Neurotransmitters and Receptors

NXT and Neurons

The somatic nervous systemThe organ system that controls body functions using electrical and chemical signals. used acetylcholinealso know as ACh A neurotransmitter that stimulates muscle contraction. to excite the skeletal muscles.  The motor neuronsNeurons that carry outgoing information from the CNS to effector organs. that did this are called cholinergic neuronsThe functional cells of the nervous system that transmit signals. due to their association with acetylcholine.  This is still true in the ANS.  There are cholinergic preganglionic neurons secreting acetylcholine to excite a postganglionic fiberThe axon of a neuron that originates in an autonomic ganglion and extends to a target organ or tissu.  There can also be postganglionic fibers secreting acetylcholine to excite the 3 effectors: cardiac muscle, smooth muscle, and glands. 

In the ANS, we have another type of neuron called an adrenergic neuron.  These neuron secrete norepinephrineA neurotransmitter involved in attention, arousal, and the fight-or-flight response. and sometimes even epinephrineadrenaline): Fight-or-flight hormone from the adrenal medulla..  For purposes of simplification, we will focus only on norepinephrine. It is released in much more abundance than epinephrine. Here comes the complicated part: there are postganglionic neurons that secrete both acetylcholine AND norepinephrine.  These are called paininthebutt neurons.  Kidding, there is no specific name I know of. 

Different NXT – ACh

Acetylcholine and norepinephrine are the primary neurotransmittersChemicals released by neurons to transmit signals across a synapse. of the ANS. Their actions depend on the receptorsProteins located on the surface or inside cells that bind specific molecules (e.g., neurotransmitter in the postganglionic fibers and target organsOrgans affected by specific hormones.. You can release all the neurotransmitters you want, but if there are no receptors, nothing happens. 

There are two types of receptors that can respond to acetylcholine attaching to them.  We have already discussed nicotinic receptorsCholinergic receptors found on postganglionic neurons and skeletal muscle; they respond to acetylcho.  These are the receptors present at the neuromuscular junctions. They are at all the synapses between the prefib and post fib. They are at all the synapses in the sympathetic chain gangliaA series of ganglia along the spinal column involved in autonomic responses. and collateral ganglia.   

These always excite the cell they are on.  Always. 

The ANS uses muscarinic receptorsCholinergic receptors found on the effector organs of the parasympathetic nervous system; they media too.  Muscarinic receptors can cause excitation or inhibition.  It depends on the organ.  Muscarinic receptors are located on cardiac muscle, smooth muscle and glands – those 3 effectors of the ANS. 

When acetylcholine connects to a muscarinic receptorA structure that detects stimuli. on cardiac muscle, the cardiac muscle cellsThe basic structural and functional units of life. are inhibited. This prevents them from increasing their rate of action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission.. This could slow the heart rate but it could also just prevent the heart rate from increasing. When acetylcholine connects to muscarinic receptors on smooth muscle it excites that smooth muscle to contract.

Different NXT – NE

Nor epinephrine is secreted by adrenergic neuronsNeurons that release norepinephrine as their primary neurotransmitter, typically part of the sympath. We could also say that norepinephrine is secreted by all sympathetic post ganglionic fibers. Your epinephrine is never secreted by a pre ganglionic fiber. Only acetylcholine is secreted between a pre and post ganglionic fiber. This is because any preganglionic fibre’s job is to excite the postganglionic fiber. Therefore making the only appropriate always excitatory nor neurotransmitterChemicals that transmit signals across synapses. acetylcholine the appropriate choice.

This leaves nor epinephrine to be secreted by every single adrenergic sympathetic post ganglionic fiber. As with any neurotransmitter, you can release all that you want. However, its effect is determined by the receptors present on the target cell. Adrenergic receptors have many classes and subclasses. We can consider two types of receptors alpha and beta. Alpha receptors are mostly excitatory causing depolarizationThe loss of electrical charge across a membrane, triggering an action potential. in the cell they are on. These receptors are almost on every cardiac and smooth muscle cell. They are not on the smooth muscle associated with the GI tract. As we know the GI tract and the enteric nervous system is its own entity. There are also beta receptors which are mostly inhibitory but can sometimes be excitatory. For example when norepinephrine is released in response to you perceiving a threat it circulates throughout your body. Organs with beta receptors such as the trachea make the connection between your epinephrine and their beta receptor. Norepinephrine inhibits the trachea. This allows you to take larger breaths. It helps you run away from the threat. In contrast the beta receptors on your heart produce an excitatory effect when norepinephrine connects to them. Nor epinephrine connecting to the beta receptors on your cardiac muscle will increase the heart rate. This is the reason many people with high blood pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. or anxiety are prescribed beta blockers. These drugs are a classification called beta blockers. Beta blockers sit on these adrenergic receptorsReceptors that bind norepinephrine and epinephrine; divided into α (alpha) and β (beta) types with preventing the connection of norepinephrine to the target cell. Are there alpha blockers? Yes. There are also acetylcholine blockers. But these are topics for another mini lecture.