Lecture 15 - The ANS, ENS, and Smooth Muscle

Question 1

How does smooth muscle contract?

Answer 1

Contract through thick and thin filaments, but these filaments are not arranged into sarcomeres. There is only 1 nucleus for every smooth muscle, unlike skeletal muscles

Question 2

2 subtypes of smooth muscle tissue and examples of where they can be found

Answer 2

1. Multi-unit smooth muscle - excitation occurs through synapses. They are stimulated by NT release
   Ex. Eye
2. Visceral smooth muscle (aka single-unit) - excitation occurs through gap-junction coupling and pacesetter cells. Stimulated by cell-to-cell connections
   Ex. Small intenstine

Question 3

Pacesetter cell

Answer 3

A specialized cell that can depolarize independently

Question 4

How does smooth muscle excitation-contraction coupling work?

Answer 4

It is Ca2+ dependent, but works differently than skeletal muscles. Increased Ca2+ binds to free calmodulin, which leads to myosin head activation

Question 5

True or false: Troponin is present in both smooth and skeletal muscle

Answer 5

False - caldesmon is present in smooth muscle instead of troponin

Question 6

What does the autonomic nervous system (ANS) consist of?

Answer 6

It consists of efferent pathways, originating in the brainstem and spinal cord, that control visceral effectors

NOTE: many, but not all of these effector cells are smooth muscle cells

Question 7

What is the difference between the synapses in the PNS and ANS?

Answer 7

PNS - somatic efferent pathways are monosynaptic, which is located at the NMJ

ANS - autonomic efferent pathways are typically disynaptic. Synapse #1 is the autonomic ganglion and synapse #2 is the effector cell. 2 autonomic neurons are called preganglionic and postganglionic

Question 8

Where do ANS efferent neurons often synapse in the digestive system?

Answer 8

They often synapse onto neurons of the ENS, rather than onto effector cells

Question 9

3 features of using metabotropic receptors

Answer 9

1. Longer-lasting responses
2. Diverse biochemical effects (not just changes in Vm)
3. Stimulatory or inhibitory responses to the same NT

Visceral effector cells usually use these receptors

Question 10

What are the 2 divisions of the ANS?

Answer 10

1. Parasympathetic division
2. Sympathetic division

Question 11

Describe the concept of dual innervation

Answer 11

The ANS shows dual innervation of target organs. Most of the time this is antagonistic, with each division stimulating opposite effects

Question 12

What are the 2 different physiological states associated with SNS and the PSNS?

Answer 12

SNS is associated with fight or flight while PSNS is associated with rest and digest. In general, if one division becomes more active, the other will usually become less active

Question 13

True or False: the 2 divisions of the ANS are not always antagonistic

Answer 13

True - in some cases, they work together to control distinct components of a complex response

Ex. 1: Sexual responses
Parasympathetic neurons mediate arousal responses in erectile tissue. Sympathetic neurons stimulate contractile ejaculatory/orgasmic reflexes

Ex. 2: Pupillary reflexes
The 2 divisions have opposite effects on pupil size, but pupillary reflexes are specifically driven by light intensity, not stressful or relaxing situations

Question 14

What does sympathetic activation mean?

Answer 14

It means sympathetic ganglia can be activated together, amplifying each others’ activity. This is because sympathetic ganglia form an interconnected chain, meaning the entire division can be activated at once

Question 15

Location of preganglionic neuron cell body in the SNS and PSNS

Answer 15

SNS - located in the lateral gray matter in thoracic and superior lumbar spinal cord

PSNS - brainstem and lateral gray matter in sacral spinal cord

Question 16

Relative length of pre- and post-ganglionic axons in SNS and PSNS

Answer 16

SNS - sympathetic ganglia are farther from the target organ (mostly in the sympathetic chain)

PSNS - parasympathetic ganglia are within or near the target organ

Question 17

NT released in SNS and PSNS

Answer 17

SNS - releases norepinephrine

PSNS - releases acetylcholine

NOTE: all preganglionic neurons release ACh (like motor neurons)

Question 17

Function of NTs in the ANS

Answer 17

Each NT released by the SNS and PSNS can activate a different suite of NT receptors that produce different metabotropic effects (ie. muscarinic and adrenergic receptors)

Question 18

True or False: The same NT can only be either excitatory or inhibitory. It cannot be both

Answer 18

False - it can be excitatory or inhibitory depending on the subtype of receptor protein expressed by the postsynaptic cell (ie. the effector organ)

Question 19

Visceral reflexes and what are the 2 types

Answer 19

Visceral functions are mostly controlled by visceral reflexes, which do not always involve the CNS

1. Long visceral reflex - involves the CNS
2. Short visceral reflex (found in the digestive system, associated with the ENS) - does not involve the CNS because it doesn’t need to send info to the CNS to stimulate a response

Question 20

Name an example for each long visceral and short visceral reflexes

Answer 20

Long visceral - the consensual pupillary light response involving the parasympathetic division. In response to bright light, both pupils constrict

Short visceral - gastroenteric reflex and gastroileal reflex