The nervous system

Nervous System Overview

The nervous system, together with the endocrine system, controls all the other organ systems. The nervous system sends one type of signal around the body, and the endocrine system sends another type of signal around the body.

The nervous system works by sending and receiving electrical signals. The main organs of the nervous system are the brain and the spinal cord. The signals are carried by nerves in the body, similar to the wires that carry electricity all over the house. The signals travel from all over the body to the spinal cord and up to the brain, as well as moving in the other direction. The messages released by the nervous system traveled through nerves. Similar to the electricity that travels through wires, nerves quickly carry electrical messages around the body.

Cells of the Nervous System

Neurons are the cells of the nervous system.  They are the basic unit of communication with the brain, spinal cord and the nerves. 

There are three classes of neuron cells:

     1. Sensory Neurons- specialized to respond to certain stimuli ( light, pressure, sound) (Receptors)

     2. Interneurons-receive sensory input, integrate the information, and create a response (integrators)

     3. Motor Neurons-relay info away from the brain and spinal cord to muscles and glands (Effectors)

There are a lot of other cells in the spinal cord and branches of nerves that support and  protect the nerve cells, they make up structures like the nerve sheath that surrounds a nerve. 

Nerve cells have numerous little finger like extensions called dendrites that come out of the cell body.  They also have one AXON- a long cylindrical extension, with more little endings called axon terminals.  The dendrites are an input zone, the axon is the conducting zone , and the axon terminals are the output zone. 

HOW A NEURON RESPONDS TO STIMULATION

When a neuron is not being stimulated it keeps a slight negative charge in the cytoplasmic fluid, just inside the membrane, compared to the interstitial fluid outside the membrane.  RESTING MEMBRANE POTENTIAL- is the steady voltage difference between the inside and the outside of the cell.  When a weak signal reaches the input zone, the voltage difference changes just slightly.  If a strong signal reaches the input zone it triggers an ACTION POTENTIAL- an abrupt, short lived reversal in the voltage difference across the cell membrane.  In other words, for a fraction of a second the inside becomes positive and the outside negative.  This fast reversal travels down the neuron from input to output zone. The neuron then returns to its resting potential charges.  This difference in the charge inside a cell and outside a cell can be maintained because potassium and sodium ions can not  diffuse across the membrane.  They need a channel opened to do so.  When the neuron is stimulated protein channels open that allow K+ ions and Na+ ions to diffuse across, moving down their concentration gradient.  An action potential lasts only a millisecond.  The action spreads in one direction because after the protein channels close, they remain insensitive for a fraction of a second. The axon is wrapped in a myelin sheath, made of cells called SCHWANN CELLS.  This sheath is very important.  Without it people experience debilitating diseases which can cause paralysis and pain, Guillan- Barre disease is one of these. 

Chemical Synapses

There is a gap between every nerve cell, a NODE/ SYNAPSE/ SYNAPTIC CLEFT.  The axon terminals are loaded with voltage-sensitive gated sodium channels where the action potential jumps the gap from neuron to neuron. The chemicals that make the jump are called NEUROTRANSMITTERS- signaling molecules that diffuse across the SYNAPSES between cells.  These gaps can be between to neurons or between a neuron and a muscle cell or a neuron and a gland cell. The first cell is called the presynaptic cell.  The second cell is called the postsynaptic cell. Depending on which channels open up, the nuerostranmitter may excite or inhibit the postsynaptic membrane.  When the signal is complete, the presynaptic cell resorbs the nuerotransmitter.  Sometimes people get too little neurotranmitter made...like with Parkinson's disease which is a lack of Dopamine neurotransmitter.  Sometimes they get to much, or with the use of drugs manage to desensitize their nerve membranes.  Neurotransmitters are a whole field of study in biology, called Neurochemistry, and we will not go far into them, but you should know there are several categories of neurotransmitter that impact the way we feel and behave, and respond to the world around us. 

Also, we can increase those synapses.  Repeated synaptic activities can have long-lasting effects on the receptor neuron, including structural changes such as the formation of new synapses, alterations in the dendritic tree, or growth of axons. An example of this is the learning process – the more you study and repeat, the more synapses are created in your brain and enable you to retrieve that information when needed.  Addictive online activities such as social media, pornography, and gaming, literally rewire your brain.  It is only with great effort that a person can change this wiring and choose a new course.  The message here is...intentionally choose what you spend your time doing, because you are training your brain to be focused  on what you do all the time. You can in essence make yourself a slave to what you teach your brain.  Living a well balanced life is important to brain development, such as spending time reading, doing physical activities, and positively socializing with others form long lasting effects for the good in your brain. 

The Nervous System

This system includes your brain, the spinal cord, and peripheral nerves.  We are called vertebrates because we have a spinal cord.  Invertebrates do not have a spinal cord. OUr peripheral nerves lead to the spinal cord, which leads to the brain. A REFLEX- is a simple stereotyped movement that stimulates a single group of nerves. No other nerves are disturbed by the stimulation.  This is called a reflex pathway.  Your doctor may have tapped a rubber hammer just under your knee, and your leg kicks forward automatically.  When nervous systems first evolved, signals were reflexive.  The oldest part of our brains are reflexive and deal with things like breathing.   The human brain is now complex, and we have the ability to make voluntary choices, brand new responses.   We divide the nervous system into the CENTRAL NERVOUS SYSTEM- which is the spinal cord and brain and PERIPHERAL NERVOUS SYSTEM- which is the nerves in the rest of the body. 

Peripheral-includes 31 nerves which connect with the spinal cord  and 12 cranial nerves which connect directly with the brain.

We also divide nerves by their function:

Somatic Nerves- the nerves that help you move

Autonomic Nerves- the nerve that control the smooth muscle, adjust heart rate,  and signal glands.

     Under autonomic nerves there are two divisions:

       1. Sympathetic Nerves- dominate during heightened awareness, fear, or excitement. They stop the housekeeping tasks and prepare for

            fight or flight

       2. Parasympathetic- the body is not receiving much outside stimulation- slow down the body and divert energy to the basic housekeeping jobs like 

            digestion and waste removal. 

Central Nervous System

The spinal cord is made of WHITE MATTER - axons bundled together, and Gray Matter- dendrites and cell bodies. 

Divisions of the Brain

The spinal cord merges with the brain, an organ which can receive, integrate, store, and retrieve information. 

There are three divisions of the Brain:

1. Hindbrain- consists of medulla oblongata ( reflex for breathing and circulation and complex reflexes like coughing), cerebellum (posture and movements), and pons (bridges the two halves of the brain)

2. Midbrain- coordinates reflex responses to sights and sounds - optic lobes

TOGETHER MIDBRAIN, PONS, and MEDULLA OBLONGATA MAKE UP THE BRAINSTEM

3.  Forebrain-Cerebrum, hypothalmus, olfactory lobes, Thalamus, Pituitary Gland, Pineal Gland

The average brain weighs about 3 pounds and contains about 100 Billion neurons. 

What makes us human.......the cerebrum.  Its folded a lot - like a walnut.  A deep fissure divides the right and left half. Each half is called a HEMISPHERE.  Each half receives and coordinates information mostly from the opposite half of the body.  The part of your brain that deals with speech in the left half along with mathematics and analytical skills.  Nonverbal, abstract skills, spatial abilities, and music reside on the right half. THe cerebral cortex, a thin layer of gray matter at the surface of the cerebrum deals mostly with tongue and thumb movements.

WHERE IS MEMORY?

The ability to store memory gives us the capacity to learn. 

Memory is stored in stages:

Short-term Memory- limited amount-lasting from a few seconds to a few hours- these are stored in the hippocampus which then helps facilitate long term memory storage

Long-term Memory- occurs through chemical and structural changes- memory is stored on a permanent basis- short term memories transfer from the hippocampus to the neocortex- part of the cerebrum. 

In 1953 a patient named Henry Molaison had his hippocampus removed.  He was not able to create new memories. He only had memories from his past formed before the surgery.