Neurophysiology 3

Motor systems

Efferent output

homeostatic output
nonhomeostatic output

agents provocateurs

neurotransmitters

1. acetylcholine (Ach)

2. norepinephrine (NE)

3. dopamine (D)

hormones

1. epinephrine (Epi)

2. norepinephrine (NE)

enzyme degraders

1. cholinesterase (ACh-esterase)

2. monamine oxidase (MAO)

3. catechol-O-methyltransferase (COMT)

receptors

1. M1/M3 muscarinic

ACh

Gq activates phospholipase C

release of IP3 and DAG

increased Ca²⁺

M1: CNS, ganglia, parietal cell secretion

M3: contraction of smooth muscle/gland secretion

2. M2 muscarinic

ACh

coupled to K⁺ channel

slows heart rate

atropine

3. N1 nicotinic ligand-gated ion channel (Nn)

ACh

rapid opening of Na⁺ channel

depolarization of ganglia cells

trimethaphan

4. N2 nicotinic ligand-gated ion channel (Nm)

ACh

rapid opening of Na⁺ channel

depolarization of skeletal muscle

turbocurarine

5. alpha1 adrenergic

Gq activates phospholipase C

release of IP3 and DAG

increased Ca²⁺

contraction of smooth muscle

6. alpha2 adrenergic

dissociation of inhibitory Gi

inhibition of adenylyl cyclase

decreased activity

7. beta1 adrenergic

activation of stimulatory Gs

activation of adenylyl cyclase

increased cardiac activity

8. beta2 adrenergic

activation of stimulatory Gs

activation of adenylyl cyclase

relaxation or dilation of smooth muscle

9. D1 dopaminergic

dopamine

activation of stimulatory Gs

activation of adenylyl cyclase

vasodilatation in kidney

10. histamine

H1: stimulation of phospholipase C activity, and the subsequent contraction of smooth muscle (bronchiolar)

H2: stimulation of adenylyl cyclase activity, and in the stomach leads to release of acid into the lumen

H3: signal transduction pattern remains to be identified; it functions, however, to reduce the release of many neurotransmitters, including histamine itself

Autonomic vs. somatic motor systems

CNS	preganglionic fiber	ganglion	postganglionic fiber	effector
brain stem parasympathetic	long	ACh .. N	short	ACh .. M cardiac & smooth muscle gland cells nerve terminals
spinal cord sympathetic	short		long	ACh .. M sweat glands
				NE .. a, b cardiac & smooth muscle gland cells nerve terminals
				D .. D₁ renal vascular smooth muscle
		ACh .. N adrenal medulla	hormonal release of Epi & NE
spinal cord parasympathetic	long	ACh .. N	short	ACh .. M smooth muscle gland cells
spinal cord voluntary motor nerve	long			ACh .. N somatic skeletal muscle

Autonomic nervous system

two-neuron motor chain

CNS
preganglionic fibers

ganglion
postganglionic fibers

effector

sympathetic

sympathetic chain ganglia

collateral ganglia

adrenergic

alpha (a)

excitatory response in effector organ

beta

b₁

excitatory

b₂

inhibitory

parasympathetic

terminal ganglia

cholinergic

nicotinic receptors

autonomic ganglia

muscarinic receptors

effector cells

atropine is selective muscarinic blocker

release of neurotransmitter by varicosities

dual innervation of most visceral organs

sympathetic and parasympathetic tone

sympathetic and parasympathetic dominance

control levels of autonomic activities

spinal reflexes

urination, defecation, erection

medulla

cardiovascular, respiratory, digestive centers

hypothalamus

integration of autonomic, somatic, endocrine responses

frontal cortex

emotional autonomic responses

Somatic nervous system

motor neurons act as final common pathway

end plate potential (EPP) is larger than EPSP, thus resulting in action potential

acetylcholinesterase

short duration of ACh binding: ~10^–9 sec

vulnerabilities of the NMJ

black widow spider venom

causes explosive release of ACh

botulinum toxin

blocks release of ACh (~10^–4 mg is lethal)

curare

competitively binds with ACh receptor

organophosphates

irreversibly inhibit AChE

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