Neural Communication part 1 - Ion Channels and Action Potentials

What is an Action Potential?

Simply, action potentials are the way in which nerve cells communicate using signals generated through both electrical and chemical means. Upon receiving an external stimulus, neurons generate and fire action potentials to relay a message to the next targeted neuron, muscle fiber, organ, or gland. 

Before diving into this concept, it is important to understand the properties of ions and ion channels. Ions are atoms that carry an electric charge due to the loss or gain of electrons. The presence of ions can cause either a net positive or negative charge. The main ions responsible for action potentials are Sodium and Potassium Ions. A relatively larger concentration of potassium is present inside the cell as opposed to a larger concentration of sodium present outside the cell. Although both ions are positive in nature, the resting potential of potassium is negative 80 millivolts (mV) as opposed to sodium whose resting potential is positive 62 mV.  

How Does this Process Work?

To initiate an action potential, a neuron must be excited by some kind of stimulus or impulse. A sensory neuron receives stimulus from external sources (such as light) while interneurons and motor neurons receive impulses from other neurons. In its normal state, a neuron will bring in   2  potassium ions for every 3 Na+ ions that it sends out. The cell maintains this ratio of ions through a process called active transport. This is where the cell expends energy to pump ions in and out of the cell with the use of ATP. By keeping a higher potassium concentration inside the cell, the cell holds a charge of -70mV at its normal resting state. When excited, some sodium ion channels open and allow sodium to diffuse. If the impulse is strong enough to cause the cell's internal charge to rise to -55mV, neighboring sodium ion channels open causing more Na+ ions to diffuse through at a higher rate depolarizing the cell. The sequence of sodium ions channels opening, depolarizing the nearby area, and causing nearby sodium ion channels to open up occurs down the axon.

Individual areas of the cell go from an internally negative to positive state due to all the sodium ions until the reach the charge of +30-40mV. This point is known as the overshoot, after which, the sodium ion channels close, potassium ion channels open hyperpolarizing the cell to a negative environment.  This transition to a negative state is known as the absolute refractory period and subsequent action potentials cannot be fired. This is because the rate at which potassium ions enter the cell makes it impossible for depolarization to occur.

The absolute refractory period is followed by a relative refractory period where potassium ions cause the cell to undershoot its resting potential and, instead, reach an internal state of -90mV. At this time, an action potential can still be fired, however, a stronger stimulus would be required to reach the -55mV threshold.

The Organization of the Human Nervous System

    I think of it this way. Cells. Multiple cells make up tissues. Multiple tissues make up organs and organ systems consist of multiple organs. Similarly, the nervous system is organized in the same way. Neurons and axons make up nerves, nerves make up neural circuits, and neural circuits create neural systems. Neural circuits which share similar tasks, come together to make neural systems.

  

    Such neural systems include sensory systems (observe and process information about the environments like the visual system, auditory system etc.) and the motor systems (reactions to certain input information. They include reflexes and counter balancing some moves).  Associational systems are composed of large numbers of cells and circuits which are located between both motor and sensory systems. Associational systems also carry out some of the most intricate processes and brain functions.

    As you know, the Central Nervous System (CNS) is composed of the brain (all 4 regions), and the spinal cord. And then, there is also the peripheral nervous system (PNS) which is all the cranial nerves which branch off the brain, and the many nerves which branch off the spinal cord. The PNS also includes sensory neurons which link sensory receptors (parts of sensory neurons which receive sensory information) to circuits in the CNS (missed information about neural circuits? Click here to view my previous post about them).

    The movement (motor) portion of the PNS is divided into 2 branches which both rely on one another in various ways. There is the somatic motor division which consists of nerves and axons which connect the brain and spinal cord to skeletal muscles. And, an autonomic nervous system which controls smooth muscles (which are muscles that move with conscious control ex: Biceps) and involuntary muscles. Both are crucial for survival, as they provide you, your muscle controls and gland controls to live. Without the somatic motor system, the autonomic system cannot move arms and legs, and without your autonomic system, you will need to keep remembering to execute many of your involuntary muscle movements!

    The neurons which lie within the PNS are located in ganglia which are lumps of nerve cells and supporting cells. Bundles of peripheral axons come together to make nerves, just like how thread is made of many thin fibers bundled and twisted. Those fibers are like axons. In nerves, all-around the body, lie glia. The only glia in the periphery though, are Schwann cells which lie layers of Myelin to help speed up neural messages.

In the PNS it is ganglia but in the CNS it is known as nuclei. Local accumulations of cells in the CNS are known as nuclei (not to get confused with the other meaning of nuclei – control center in most cells). Nuclei are found throughout the CNS, including the Spinal cord and division of the main area of the brain. In addition, there are cortices (cortex-singular) which are flat sheets of nerve cells. 

Tracts in the CNS consist of axons, just like those in the PNS. And those tracts that cross the midline division of the brain are called commissures. The area that contains axons, tracts, and commissures is known as white matter (named so because of its color due to its heavy content of myelin). On the other hand, gray matter is the large clumps of cell bodies and neuropil (click here for more info about neuropil). It is located in the brain and spinal cord.

Somatic and Autonomic Nervous Systems

Within the peripheral nervous system, there are 2 subdivisions: The Somatic and Autonomic nervous systems. These 2 are have opposite functions but both come in the same importance for your survival. Here's more...

The Somatic Nervous System

The Somatic nervous system (SNS) is all the nerves that connect to your skeletal muscles and control voluntary muscle movements including the movement of all sensory organs. The SNS’s job could be as simple as moving your arm up and down. It sends electrical and chemical messages from the brain to the skeletal muscles that needs to be moved based on information about your environment (known as stimuli). 

Almost all of the action that the somatic nervous system does is voluntary, but there are exceptions. These are known as reflex arcs. A muscles will move in response to stimuli without much activity in the brain, and almost no voluntary control. This is when your peripheral nerve and axon pathways only reach until your spinal cord. Just like the action of pulling your hand away from something hot. 

As a result of diseases and injuries to the spinal cord, the somatic nervous system will not be able to control parts of your body. Paraplegia is the term given for paralyzed legs due to damage to your lower spinal cord (damage in that area terminates transmitting signals which results in no control). Quadriplegia is the term given for the paralysis of all 4 limbs consequently to upper spinal cord or neck damage.

The autonomic nervous system 

The autonomic nervous division (ANS) is another main sub division of your periphery and plays a huge role for your survival. It is in charge of all involuntary actions, like your heart rate, basic reflexes, breathing and other movements or actions that you cannot really control. It sends impulses from the center of the brain (the diencephalon area) to many major involuntary areas such as the heart, lungs, and even blood vessels (for altering blood pressure). The main regions of the brain which control these involuntary actions include: The hypothalamus (in the diencephalon), the medulla, and the pons areas (both medulla and pons located within the brain stem). 

Looking further into the autonomic nervous system, there are 2 more divisions! They are well known as the sympathetic and parasympathetic systems. These 2 major components of the ANS prepare us for action and rest. The nerves from both systems carry impulses from sensory organs, to the 3 regions (hypothalamus, medulla, and pons). 

The sympathetic nervous system gets us ready for activity. It includes the command of your adrenal glands to produce adrenaline, meaning that it gets us ready for the famously known "fight or flight" type situations. Operating through a series of interconnected neurons, the sympathetic system also controls eye dilation (enlarging it), increasing heart rate + blood pressure, and contracts our bladder before while we get involved with a motion activity, lessening the urge to urinate.

As the sympathetic system is important for our survival in fight related situations, our parasympathetic system is also vital in our lives. It is responsible for healing/resting. This systems also acts through connections between nerves and organs; nevertheless, unlike the sympathetic system, its sensory signals are consciously received. Functions include contracting your eyes, lessening heart rate (therefore lessening breathing and blood pressure), and dilating your bladder, increasing the urge to pee!

Diseases that the ANS is responsible for includes...

• High blood pressure levels
• difficulty in breathing
• Heart problems and etc.

The Human Nervous System

    As a collection of pathways for neurons (brains cells) that branches and monitors every crevice of your body, the nervous system  is a substantial part of your body which controls your day-to-day tasks. Breathing, eating, sleeping, and knowing your surroundings are all controlled by your nervous system and it is designed to do much more. Let me explain.

The nervous system is divided into 2 parts: The Central nervous system and the Peripheral nervous system. The two systems, Central and peripheral (CNS and PNS) act as a single unit depending on each other for our survival. Here is more.

    The CNS is simply composed of your Brain, and your spinal cord. This is the area that receives, processes and sends information in response to changes in the environment or situation. 

    The Brain receives and processes sensory information, does the thinking, judging, and your cognitive skills (cognition- things you remember from past experiences, such as the taste of pizza). Other regions of the brain are responsible for movement, voluntary and involuntary functions such as heart beat, breathing and digestion. 

    The spinal cord is the 2nd component of the CNS, and is a thick wire-shaped nerve that stretches along your spine. The spinal cord is the foundation shape for many nerves known as spinal nerves which branch off of your cord’s body and attach themselves to organs and skeletal muscles, or branch off from muscles and attach to your cord. It receives information from those many nerves and transmits signals to and from the brain.

    Moving on, the Peripheral system is the other main nervous system of the human nervous system. It includes all of the nerves and axons that branch off of the brain (cranial nerves) and the spinal cord (spinal nerves). Examples of cranial nerves are optic nerves (the nerves that connect the eye to the brain), the auditory nerve (carries sound information), and many others. 

    They bring in sensory information (stimuli) to send to your brain about the surroundings. The spinal nerves branch off of the spinal cord and connect to organs (to operate), skeletal muscles (for movements), and receptors (for sensory information), such as those on your fingertips, and tongue. 

    The peripheral system is further divided into 2 systems. The somatic nervous system, and the autonomic nervous system. (There will be information about both in the next post.)

    Now for an example of the PNS and CNS working together… let’s say your hand was placed on a hot pan. Neural receptors feel your hand burning and send information down spinal nerves to the spinal cord. Then, from the spinal cord the information gets sent to the brain. "Your hands are burning!" is the message. Your motor cortex in your CNS of the brain quickly responds by sending an impulse to the somatic nervous system in the periphery, telling it to move your hand. The peripheral nervous system’s SNS (somatic nervous system) then executes what the message says, helping your body avoid injury or damage to your hand.