasympathetic nervous system
• Somatic nervous system: voluntary e.g. skeletal muscles
• Allows CNS to control muscles, glands, secretions, movement: homeostasis by being connected to the CNS

Question 2

AUTONOMIC NERVOUS SYSTEM

Answer 2

Consists of two neurons: preganglionic fiber and postganglionic fiber

central nervous system with the cell body of preganglionic cell

preganglionic fiber

in autonomic ganglion:
preganglionic terminal releases preganglionic neurotransmitter onto the cell body of the postganglionic fiber

postganglionic varicosity releases
postganglionic NT onto effector organ

Releases only two different neurotransmitters on the structures it controls: acetylcholine and norepinephrine

Question 3

SUBDIVISIONS OF ANS

Answer 3

Sympathetic nervous system:
– Nerve fibers originate from thoracic and lumbar regions of spinal cord
– Preganglionic fibers: short, release acetylcholine (ACh)
– Postganglionic fibers: long, release norepinephrine (adrenergic)
– Synapse within the sympathetic ganglion chain/collateral ganglion

• Parasympathetic nervous system:
  – Nerve fibers originate from the cranial and sacral levels of the CNS
  – Preganglionic fibers: long, release acetylcholine (ACh)
  – Postganglionic fibers: short, release acetylcholine (cholinergic)
  – Most synapse at terminal ganglia near effector organs

Question 4

spinal cord

Answer 4

8 cervical nerves

12 thoracic nerves

5 lumbar nerves

5 sacral nerves

1 coccygeal nerve

cauda equina is made up of the 5 lumbar nerves & 5 sacral nerves

Question 5

autonomic effectors and nerves

Answer 5

parasympathetic preganglionic
- releases ACh onto the nicotinic receptor
- it then releases ACh onto muscarinic receptor on cardiac muscle

sympathetic preganglionic –this is the sympathetic ganglion chain
- releases ACh onto the nicotinic receptor
- it then releases NE onto alpha receptor onto stomach (smooth muscle)

sympathetic preganglionic –this is the sympathetic ganglion chain
- releases ACh onto the nicotinic receptor in adrenal medulla
- it then releases E & NE into blood stream
- E & NE can now bind onto veins (smooth muscle)

sympathetic preganglionic –this is the collateral ganglion
- releases ACh onto the nicotinic receptor
- it then releases E & NE to bind onto most exocrine gland and some endocrine glands onto the B1 & B2 receptors

parasympathetic preganglionic fiber –this is the collateral ganglion
- releases ACh from terminal ganglion onto the nicotinic receptor
- it then releases ACh to bind onto muscarinic receptors of adipose tissues

Question 6

CONTROL OF ANS

Answer 6

Involuntary responses of visceral organs: e.g. circulation, digestion, sweating, urination

• Visceral organs receive dual innervation: signaled by both subdivisions of the ANS
• Both subdivisions are active in controlling the activity of an internal organ, one may predominate at a given moment: sympathetic or parasympathetic tone, exert opposite effects!

Question 7

SYMPATHETIC DOMINANCE

Answer 7

Prepare the body for emergency or stressful situations: flight or fight response
– Dilates blood vessels supplying skeletal muscle
– Increases heart and respiratory rate
– Converts glycogen to glucose in liver
– Promotes sweating
– Dilates pupils of the eye and bronchioles
– Inhibits digestion and urination

• Aimed at increasing flow of oxygenated, nutrient rich blood to skeletal muscles: strenuous physical activity

Question 8

PARASYMPATHETIC DOMINANCE

Answer 8

Dominates in quiet, relaxed situations – Decreases heart and respiratory rate
– Promotes digestion and urination
* General housekeeping activities, conserves energy

Question 9

DUAL/RECIPROCAL INNERVATION

Answer 9

Advantage: allows precise control over the activity of an effector organ

• Exceptions:
  – Sympathetic only: arterioles and veins, sweat glands
  – Salivary glands: has both systems, but increases salivation

Question 10

TARGET SPECIFICITY

Answer 10

ACh and NE bind to several different receptors: target specificity

• Cholinergic receptors: bind to ACh
  – Nicotinic receptors: postganglionic cell bodies, ion channels, depolarization
  – Muscarinic receptors: effector organs, G-protein coupled receptors
• Adrenergic receptors: bind to NE & E, G-protein coupled receptors
  – Alpha receptors: more sensitive to NE
• Alpha 1(1): Ca2+ second messenger system, excitatory response
• Alpha 2 (2): blocks c-AMP production, inhibitory response

– Beta receptors: c-AMP pathway
* Beta 1 (1): equally sensitive to E & NE, found in heart, excitatory response
* Beta 2 (2): more sensitive to E, inhibitory response

• Effect of drugs: agonists and antagonists, specificity

Question 11

TARGET SPECIFICITY

Answer 11

Multiple receptors present for NE and ACh

• Receptors are present at different locations
• Receptors are coupled to different signaling pathways
• Different kinds of receptors may have different sensitivity to the neurotransmitters

Question 12

SOMATIC NERVOUS SYSTEM

Answer 12

Signals skeletal muscle via motor neurons
– Axons: constitute SNS
– Cell bodies: lie in ventral horn

• Different from ANS
  – Axons do not synapse in ganglion
  – Can only stimulate skeletal muscle contraction
• Somatic motor neurons: final common pathway
• Diseases: Polio, Lou Gehrig’s disease (ALS)

Question 13

NEUROMUSCULAR JUNCTION

Answer 13

Efferentneuron(myelinated)→terminal branches (unmyelinated) → neuromuscular junctions
– Terminal button: knob-like terminal at muscle fiber
– Motor end plate: muscle cell membrane below TB

• ACh is the chemical messenger (NT), released when AP reaches the terminal button
• Leads to the generation of end plate potential

Question 15

GENERATION OF END PLATE POTENTIAL

Answer 15

An AP at the axon terminal opens voltage-gated calcium channels in the terminal button, allowing calcium ions to diffuse into the terminal button from the ECF

• ACh is released by exocytosis from vesicles in the TB
• ACh diffuses through the space between the nerve cell and muscle cell & binds to receptor sites on the motor end plate of the muscle cell
• This opens channels for sodium influx into the ICF of the muscle cell, which produces a local current flow that opens adjacent sodium channels in the motor end plate
• An action potential is initiated through the muscle fiber
• Acetylcholinesterase (AChE) terminates the activity of ACh at NMJ

so ACh allows for the opening of sodium channels and allows Na+ to flow into the cell to generate an AP

Question 16

NEUROMUSCULAR JUNCTION

Answer 16

Vulnerable to chemical agents and diseases

• Toxins: alter the release of ACh, respiratory failure
  – Black widow spider venom: explosive release of ACh
  – Botulinum toxin: blocks release of ACh
• Curare: blocks the receptors at motor end plate
• Organophosphates: irreversibly inhibit AChE
• Myasthenia gravis: loss of ACh receptors at MEP, auto- immune disease, extreme muscular weakness