November 28, 2023


International Student Club UK

Autonomic Nervous System, Sympathetic vs Parasympathetic Nervous System, Gray and White Matter, Upper Motor Neurons, and Somatosensory Tract [MCAT, USMLE,


In this lesson, we explore the nervous system and share notes as part of the study guide series. We will explore the awesome brain and nerves! Topics include the autonomic nervous system, sympathetic vs parasympathetic nervous system, gray and white matter, upper motor neurons, and somatosensory tract.

Check out our popular nervous system notes.

Autonomic Nervous System

  • ANS controls things without involving the consciousness.
  • It consists of efferent neurons in the peripheral nervous system that control three types of cells:
  • We can divide the autonomic nervous system into sympathetic & parasympathetic.
  • The autonomic nervous system has two functional differences. The sympathetic nervous system is associated with fight or flight, and the parasympathetic nervous system is associated with rest and digest.
    • Rest and digest means that body functions promoting homeostasis are activated.
    • Fight or flight means that body functions promoting survival are activated.
    • When one system is activated, the other decreases activities.
    • Sympathetic nervous system:
      • Starts in the middle part of the spinal cord
      • The first neuron off the soma of the spinal cord has a short axon and synapses in a ganglia close to the spinal cord. The second neuron has a longer axon that then goes toward the desired target — i.e. a tissue that contains smooth, cardiac muscle, gland cells
      • The chain of ganglia coming out of the first, short axons off the spinal cord is called the sympathetic chain.
      • Sympathetic nerves originate in the center of the spinal cord and have a short axon to the synapse of another neuron. From there, there is a long axon to the target neuron.
    • Parasympathetic nervous system:
      • Starts either in the brain stem or at the bottom of the spinal cord
      • First neuron sends a long axon out to synapse in a ganglion at a lengthy distance from the first neuron soma. The second neuron then sends a shorter axon to the target cell.
      • The parasympathetic nerves originate in the brainstem or at the bottom of the spinal cord. Parasympathetic nerves have long axons to the synapse of another neuron, then a short axon to the target neuron.
  • Functions of sympathetic nervous system: “Fight or flight”
    • causes changes that will help us fight or run away when threatened
  • Functions of parasympathetic nervous system: “Rest and digest”
  • causes changes more important for homeostasis
  • Consider the blood flow to the the gastrointestinal system, which plays a big role in the amount of digestion that can happen and the amount of blood available for other muscles.
    • When the sympathetic nervous system “fight or flight” is activated, blood flow to the intestines is decreased and redirected towards skeletal muscle
    • Most of the time, though, when you’re in a non-threatening situation and can “rest and digest,” the parasympathetic nervous system is activated which diverts blood away from skeletal muscle and brings it towards the intestines to help you digest food.
  • Consider the heart output, or how much blood is pumped out in a give time period:
    • When sympathetic nervous system is activated, heart output is increased to increased blood availability for skeletal muscle.
    • When parasympathetic nervous system is activated, heart output is decreased because you don’t need as much blood flow to the rest of the muscles if you’re not using them.
  • These examples of blood flow involve the activity of smooth muscle, which makes up blood vessels, and cardiac muscle, which obviously affects the heart.
  • Consider the glands:
  • When sympathetic nervous system is activated, sweat glands are activated — this cools us down and allows us to move faster and farther.
  • When the parasympathetic nervous system is activated, salivary glands are activated — helps us digest food.
  • Most of the things the SNS does increases the ability of the body to turn stored energy into movement! Whereas most of the paraSNS activities allows us to conserve energy and digest food.
  • Autonomic neurons also play a role in changing pupil size of the eyes, in sexual functions, and more.

Gray and White Matter — grey = soma; white = axons

  • In the CNS, which is mostly the brain and spinal cord, gray matter contains most of the neuron somas, and white matter contains most of the myelinated axons.
  • Looking at different cross-sections of the spinal cord, we see that most of the gray matter (butterfly or H-shape) is on the inside of the spinal cord, and most of the white matter is on the outside.
  • Looking at cross sections of the brain, we see that gray matter is mostly on the outside of the brain! This is called cortex. The gray matter over the cerebrum is thus called the cerebral cortex, while gray matter on the cerebellum is called cerebellar cortex. Most of the neuron somas are here.
    • Most of the white matter of the brain is on the inside of the brain, under the cerebral cortex.
    • There are some other areas deep in the brain that have also gray matter, which we call nuclei.
  • In the white matter of the CNS are collections of axons that travel together through the CNS; we call them tracts. (One tract can have many axons in it, often carrying similar sorts of information from one part of the CNS to another part).
  • In addition to neurons involved in motor, sensory, and automatic functions, the CNS also has many neurons participating in higher functions of consciousness, cognition, and emotion. These take place particularly in the cerebral cortex.

Upper Motor Neurons

  • Recall, the LMNs have their somas in the brain or spinal cord and they send nerves out to skeletal muscles. LMNs that pass through spinal nerves primarily control cells in the limbs and trunk, while LMNs that pass through cranial nerves primarily control cells in the head and neck.
  • Turns out that while the LMNs control what muscles contract and when, the Upper Motor Neurons (UMNs) control the lower motor neurons!
  • Somas of the UMNs are found mainly in the cerebral cortex (gray matter over cerebrum), and their axons descend down to synapse on LMNs in the brain stem or the spinal cord, depending on tract.
  • Let’s think about a LMN at the top of a spinal cord on the left side. The soma will be in the spinal cord, and send an axon out into the muscles. The UMN that controls this LMN will start in the cerebral cortex on the opposite side, and send its axon down through the deep white matter, and into the brain stem (through the midbrain, pons, and medulla). Where the brain stem meets the spinal cord, most of these axons will cross over to the other side and travel down the appropriate left side until they reach the LMN to synapse on it and control it.
    • We call this the corticospinal tract.
    • The left side of the brain controls, for the most part, the right side of the body.
    • Because of the crossover, we see that if there’s dysfunction of a tract at the spinal cord site, there will be weakness on that same side of the body. If, however, there’s dysfunction in a neuron at the site of the cerebral cortex, there will be weakness on the opposite side of the body.
  • Ex of LMN in the brain stem — one extends to each side of the head or neck. To reach these, some UMNs start in the cerebral cortex and send an axon down in a similar way as the corticospinal tract and they will similarly cross over and affect the LMN on the other side of the brain stem. We also see, however, that some UMNs will travel down to affect an LMN on the same side of the brain stem.
    • We call this the corticobulbar tract. It includes UMN axons that innervate LMNs in brain stem. We can get different patterns of weakness with abnormalities of this tract.. more on that later.
  • Dysfunction in either the UMNs or the LMNs can cause weakness.
    • Upper Motor Neuron signs can occur with or without weakness. These signs can help us understand, if a patient does have weakness, whether the problem is in the upper or lower motor neurons.
      • Hyperreflexia — an increase in the muscle stretch reflexes (opposite of LMN sign hyporreflexia). Would cause a patient to have an exaggerated response to a knee tap.
        • Cause of hyperreflexia is not known. But apparently when muscle spindles (receptors in skeletal muscle which are activated and their info is carried back by somatosensory receptors to elicit a response by UMNs), are not periodically stimulated by the UMNs, the LMNs may become super sensitive. This may mean a normal signal from a UMN causes an LMN to have an exaggerated reflex.
        • Clonus — rhythmic contraction of antagonist muscles, which have an opposite effect on a joint.
      • ex: Antagonist muscles in the front of your shin cause you to pull your foot up, while the counter muscles in the back of the leg cause you to push your foot down (like a gas pedal). If a doctor grabs the foot of a patient who has UMN dysfunction and rapidly pulls it upward, the foot may go into this involuntary movement (clonus) where it starts going up and down and up and down over and over.
        • The cause of clonus is likely just hyperreflexia… each time the foot goes one way the muscles on the other side are stretched; and the muscles end up triggering each other
        • The involuntary rhythmic contraction of antagonist muscles is known as clonus and is a sign of upper motor neuron dysfunction.
      • Hypertonia — Increased tone (resistance) of skeletal muscles (opposite of LMN sign hypotonia).
      • This can cause muscle spasms, different from the fasciculation of LMN degeneration.
        • Muscle spasticity is a feature of hypertonia, or increased tone of skeletal muscle, and is an upper motor neuron sign.
      • Extensor Plantar Response (aka Babinski sign) — If you take a hard object and scrape along the bottom of the foot, the normal plantar response is flexor, to have the toes curl down towards the bottom of the foot. If a person has UMN dysfunction and you do this to them, though, the foot will respond with extensor, meaning the toes will extend away from the bottom of the foot. The extensor plantar response (or babinski reflex) is a sign of upper motor neuron dysfunction and can be seen when a noxious stimuli is placed on the bottom of the foot, causing the toes to go into extension away from the bottom of the foot, rather than flexing down in the direction of the bottom of the foot.

Somatosensory Tracts

  • Somatosensory tracks are groups of axons that carry information about the environment back to CNS.
  • Recall, the different types of somatosensory information tend to travel in different pathways:
    • Position sense, Vibration sense, and fine touch sense — these signals travel in large diameter, heavily myelinated axons. Fast response.
    • Pain sense, Temperature, and Gross, or less precise, touch sense — these signals travel in smaller diameter, thinly myelinated (if at all) axons. Slower response.
  • Somatosensory info from most of the body travels to CNS through (afferent) nerves in the PNS and then through spinal nerves that enter the spinal cord and deliver that info.
    • Somatosensory info from the face will usually travel into the brainstem through cranial nerves.
  • What happens once the info is delivered into the brainstem or spinal cord?
    • For the somatosensory pain, temperature, and gross touch info — Inside the spinal cord, neural axons carry that information up to the brain in one of the somatosensory tracts that’s specific to that type of sensation.
    • If, e.g. a noxious stimuli is experienced on the left side of the body, an axon will carry that pain sensation across to the right side of the spinal cord, and then up through the brain stem until it comes to a place deep down in the cerebrum. It enters the cerebral hemisphere on the other side from the part of the body where the receptor is on.
    • The same is true for the somatosensations of the other category (position, vibration, fine touch), although their axons cross to the other side a little further up the body in the brain stem (instead of in the spinal cord)
    • Pain, temp, gross touch, etc. sensations from receptors in the face (and some other parts of the head) can come into the brainstem through cranial nerves that will travel through axons that go down first, and then cross and then go up to about the same place in the cerebral hemisphere that the info from the rest of the body came from.
      • This tract is also the case for position, vibration, and fine touch. They come into the brain stem from cranial nerves, go down first and then cross and then go up to about the same place.
    • In this place deep in the cerebral hemisphere, all these different types of somatosensory information come back together and stay close to each other as they send that information on to areas of the cerebral cortex that will do more processing of the information.
    • Because these somatosensory tracts have this sort of anatomy, if there’s an injury or disease to one side of the brain’s hemisphere, the other side of the body can have somatosensory loss.

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