The Nervous System

Anatomy and Physiology

Nervous System Functions

• 1. Sensory-receptors gather information and pass it on toward the CNS

• 2. Integrative-in the spinal cord or brain, we put information together and make sense of it

• 3. Motor-carry impulses to effectors such as muscles and glands

Basic Structures

• Central nervous system (CNS) consists of brain and spinal cord only

• Peripheral nervous system (PNS) are nerves outside the CNS

Basic Structures

• Neurons are the nerve cells that can transmit the electrical impulse

• Neurons are surrounded by support cells called neuroglial cells

• There are 5 types of neuroglial cells

Neuron Structure

• Cell body with nucleus

• Dendrites: branching fibers that RECEIVE impulses

• Axon: long fiber that SENDS the impulse on

Neuron Structure

• Axon is often wrapped in cells called Schwann cells

• A Schwann cell has its own nucleus and is made of myelin

• Nodes of Ranvier are the spaces between Schwann cells

Types of Neurons

• Sensory neurons carry information from sensory receptors to the CNS (AFFERANT)

• Interneurons relay impulses within the CNS

• Motor neurons carry impulses from CNS to effectors ( EFFERENT)

Types of Neurons

• Bipolar: only two fibers—one dendrite and one axon

• Unipolar: single fiber from the cell body which splits into dendrite and axon

• Multipolar: many dendrites; one axon

Identify which neuron is unipolar, bipolar, and multipolar.

Identify which is the sensory neuron, the interneuron, and the motor neuron

The Nerve Impulse• The membrane of

a resting neuron is POLARIZED

• This means that there is a different electrical charge on the outside of the membrane as compared to the inside

Resting Potential

Resting Potential• More positive ions outside than inside• Result of actively pumping out sodium ions (Na+)• Potassium ions are also involved (K+)• Three Na+ ions are pumped out for every two K+ ions

pumped in.

Becoming Polarized

Sodium Potassium Pump

Resting Potential• More positive outside the membrane than inside

(POLARIZED)

• This charge difference is called a potential difference

• Holds potential energy which can be released in the form of an electrical impulse

Action Potential

• When stimulated, the membrane of the neuron opens the channels and lets the Na+ rush inside

• This is DEPOLARIZATION

• This releases the potential energy in the form of a current down the neuron membrane

Like a Wave

Depolarization of one section stimulates the next section of membrane

Action Potential

Back to Resting Potential

After the wave, or current, of depolarization has passed, ions are restored to resting potential

This means Na+ are again pumped out, setting up a more positive charge outside the neuron

Odds and Ends

• Threshold must be reached to trigger depolarization

• Each nerve fiber has an ALL OR NONE RESPONSE

Reflex Arc

• Minimum of three neurons

• Sensory to inter to motor neuron

• Fastest responses go only to the spinal cord, not all the way to the brain

Reflex Arc

Reflex Arc

Reaction Time

• Is the time between stimulation and the beginning of the response

• Sensory neuron to interneurons in brain, then motor neuron

Reaction Time

• Can get shorter, as we practice

• Use the same pathway over and over

• Learned behavior

Bell Ringer

• Which nerve would be faster at transmitting an impulse: 2 mm in diameter, or 4.5 mm in diameter

• Which neuron would be faster at transmitting an impulse: one with Schwann Cells, or a bare axon with no Schwann cells

• How is multiple sclerosis related to Schwann cells?