Download 6.1 Overview of the Nervous System

Chapter 6 The Nervous System
6.1
Overview of the Nervous System
- Italian Scientist Luigi Galvani in the 18th century detected electrical current in
human muscle when it developed tension
- The human body is a biological machine running on tiny electrical charges
A. Organization of the Nervous System - Two major divisions
1. Central Nervous System (CNS)
1. brain
2. spinal cord
- key components, damage or destruction is lifethreating
- both are protected: brain = skull
spinal cord = vertebral column
2. Peripheral Nervous System (PNS)
a. All parts of the nervous system that are NOT brain or spinal cord
b. Specialized nerve endings called sensory receptors (pain, pressure, temp)
1. Afferent nerves (sensory) – transmit impulses from sensory receptors
to CNS (afferent nerves tell how body is affected by stimuli)
2. Efferent nerves (motor)– transmit impulses from CNS to muscles or
glands (efferent nerves stimulate muscles to produce effort)
-two divisions of motor nerves
a. somatic nervous system (voluntary) – stimulate muscles
b. autonomic nervous system (involuntary) – controls smooth
muscles, cardiac muscle
a. Autonomic takes care of daily functions without
thinking
- parasympathetic (routine)
- sympathetic (high alert)
B. Nervous Tissue – two categories of tissues (support and nerve)
1. Neuroglia (aka glial cells) – perform support functions
a. CNS has 4 types of glial cells
1. Astrocytes – lie between neurons and capillaries, control the nutrients from
blood supply to the nerves and filter harmful substances in the blood.
Accounts for about half of all nervous tissue.
2. Microglia – absorb and dispose of dead cells and bacteria
3. Ependymal – form protective covering around spinal cord and brain cavity
4. Oligodendrocytes – wrap around nerve fibers and produce fatty insulation
material called myelin
b. PNS has 2 types of glial cells
1. Schwann cells – form myelin sheaths around nerve fibers
2. Satellite cells – cushion and protect neurons
2. Neurons – transmit information in the form of nerve impulses
a. typical neuron = cell body, branching dendrites, long tail-like axon
1. cell body – nucleus, mitochondria, typical structures of all cells
2. dendrites – collect stimuli and transmit to the cell body
3. axons – transmit impulses away from the cell body
b. myelin sheaths cover axons and increase the rate of impulse transmission,
appear white (White matter); Gray matter is non-myelinated nerve fibers
c. insulated gaps between Schwann Cells are call nodes of Ranvier
d. axon terminals contain vesicles with chemical messengers called
neurotransmitters
- Axon terminals do not touch the other neuron or muscle, there is a
microscopic gap known as the synaptic cleft
- The intersection of an axon terminal of one neuron, the cleft, and the
dendrite of another neuron or muscle is known as a synapse
e. Classified by function: 3 types of neurons
1. Sensory neurons – carry impulses from the skin and organs to spinal cord
and brain, carrying environmental and internal information (input)
2. Motor neurons – transmit impulses from the brain and spinal cord to
muscles and glands
3. Interneurons (associated neurons) – transmit impulses between other
neurons.
f. Classified by structure: 3 types of neurons
1. Bipolar neurons – one axon, one dendrite = found in nose and eyes
2. Unipolar neurons – single axon with dendrites at one end = some sensory
3. Multipolar neurons – one axon, multiple dendrites = all motor and
interneuron.