Download Chapter 33 Nervous System

Chapter 33 Nervous System
A. Structure of the Nervous System
a. Neurons
i. Neurons
1. Specialized cells that help gather information about the environment, interpret and react
2. Make up nervous system
ii. Dendrites
1. Receive signals called impulses from other neurons and conduct impulses to cell body
2. A single cell might have many dendrites
iii. Cell Body
1. Is where nucleus of neuron and many cell organelles are located
iv. Axon
1. Carries nerve impulse from cell body to other neurons and muscles
Dendrites
Cell Body
Axon
Nucleus
b.
v. 3 Types of Neurons
1. Sensory neurons
a. Send impulses from receptors in skin and sense organs to brain and spinal cord
b. Signal interneurons
2. Interneurons
a. Carry impulse to motor neurons
b. Located in spinal cord and brain
3. Motor neurons
a. Carry impulses away from brain and spinal cord to a gland or muscle
b. Causes a response
vi. Reflex Arc
1. Nerve pathway that consists of a sensory neuron, an interneuron, and a motor neuron
2. Brain is NOT involved
3. Basic structure of nervous system
A Nerve Impulse
i. An electrical charge traveling the length of a neuron
ii. Impulse results from a stimulus – touch, loud noise, etc.
iii. A neuron at rest
1. Not conducting an impulse
+
2. More sodium ions (Na ) outside the cell than inside cell
+
3. More potassium ions (K ) inside the cell than outside cell
4. For every 2 potassium ions pumped into neuron, 3 sodium ions pumped out
iv. An action potential
1. Also called a nerve impulse
2. Threshold
a. Minimum stimulus to cause action potential
3. All or nothing
a. Nerve impulse is either strong enough to travel along neuron or isn’t
v. Speed of an action potential
1.
2.
B.
Speed of action potential varies due to myelin sheath
Neurons with myelin
a. Carry impulses associated with sharp pain
3. Neurons without myelin
a. Carry impulses associated with dull, throbbing pain
vi. The synapse
1. Gap between axon of one neuron and dendrite of another axon
2. When a motor neuron synapses with muscle cell, the released neurotransmitter crosses synapse
and causes muscle to contract
3. Neurotransmitter
a. Chemical that diffuses across a synapse and binds to receptors on the dendrite of a
neighboring neuron
b. Over 25 known neurotransmitters
c. A single neuron can communicate with many other neurons
Organization of the Nervous System
a. The Central Nervous System (CNS)
i. The brain
1. Contains 100 billion neurons
2. Control center of the body – maintains homeostasis and involved with almost all body activities
ii. The spinal cord
1. Nerve column that extends from brain to lower back (protected by vertebrae)
2. Spinal nerves extend from spinal cord to parts of body and connect to central nervous system
3. Reflexes processed in spinal cord
b.
The Peripheral Nervous System (PNS)
i. Nerves are NOT neurons they are bundles of axons
ii. 12 cranial nerves that lead to and from brain
iii. 31 spinal nerves that lead to and from spinal cord
iv. Contains all neurons that are not part of central nervous system, including sensory neurons and motor
neurons
v. Neurons in peripheral nervous system can be classified as part of somatic nervous system or autonomic
nervous system
vi. Somatic nervous system
1. Voluntary
2. Relays information to and from skin and skeletal muscles
vii. Autonomic nervous system
1. Involuntary
2. Relays information to internal organs
viii. Sympathetic nervous system
1. Controls organs in times of stress
ix. Parasympathetic nervous system
1. Controls organs when body is at rest
C.
The Senses
a. Taste and Smell
i. Sensory receptors are specialized neurons which allow you to taste, smell, hear, see, touch, detect motion
and temperature
ii. Taste buds are specialized chemical receptors on tongue that detect sweet, sour, salty, and bitter
iii. Taste and smell stimulated by chemicals and function together to detect different combinations of
chemicals in food and send information to another part of brain
b. Sight
i. Light path
1. Light enters eye through cornea
2. Cornea focuses light through pupil (size regulated by muscles in the iris)
3. Behind iris is the lens which inverts image
4. Projected to retina
5. Passes through vitreous humor (colorless gelatin-like liquid between lens and retina)
6. Retina contains rods and cones
7. Receptors send action potentials to brain via neurons in optic nerve
8. Brain interprets combination of signals to form visual image
c.
Hearing and Balance
i. Hearing
1. Sound waves enter ear canal, causing eardrum to vibrate
2. Vibrations move through middle ear (malleus, incus, stapes)
3. Stapes vibrates causing oval window (separates middle from inner ear) to move back and forth
4. Cochlea is filled with fluid and lined with tiny hair cells
5. Vibrations cause fluid inside to move like a wave against hair cells
6. Hair cells generate nerve impulses in auditory nerve and transmit to brain
ii. Balance
1. Semicircular canals (located in inner ear) transmit information about body position and balance
to brain
2. Fluid filled and lined with hair cells
3. Head position changes = fluid moves through canals causing hair cells to bend, sending impulse
to brain
d.
Touch
i. Many types of sense receptors which respond to temperature, pressure and pain are found in epidermis
and dermis of skin
ii. Fingertips detect light touch
iii. Sole of feet respond to heavy pressure
iv. Pain receptors are simple (consist of free nerve endings) and found in all tissues except for brain
D. Effects on Drugs
a. How Drugs Work
i. Those that cause changes in the nervous system work in one or more of the following ways
1. Cause an increase in amount of neurotransmitter released into synapse
2. Block receptor site on a dendrite, preventing neurotransmitter from binding
3. Prevent neurotransmitter from leaving synapse
4. Imitate a neurotransmitter
ii. Drugs that affect nervous system
1. Dopamine (neurotransmitter) found in brain and is involved with control of body movements and
other functions
2. Also strongly involved in feelings of pleasure or reward
3. Normally removed from synapse by being reabsorbed by neuron that released it