I. What are the Major Functions and Organization of the Nervous System?
If youโre a fan of Grey's Anatomy and particularly a Dr. Derek Shepherd (i.e. McDreamy) stan, then this section of our MCAT biological review series may be your favorite one to cover! From the brain to the spinal cord to the various nerves of the body, itโs no wonder why Dr. McDreamy chose to specialize in this field of surgery!
The nervous system is definitely up there in regards to it being a more complex and intricate physiological set of organs, tissue, and cells. In this article, weโll introduce you to the nervous system on a more grand scale focusing on its main function as well as its main organizational features.
II. The Main Control Center
As mentioned, in order to get a more macroscale view of the nervous system, weโll go ahead and divide our discussion into getting a main grasp of the nervous systemโs major functions and organizational structure.
A. Major Functions of the Nervous System
Overall, we can take different processes and functions of the nervous system and put them into one of these major categories: sensation, movement, cognition, and homeostasis.
Sensation refers to the receiving of sensory stimuli from the peripheral nervous system (PNS) which is then processed via the central nervous system (CNS). Movement obviously refers to the conscious and unconscious motor excitation of muscles from the brain allowing for us to move our body.
Cognition is primarily done by the brain and refers to more high order thinking including problem solving, decision making, emotional awareness and much more! Finally, the CNS constantly and autonomously regulates our body by sending signals via the PNS to maintain physiological homeostasis.
Weโll primarily focus on the autonomic, homeostatic functions of the nervous system when we get into the different divisions of the peripheral nervous system, particularly the sympathetic and parasympathetic nervous systems!
Weโll also briefly give an introduction to understanding how sensation and movement occur within the nervous system but weโll ultimately go into further detail for these topics in our psychology articles.
B. Major Divisions of the Nervous System
Letโs go ahead and get into the main divisions of the nervous systems. In its simplest form, the nervous system is divided into the central and peripheral nervous system.
I. Central and Peripheral Nervous System
The central nervous system is simply composed of the brain and spinal cord. The brain is our organ responsible for the sensual processing of the external environment and also allows for the initiation of various actions throughout the body.
This is done via the spinal cord as it takes in incoming sensory information from the peripheral nervous system via afferent, sensory neural fibers to be processed by the brain.
Initiating signals towards the rest of the body exits the brain and the spinal cord via what are called efferent, motor neural fibers. Take a look at the diagram below showing a cross section of the spinal cord with the CNS and PNS.
Finally, to illustrate and get the point home even further, the peripheral nervous system basically comprises all the nerves outside the central nervous system!
These are the nerves that again take in incoming sensory information towards the CNS and receive initiating stimuli exiting the CNS!
II. Somatic and Autonomic Nervous System
While the central nervous system can be broken down into the brain and spinal cord, the peripheral nervous system can be further broken down into the somatic and autonomic nervous system.
The main word/phrase associated with the somatic nervous system is voluntary, muscle contraction for movement as the motor portion of the brain sends out the voluntary, excitatory signals towards the peripheral nerves which stimulate the muscle to contract!
Specifically, the peripheral nerves that stimulate the muscles via these voluntary, excitatory stimuli at what are called neuromuscular junctions, which we expand upon more in our โMuscular Contraction: Processesโ article.
Furthermore, as its name implies, the autonomic nervous system is the division of the peripheral nervous system which deals with the autonomous, homeostatic regulation of our body and its various processes.
When referring to the ANS, think about all the bodily processes we donโt have conscious control over such as changing our heart rate, our digestion, etc. Letโs go ahead and explore this even further talking about the sympathetic and parasympathetic nervous systems!
III. Sympathetic and Parasympathetic Nervous System
The final major subdivisions of the nervous system is the sympathetic and parasympathetic nervous system, which are subsets of the autonomic nervous system which again is a branch of the peripheral nervous system.
Your sympathetic nervous system changes the autonomic functioning of your body in response to high stress, intensity situations, also known as your โfight or flightโ response. The main neurotransmitters responsible for initiating this sympathetic response are norepinephrine and epinephrine which bind on to what are called adrenergic receptors.
Think about a circumstance where you suddenly see a dog barking and running at you โ in order for your body to accommodate this type of high stress situation, your sympathetic nervous system will adjust automatically adjust your body processes via the following ways:
If we think a little more closely about these physiological responses, they make sense for a high stress situation!
Pupillary dilation would expand our visual field to take in more visual information while an increase in heart rate would increase blood flow throughout the body to deliver oxygen and nutrients as increased metabolism and energy is needed during this stress response!
However, processes such as digestion and urination would want to be inhibited as these processes wonโt contribute to the necessary demands of the body during a stressful situation.
Conversely, your parasympathetic nervous system changes the autonomic functioning of your body in response to low stress, resting circumstances also known as your โrest and digestโ response. The main neurotransmitter responsible for initiating this parasympathetic response is acetylcholine which binds to what are called cholinergic receptors.
In this case, think about a situation where youโre simply relaxing at home watching T.V. โ in this case, the bodyโs parasympathetic response wants to focus on maintaining a relaxed physiological state which promotes the following autonomic changes:
Finally, another key, high yield fact to learn about the parasympathetic nervous system is that much of the primary parasympathetic stimulation comes from the vagus nerve which is actually your 10th cranial nerve.
This is due to the fact that a majority of the preganglionic parasympathetic fibers branch off of the vagus nerve!
The great thing about learning the different autonomic functions of the sympathetic and parasympathetic nervous systems is that the processes are essentially the opposite of one another!
III. Bridge/Overlap
As weโre talking about how different neurotransmitters are utilized in the different sympathetic and parasympathetic nervous systems, it may be helpful to recollect on protein signaling as these are how the neurotransmitters eventually enact the specific physiological responses.
I. Intricacies of G-Protein Coupled Signaling
As mentioned before, the neurotransmitters norepinephrine/epinephrine and acetylcholine all participate in G protein coupled receptor signaling.
The components of this receptor are exactly within the name โ it contains a transmembrane receptor, which actually crosses back and forth on the membrane 7 times, and a G protein complex which contains an alpha, beta, and gamma subunit.
When a signaling molecule, in this case a neurotransmitter, binds onto the receptor, it triggers the dissociation of the alpha subunit from the beta and gamma subunits.
The alpha subunit then goes on to activate adenylate cyclase which allows for the conversion of ATP to cyclic AMP. cAMP then goes on to activate protein kinase A which will then activate various other proteins depending on what cellular/physiological response is needed.
IV. Wrap Up/Key Terms
Letโs take this time to wrap up & concisely summarize what we covered above in the article!
A. Major Functions of the Nervous System
In general, we can put all the major processes and functions of the nervous system into 4 main categories: sensation, movement, cognition, and homeostasis.
B. Major Divisions of the Nervous System
There are various subdivisions by which we can divide the nervous system โ the most simplest way to organize the nervous system is via the central nervous system and peripheral nervous system.
I. Central and Peripheral Nervous System
The central nervous system simply contains the brain and spinal cord while the peripheral nervous system basically contains all the nerves outside of the nervous system.
In general, information can be taken from the peripheral nerves from the PNS and be processed by the CNS via what are called afferent. sensory nerve fibers.
Conversely, initiating information and signals coming from the brain can exit the CNS to stimulate the PNS via what are called efferent, motor nerve fibers.
II. Somatic and Autonomic Nervous System
The peripheral nervous system can be further broken down into the somatic and autonomic nervous system.
The main word/phrase to associate with the somatic subdivision is voluntary, motor movement as this division of the PNS is concerned with active stimulation of the muscle in order to move the body.
The autonomic nervous system is also what it sounds like as itโs involved in the autonomic regulation of the processes that we donโt have voluntary control over such as digestion, hormone secretion, etc.
III. Sympathetic and Parasympathetic Nervous System
Finally, the autonomic nervous system can be broken down into the sympathetic and parasympathetic nervous systems.
Your sympathetic nervous system is your bodyโs โfight or flightโ physiological response during high stress situations and is mediated by the hormones norepinephrine and epinephrine.
Your parasympathetic nervous system is your bodyโs โrest and digestโ physiological state in settings when the body wants to relax and conserve energy. Much of the parasympathetic response originates from the vagus nerve with the main neurotransmitter being acetylcholine.
V. Practice
Take a look at these practice questions to see and solidify your understanding!
Sample Practice Question 1
The spinothalamic tract is a type of sensation pathway which allows for the transmission of pain and temperature sensory information to be processed by the brain. In which division of the nervous system does this processing occur and what type of neural fibers transmit this information?
A. Central Nervous System, Afferent Neurons
B. Central Nervous System, Efferent Neurons
C. Peripheral Nervous System, Afferent Neurons
D. Peripheral Nervous System, Efferent Neurons
Ans. C
The processing of the sensory information which comes from the peripheral nervous system occurs at the brain which is a part of the central nervous system in addition to the spinal cord.
Additionally, this sensory stimuli enters into the CNS from the PNS utilizing incoming afferent, sensory neural fibers. Recall that efferent, motor neural fibers allow for initiating neural signals which originate from the CNS to exit and stimulate external target organs.
Sample Practice Question 2
Clostridium Botulism is a type of bacteria which releases its own botulism toxin during infection. The botulism toxin is particularly devastating towards the human body as it prevents the release of acetylcholine at neural synapses by preventing the release of the vesicles. Which of the following physiological outcomes would you expect to occur as a result of this toxin release?
I. Mydriasis (i.e. Pupil Dilation)
II. Increased Urination
III. Increased Bowel Movement
A. I only
B. II and III
C. II only
D. I and III
Ans. A
This question is much more similar in regards to a more difficult type of question the MCAT might ask. First, itโs important to recall that acetylcholine is the main neurotransmitter which initiates the parasympathetic nervous system.
If the botulism toxin prevents acetylcholine release, then physiological response of the parasympathetic nervous system will be decreased. This is why mydriasis (i.e. pupillary dilation) occurs as you have decreased parasympathetic activity to promote pupillary constriction. Conversely, urinary excretion and bowel movement are also inhibited with the botulism toxin because these are physiological response which are active in parasympathetic stimulation.
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