Download The Nervous System - FW Johnson Collegiate

The Nervous System
2 Main Divisions
- Central Nervous System – contains the brain and spinal cord
- Peripheral Nervous System – carries information between the organs of the body and
the central nervous system
2 Divisions of the Peripheral Nervous System:
- Somatic Nerves – control the skeletal muscle, bones, skin, and sensory organs
- Autonomic Nerves – control internal organs of the body
3 Groups of Neurons
1. Sensory neurons – neurons that allow you to respond to your external environment
2. Interneurons – neuron that communicates to other neurons
3. Motor neurons – neurons that control muscle movement
All neurons contain:
- cell body: contains nucleus
- axons: extension of cytoplasm, very thin
o covered with myelin sheath (insulates)
- dendrites – receive information
How Neurons Work: Bristol University Page- http://www.bris.ac.uk/synaptic/public/basics_ch1_2.html
- the membrane of a neuron carries a charge
- when at rest the charge is -70mV
- when the nerve becomes excited, the charge becomes +40mV
- no other cells carry a charge
- the electrochemical event (charge) is caused by an unequal concentration of positive
ions across the cell membrane
- The reversal of charges (from negative to positive) is described as an action potential
- A potassium pump pulls K+ into the cell and a sodium pump pushes Na+ outside the
nerve cell
- In the resting state, more channels in the membrane are open so more K+ gets out
than Na+ gets in
- Polarized – when the resting membrane is charged
o Energy comes from ATP
o When the nerve becomes excited, Na+ gates are opened and K+ gates close
- Depolarization – charge reversal (negative to positive) Na+ gates close and K+ gates
open
- Repolarization – restoring the original polarity of the nerve membrane
- Refractory period – the time it takes for the nerve to become repolarized
-
The cycle of depolarization and repolarization takes only about
2 milli-seconds (0.002 seconds)
Resting
K+ in
Na+ out
++++
++++
Excited (Polarized)
Na+ out
K+ gates closed
++++
----
Depolarization
Na+ gates close
K+ gates open
---++++
Repolarization
K+ in
Na+ out
++++
++++
Nerve cells respond to:
- changes in pH
- changes in pressure
- specific chemicals
Threshold Levels
- the stimulus must be above a critical value in order to produce a response
- if the stimulus is not strong enough, there will be no response
- although stimuli above threshold levels produce nerve impulses of identical speed and
intensity, variation with respect to frequency does occur
- a glass rod at 40˚C may cause a single neuron to reach threshold level while the same
glass rod at 50˚C will cause 2 or more to fire. The greater the number of impulses,
the greater the intensity of the response
The Sequence of Events that Would Occur When a Nerve Impulse is Transmitted to the brain:
1. Impulse travels along the axon
2. Neurotransmitters (chemicals that allow an impulse from one nerve cell to pass to
another nerve cell) are released from the presynaptic neuron
3. Neurotransmitters diffuse across the synapse
4. Neurotransmitters attach to receptor molecules on the postsynaptic neuron
5. Neurotransmitters are destroyed by an enzyme
6. This continues rapidly until the message reaches the brain
7. The brain processes the information and sends a reflex message back to the same site in
a similar fashion
Messages can travel in neurons up to 431.3 kilometers per hour.
Nervous System Definitions:
Vagus Nerve
- an important cranial nerve, part of the cranial nerves that exit directly from the brain
- branches off the brain down to the abdomen
- it regulates the heart, bronchi of the lungs, liver, pancreas, and digestive tract
Meninges
- 3 protective membranes surrounding the brain to protect the central nervous system
Cerebrospinal Fluid
- Circulates between the innermost and middle meninges of the brain and through the
central canal of the spinal cord
- Acts both as a shock absorber and a transport medium carrying nutrients to cells of
the brain while relaying wastes from the cells to the blood
Interneurons
- Neurons that communicate with other neurons and are organized into nerve tracts
that connect the spinal cord with the brain
Spinal cord
- Carries sensory nerve messages from receptors to the brain and relays motor nerve
messages from the brain to muscles and glands
Brain
-
composed of 3 distinct regions
o forebrain – contains olfactory lobes, which are centers that receive information
about smell
 also contains the cerebrum (2 giant hemispheres that act as the major
coordinating center for which sensory information and accompanying
motor actions originate)
 the surface of the cerebrum is known as the cerebral cortex
(it increases surface area)
 corpus callosum – allows communication between the 2 hemispheres
o midbrain – less developed than the forebrain
 acts as a relay center for some eye and ear reflexes
o hindbrain – joins with the spinal cord
 contains the cerebellum, pons, and medulla oblongata
 cerebellum – largest section of the hindbrain – controls limb movements,
balance and muscle tone
Pons – a relay station that passes information between the 2 regions of the cerebellum and
between the cerebellum and the medulla
Medulla Oblongata – acts as the connection between the peripheral and central nervous systems
- regulates involuntary muscle action
Olfactory Lobes- located in the front of each cerebral hemisphere (in the cerebrum)
Neurological Disorder:
Without the myelin sheath (protective layer around the axon), we cannot function. This is
demonstrated by the devastating effects of Multiple Sclerosis, a demyelinating (destroys the
myelin sheath) disease that affects bundles of axons in the brain, spinal cord and optic nerve,
leading to lack of co-ordination and muscle control as well as difficulties with speech and vision.