Module 1 The Nervous System

1.1.3 page 5

Reflect and Connect

When you were introduced to that special person, you quickly withdrew your hand from the handshake and then became embarrassed and blushed. To reflect on your knowledge from this lesson, trace the reflex arc that occurred when you withdrew your hand. Include a description of all the parts of the motor neuron that communicated with your muscles, and include a description of the role of the myelin sheath. Reflect on why your embarrassment and blushing happened later.

Lesson Summary

In this lesson you have explored the following focusing questions:

• What are the structures and functions of the neuron? How do they support communication?
• What are the components of the reflex arc?

You have examined the dendrites that receive information and pass it to the cell body. The cell body performs life functions and passes the information to the axon. The axon then passes information to its terminal. The axon has many specialized parts. The Schwann cell is a special support cell that wraps around the axon and produces an insulating fatty layer called myelin. Myelin insulation increases the rate of communication. The nodes of Ranvier also increase the rate of communication. You will study how communication rate is increased in Lesson 7.

 

Whether you withdrew your hand from the handshake or from a hot stove, or you stopped a fast-moving puck as a goalie, the response involved three types of neurons. The sensory neuron is able to receive stimuli from the sensory receptor and pass this information to the interneuron in the grey matter of the spinal cord. The interneuron is structured so that it can send nerve impulses to the brain for further processing, or it can stimulate a third type of neuron—the motor neuron. Motor neurons stimulate muscles and glands. A motor neuron also initiates quick involuntary response called a reflex. The reflex behaviour gives protection to the body and enables survival.

 

Diseases, such as Alzheimer’s, alter neurons in the brain and interrupt communication between nerve cells. Other diseases, such as multiple sclerosis, interrupt communication by destroying the myelin sheath and slowing or stopping nerve impulse transmission. You will learn more about these diseases as you study the mechanism of electrochemical communication through and between neurons in Lessons 7 and 8.

 

Injuries can also interrupt communication. If a sensory neuron were damaged, would you be able to detect the heat of the hot stove? If a motor neuron were damaged, would you be able to block the shot on net? Physiotherapy currently involves technologies to speed up the development of new sensory and motor neuron communication pathways.

 

Glossary

axon: the process that emerges from the cell body and conducts the nerve impulse away from the cell body; the axon may be a metre long in motor neurons

 

axon terminal: numerous endings found at the end of an axon; axon terminals are enlarged into knobs that are specialized for producing neurotransmitters

 

cell body: a part of a neuron that contains the nucleus and other cell organelles for carrying out the metabolic reactions of the nerve cell; relays the nerve impulse from the dendrites to the axon

 

dendrite: a branching  process of a neuron that receives input from other neurons or sensory receptors and transmits a nerve impulse toward the cell body

 

effector: one of the three types of muscle or a gland that responds to a nerve impulse

 

interneuron: a type of nerve cell found either in the brain or spinal cord that transmits nerve impulses from sensory neurons within the various parts of the brain or to motor neurons

 

motor neuron: a type of nerve cell that transmits nerve impulses toward an effector, which can be a muscle or a gland

 

myelin sheath: a fatty insulating layer that surrounds axons that greatly increases the rate of impulse transmission

 

neuron: a cell in the nervous system that generates and transmits nerve impulses; consists of dendrites, cell body containing the nucleus, and axon that may or may not have a myelin sheath

 

node of Ranvier: a tiny gap in the myelin sheath surrounding the axon of myelinated neurons; nerve impulse transmission occurs between nodes of Ranvier in what is called salutatory conduction which causes the increase in the speed of impulse transmission

 

reflex: an inborn, unlearned behaviour that results from the stimulation of a special neural pathway called the reflex arc

 

reflex arc: an involuntary neural pathway that consists of a sensory receptor, a sensory neuron, a control centre that can be either the brain or spinal cord, a motor neuron, and an effector that results in a reflex behaviour that usually has survival value

 

Schwann cell: a type of supporting nerve cell that is found in the peripheral nervous system and wraps around axons of neurons and produces the myelin sheath

 

sensory neuron: a type of nerve cell that receives stimuli and conducts an impulse toward the brain and spinal cord (central nervous system)