Introduction to the Autonomic Nervous System

Autonomic Nervous System

The autonomic nervous systemThe part of the peripheral nervous system that controls involuntary functions such as heart rate, di can sometimes be called the visceral nervous systemThe organ system that controls body functions using electrical and chemical signals.. This is because neuronsThe functional cells of the nervous system that transmit signals. of the autonomic nervous system innervateTo invade a tissue with nervous tissue is to innervate it. our squishy organs.  The ANS is a motor system including only outgoing neurons from the CNSComposed of the brain and spinal cord; integrates and processes information. .  We have previous discovered that motor neuronsNeurons that carry outgoing information from the CNS to effector organs. innervate skeletal muscle and release acetylcholinealso know as ACh A neurotransmitter that stimulates muscle contraction. to excite the muscle.  That was the somatic nervous system.  Here, neurons innervate cardiac muscle, smooth muscle, or glands and excite them using acetylcholine and norepinephrineA neurotransmitter involved in attention, arousal, and the fight-or-flight response.. 

You might be thinking that these autonomic neurons fully control these visceral organs.  Untrue.  These organs have their own ways of establishing baseline activity.  For example in the heart there are pacemaker cellsThe basic structural and functional units of life. that establish a baseline heart rate. A baroreceptor is located in your aorta. It senses the pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. of the blood coming from the heart as it pumps.  This baroreceptor is connected to cranial nerve IX the glossopharyngeal nerve.  This nerve travels to the medulla oblongataThe lowest part of the brainstem controlling vital functions like breathing and heart rate. to synapseThe junction between two neurons where communication occurs. with an interneuron.  This interneuron evaluates the incoming info and determines an output.  This output leaves the medulla oblongata by way of cranial nerve X the vagus nerve.  This nerve travels to the heart where it adjusts the firing of pacemaker cells.

Visceral Reflexes

Just as with somatic reflexesAutomatic responses to stimuli. we have visceral reflexesReflexes that control involuntary functions of visceral organs (e.g., blood pressure, digestion) thr.  These reflexes have a much slower response. The response is slower than the somatic reflexes we’ve previously discussed. These include the withdrawal reflex and the cross extensor reflex.  Somatic reflexes and visceral reflexes can sometimes use the same sensory receptorsProteins located on the surface or inside cells that bind specific molecules (e.g., neurotransmitter and unipolar sensory neuronsNeurons that carry information from sensory receptors to the CNS.. For example, thermoreceptorsSensory receptors that respond to temperature changes. in the skinThe body’s largest organ, providing protection and regulation. of your hand can detect when you are burning your hand on a hot surface. This detection causes the withdrawal reflex using your skeletal muscles. That is a somatic reflex. A visceral reflex may also use thermoreceptors most likely embedded in a visceral neuron. Instead of activating somatic tissues or skeletal muscle visceral reflexes activate smooth muscle cardiac muscle or glands. These three items are known as the effectors of visceral reflexes. This motor neuron will be a neuron of the autonomic nervous system. It will not be a neuron of the somatic nervous system.

Autonomic Output

Let’s take a moment to review output of the somatic motor system which activates skeletal muscles. There is commonly 1 multipolar neuronA neuron with one axon and multiple dendrites; the most common type in the CNS. that originates in the primary somatic motor cortex. This motor neuron descends the spinal cordThe central nervous system structure that relays signals between the brain and body. in a motor tract. It leaves the spinal cord by way of the ventralRelating to the front or belly side of the body. root spinal nerve and then the ventral remus. Therefore this one motor neuron then reaches all the way from the brain to the skeletal muscle. At the junction of the neuron and the skeletal muscle, called the neuromuscular junctionThe connection between a motor neuron and a muscle fiber., the motor neuron uses acetylcholine. This chemical excites the skeletal muscle. There are other examples of somatic motor output that involve 2 neurons. We would have one neuron leaving the primary motor somatic cortex of the brain and descending the spinal cord. At the appropriate level, that motor neuron would synapse with another neuron. This new neuron then leaves the spinal cord and travels all the way to the skeletal muscle. Here, ACH is used to excite it.

Autonomic output uses 2 neurons. In this class, the neurons will not originate in the brain. Instead, they will originate in the lateral hornA part of the spinal cord’s gray matter, containing autonomic motor neurons. of the gray matterThe inner portion of the spinal cord composed mostly of neuron cell bodies and synapses. of the spinal cord. As you might recall the lateralAway from the midline of the body. horn was designated as an area of synapse for V several motor neurons. This neuron then leaves the spinal cord by way of the ventral root. It travels through the spinal nerve and a ventral ramus out to a structure called a ganglionA cluster of neuron cell bodies located in the peripheral nervous system (PNS).. A ganglion is simply an area of synapse. At this ganglion, the first neuron synapses with a second neuron. Then, this second neuron travels all the way to the target cell. As you can see here we have our three targets smooth muscle glands and cardiac muscle cells. The two neurons are given names in reference to their location to the ganglion. The preganglionic fiberThe axon of a neuron that originates in the central nervous system and synapses in an autonomic gang occurs in sequence before the ganglion and the post ganglionic fibre occurs in sequence after it.  These can sometimes be referred to the preganglionic and postganglionic neurons.  Either is correct. Before in the nervous system we called these the pre synaptic and the post synaptic neurons. However, in the autonomic system, it’s more appropriate to refer to the ganglion’s location. This location contains the synapse.

What’s a Ganglion?

The gray matter of the spinal cord houses synapses that occur within the CNS. In contrast, a ganglion houses synapses outside it.  These little bubble in these nerves are the ganglia. A ganglion contains the axon terminals of the preganglionic fiber and a cell bodyThe central part of a neuron containing the nucleus and organelles. of the post ganglionic fiber.  A ganglion can simply be one synapse between fibers, but that is a simplification.  Usually these are nerve plexuses associated with a ganglion housing many synapses. 

There are a lot of ganglia located right next to the spinal cord.  It’s like two extra spinal cords of just gray matter on either side of the real spinal cord.  These structures lateral to the spinal cord are called the sympathetic chain gangliaA series of ganglia along the spinal column involved in autonomic responses..  There are also ganglia located further out in the visceral organs.  These are called the celiac ganglion, the superiorAbove or toward the upper part of the body. mesenteric ganglion and the inferiorBelow or toward the lower part of the body. mesenteric ganglion.  The term mesenteryA fold of peritoneum that supports and connects the intestines to the abdominal wall. refers to this connective tissue of fat, nerves, and vessels that kinda binds together all your intestines. 

Divisions

The autonomic nervous system is an independent system. It is separate and different from the somatic nervous system. Within the autonomic nervous system there are two divisions that have classically been recognized. There is now a third division that is starting to be included among the divisions of the autonomic nervous system. Here is just a summary there are many lectures that go into much more detail for the first two divisions. The sympathetic nervous system is also called the thoracolumbar system. This name is due to the post ganglionic fibers originally originating in the thoracic and lumbar section A cut or slice of the body or an organ for study. of your spinal cord. This is the division that takes over when you feel threatened stressed or anxious. This system mobilizes you to either fight or flight. Recently in my courses students have also suggested that this system can make you freeze. The parasympathetic nervous system is also commonly called the cranial sacral nervous system. Ideally, this system is in control for you all the time. It helps you to rest and digest. Stress and anxiety can cause the sympathetic nervous system to override the parasympathetic leading to many of the physical symptomsSubjective experiences reported by the patient (e.g., nausea, fatigue). of stress such as gastrointestinal distress period