The Structure of the Nervous System Chapter 13.1

Overall Structure  The nervous system is separated into two main categories:  The central nervous system, which is made of the spine and brain.  The peripheral system, which makes up the rest of the nerves.

Peripheral Nerves  These nerves relay information from the organs to the spine and brain.  They can be separated into somatic (voluntary) and autonomic (involuntary).  The somatic nerves control and relay information from skeletal muscles, bones and the skin.  The autonomic nerves control the body through two systems known as the (para)sympathetic systems.  Nerves can be further classified as sensory or motor.

Structure of a nerve  If one were to take a nerve and cut it apart, they would find two distinct types of cells:  The glial cells act to provide structure for the nerves.  The neurons are the actual conductors of the electrical impulses of the nerves.

Structure of a Neuron Dendrite Axon Myelin Sheath Nodes of Ranvier

 Dendrite – Receive information (whether from environment of other neurons) and sends it to the cell body.  Axon – Cytoplasm extension from the cell that carries signals sent from the cell body.  Myelin – A fatty protein that acts as an insulator for the axon.  Nodes of Ranvier – Gaps in the myelin sheath that act to speed transmission.  Schwann Cell – A glial cell that creates the myelin sheath.

Speed of Pulses  Pulses tend to travel faster along axons with wider diameters.  They also travel faster along axons that have myelin sheaths.  This is why MS and some other autoimmune diseases are so dangerous, as they attack the myelin sheaths.

Neurilemma  This is an additional membrane found outside the myelin.  When present it will restore damaged neurons (such as a severed neuron after a minor cut).  Nerves within the central nervous system lack neurilemma, hence why spinal and brain injuries tend to be permanent.

Repairing Damaged Cells  Scientists are working on different ways to repair damage central nerves.  Stem cells can possibly be used to replace damaged nerve cells.  Scientists have also discovered little spore-cells around nerves that can transform into nerves.

Neuron Classification  Neurons can be classified as one of three groups: 1.Sensory neurons – relay information from sensory receptors to central nervous system. These are found in bundles known as ganglions. 2.Interneurons – found only in the central nervous system, work to interpret and relay information from sensory neurons. 3.Motor neurons – relay information to effectors, such as muscles, organs and glands.

Reflexes  Often we have “knee-jerk” responses to situations.  They happen so quickly that information isn’t even relayed to the brain.  This path, which goes to the spinal cord and back, is known as the reflex arc.

Examples of the Reflex Arc  The tapping of a knee is a reflex arc.  Sensory receptors detect a stretch of the tendon by the patella.  This triggers a message to be sent through the sensory neuron.  This information takes a brief stop in the spine, and is sent back out again through two motor neurons.  These neurons activate the antagonistic system around the knee, causes it to jerk.

 Another example you may have experienced is when doctors test for concussion like syndromes.  They shine a low-intensity light into your eyes.  These then detect this and enact a reflex arc that causes pupil dilation.  Specifically, light in one eye will cause both to dilate.

 Generally, a lack of response in a reflex arc test would indicate some damage to nerves, most likely the central nervous system.  This is why these simple tests are very important to doctors.

Homework  Pg  Pg