The Nervous System: Efferent Fibers

The motor systems (those neurons leaving the central nervous system and going to the periphery), can be broken down into two major groups:

somatic and autonomic

The somatic nervous system is

The autonomic nervous system is

Both systems are considered to be

is your voluntary system and innervates primarily skeletal muscle

is largely involuntary and goes to cardiac muscle, smooth muscle, and glands

to be reflexive (meaning that they can transmit reflexes).

The autonomic nervous system (ANS) can be broken down into two largely antagonistic divisions:

Parasympathetic Nervous System (PNS) – also known as the "resting and digesting" system or the “feed and breed” system, it is important for basic life functions and is restorative in nature.

Sympathetic Nervous System (SNS) – is known as the "fight, fright or flight" system that gets your body ready for action.

The ANS works below the level of consciousness meaning?

they are under tonic control, always on but at varying degrees and generally antagonistic but work together to maintain homeostasis. The ANS innervates cardiac, smooth, glands, some organs, and adipose tissue

The divisions of the ANS are generally antagonistic. What is an exception to this rule?

salivary glands, generally the PSNS increases salivation while the SNS decreases it. The exception happens when you exercise, which can increase salivation as you breathe through your mouth. Some differences in the saliva is when activated by the SNS it tends to have low digestive enzymes while the PSNS has higher

What is dual innervation?

Dual innervation is usually antagonistic (remember there are always exceptions), where both branches of the ANS target the area but do different things

What is an example for dual innervation? What is an exception?

In the heart both the SA node and the AV node are innervated by the SNS and the PSNS and each have antagonistic effects. SNS increasing the heart rate and PSNS decreasing the heart rate.

But there is an exception to this rule as the ventricles are only innervated by the SNS

What is mono innervation?

When a single branch of the ANS goes to target area

Mono innervation: Sympathetic nervous system

blood vessels

sweat glands

kidneys

adrenal medulla

arrector pili muscle

ventricles

adipose tissue

Mono innervation: Parasympathetic nervous system

• interesting fact?

The lacrimal glands are only innervated by the PSNS, but the sympathetic fibers do pass through the lacrimal gland but do NOT synapse

What is the Bell - Magendie Law?

most afferent fibers enter the spinal cord through the dorsal root

most efferent fibers exit the spinal cord through the ventral root

What is gray matter? white matter?

GM: dendrites, cell bodies, and unmyelinated axons

WM: myelinated axons of ascending and descending tracts

Sympathetic nervous system:

• origin, ganglia, axon length

SNS motor neuron originates in the lateral gray horn of the thoracolumbar region of the spinal cord

The ganglia is either pre- or para- vertebral ganglia, that are right next to the spinal cord

The preganglionic neuron will have a short axon as the ganglia is paravertebral and the post ganglionic neuron will have a long axon to reach the target cell

Ganglia

Single: Ganglion

group of cell bodies in the PNS

Parasympathetic nervous system:

• origin, ganglia, axon length

PSNS motor neurons originate in the craniosacral region of the spinal cord. Their ganglia is terminal ganglia, that is either next or inside the target organ. The preganglionic neuron will have a long axon because it has to reach the terminal ganglion, so the postganglionic neuron will have a short axon as its right next to or inside the target organ

What are varicosities?

varicosities are “beads on a string,” the axonal terminal of the post ganglionic neuron. Varicosities are filled with vesicles that contain NT, but have no designated synapse, so the NT is released over the surface of the target cell

AP traveling down the varicosities

AP will travel down the axon → reach the varicosities and depolarize their membrane → open vgated Ca2+ channels → Ca2+ will rush in and trigger exocytosis of synaptic vesicles with NT → NT is released over the target cell → NT binds to receptor on target cell → opening either ligand gated ion channels

• or g protein receptors (in glands), resulting in the formation of the secondary messenger inside of the cell, activating the enzymatic cascade

What NT are we focusing on?

Acetylcholine and Norepinephrine

What general receptor can acetylcholine bind to? What are the two types?

ACh can bind onto cholinergic receptors, either nicotinic or muscarinic and they are differentiated because we have different agonists for them

Agonist

an agonist mimics ligand and excites the receptor activating the SAME pathway

Nicotinic cholinergic receptors

• What can bind? What is the agonist? What cannot bind?

ACh can bind and excite nicotinic receptors, leading to a response

Nicotine is an agonist to nicotinic receptors, so it can also bind and activate the same pathway as ACh

Muscarine cannot bind

Muscarinic cholinergic receptors

• What can bind? What is the agonist? What cannot bind?

ACh can bind and excite the receptor, activating the pathway

Muscarine is an agonist to muscarinic receptors, so it can bind and activate the same pathway as ACh

Nicotine cannot bind

What general receptor can norepinephrine (NE) / noradrenaline bind to?

• What are the types? What is NE an agonist to?

NE can bind to adrenergic receptors. There are four types alpha 1 and 2 and beta 1 and beta 2.

NE is also an agonist to epinephrine/adrenaline they can bind to the same receptor and activate the same pathways

Parasympathetic nervous system

1. Preganglionic cell body location

2. Preganglionic axon length

3. Ganglion location

4. Preganglionic

neurotransmitter/receptor

5. Postganglionic axon length

6. Postganglionic

neurotransmitter/receptor(s)

7. Targets

8. Effects

1+2) The preganglionic neuron originates in the craniosacral region of the spinal cord and has a long axon

3+4) To reach the terminal ganglion. The NT of the preganglionic neuron will be ACh and the receptor on the post ganglionic dendrites/cell body will be nicotinic cholinergic receptors

5) Since the post ganglionic neuron is located in the terminal ganglion the axon length will be short

6) The neurotransmitter of the postganglionic neuron will be ACh but the receptor on the target will be a muscarinic cholinergic receptor

7) targets will be cardiac and smooth muscle and glands

What is the effect of the post ganglionic neuron NT and the receptor on the heart in the PSNS?

When ACh released by the postganglionic neuron binds to the muscarinic cholinergic receptors on the heart it has three effects:

1) negative chronotropy: decrease in the heart rate

2) negative inotropy: decrease in the strength of the contraction

3) negative dromotropy: decrease in the velocity of the conduction (contraction)

Sympathetic nervous system: GANGLIONIC

1. Preganglionic cell body location

2. Preganglionic axon length

3. Ganglion location

4. Preganglionic

neurotransmitter/receptor

5. Postganglionic axon length

6. Postganglionic

neurotransmitter/receptor(s)

7. Targets

8. Effects

1) Preganglionic neuron originates in the lateral gray horn of the thoracolumbar region of the spinal cord

2+3) The axon of the preganglionic neuron will be short, as the pre/paravertebral ganglia are right next to the spinal cord

4) The NT of the preganglionic neuron will be ACh, and the receptor on the postganglionic dendrites/cell body will be nicotinic cholinergic receptors

5) Since the postganglionic neuron orginates in the para/prevertebral ganglia it will have a LONG axon to reach the target cell.

6) The NT of the postganglionic neuron will be NE, and the receptor on the target cell will be adrenergic receptors a1, b1, or b2. The receptor a2 will actually be located on the varicosities of the postganglionic neuron

7) cardiac, smooth, or glands

8) varies

SNS: Where are the adrenergic receptors located?

a1, b1, and b2 are located on the target cell while a2 is located on the varicosities of the postganglionic neuron

SNS: Alpha 2 receptor

• location

• effect

a2 receptors are located on the presynaptic membrane of the postganglionic neuron. In comparison to a1, b1, and b2 receptors a2 has a lower affinity for NE. So when the concentration of NE is low, it will bind to its higher affinity receptors, but when the concentration of NE increases, it will begin to bind to a2 receptors.

Effect: activating a2 receptors due to the increase in concentration will cause feedback inhibition. Blocking the further release of NE, conserving it

SNS: Alpha 1

• location

• effect

• systemic blood vessels (essentially all blood vessels but the heart)

• will cause vasoconstriction

SNS: Beta 1

• location

• effect

located in the cardia muscle

1) positive chronotropy: increase the HR

2) positive inotropy: increase the strength of contraction

3) positive dromotropy: increase the speed of conduction (contractions)

SNS: Beta 2

• location

• effect

Location: some systemic blood vessels, coronary blood vessels, and bronchial smooth muscle

Effect: vasodilation and bronchodilation

Sympathetic nervous system: NONGANGLIONIC

1) Cell body origin

2) axon length

3) NT and Receptor

4) target

5) effect

1) cell body of the motor neuron originates in the lateral gray horn of the thoracolumbar region of the spinal cord

2) Since it is a single fiber (nonganglionic) the axon will be long

3+4) Axon will travel to the adrenal medulla and synapse with chromaffin cells. The neuron will secrete ACh and the receptors on the chromaffin cells will by nicotinic cholinergic receptors

5) when nicotinic cholinergic receptors are activated by ACh chromaffin cells will secrete NE and epinephrine, which will be taken up and carried by the blood and bind to a1, b1, and b2 receptors

What are chromaffin cells?

“modified neurons” that are secretory cells that secrete NE and epinephrine

Somatic nervous system (NONGANGLIONIC)

1) Cell body origin

2) axon length

3) NT and Receptor

4) target

5) effect

1) somatic motor neuron originate in the ventral gray horn in the cerebral cortex

2+4) they have long axons that leave through the ventral root and go to the skeletal muscle

3) NT released will be ACh and the receptor on the muscle fiber of the skeletal muscle will be nicotinic cholinergic receptors

5) the effect will be the contraction of the skeletal muscle