Anatomy and Physiology

1. Anatomy and Physiology 9.3 – Central Nervous System

2. Warm Up 01.25.07 • Explain what a reflex arc is.

3. Nerve Impulses • A nerve impulse is a self-propagating wave of electrical disturbance that travels along the surface of the neuron’s plasma membrane • They are initiated by a stimulus – a change in temperature, pressure, etc • See Figure 9.8 page 271

4. Nerve Impulses (cont’d) • The membrane of a resting neuron has a slightly + charge on the outside and a negative charge on the inside. • Because there are lots of Na+ on the outside • When the membrane is stimulated, the Na+ flows into the cell and the inside of the membrane becomes suddenly + and the outside becomes negative

5. Nerve Impulses (cont’d) • Even though the section of membrane that was stimulated immediately recovers, the electrical disturbance opens other Na+ channels and the electrical wave continues to travel down the membrane in one direction

6. Saltatory Conduction • If the impulse encounters a section of membrane that is covered with myelin, it “jumps” through the myelin. • This is called saltatory conduction and is much faster than conduction through nonmyelinated sections • Figure 9-8, Page 271

7. The Synapse • A synapse is a place where impulses are transmitted from one neuron (the presynaptic neuron) to another neuron (the postsynaptic neuron) • A synapse has three structures: a synaptic knob, a synaptic cleft and that plasma membrane of the postsynaptic neuron

8. Parts of a Synapse • The synaptic knob is a tiny bulge at the end of a terminal branch of a presynaptic neuron’s axon • Each contains many small sacs or vesicles • Each vesicle contains a very small quantity of a chemical compound called a neurotransmitter • When the nerve impulse arrives at the synaptic knob, neurotransmitter molecules are released from the vesicles into the synaptic cleft – a space between the synaptic knob and the PM.

9. Synaptic Cleft • The synaptic cleft is an incredibly narrow space – only about two-millionths of a centimeter in width • The plasma membrane of a postsynaptic neuron has protein molecules embedded in it which serve as receptors to which neurotransmitter molecules bind • This binding can initiate an impulse by opening ion channels

10. Neurotransmitters • They are chemicals by which neurons communicate • They assist, stimulate or inhibit postsynaptic neurons • At least 30 different compounds have been identified as neurotransmitters • Not distributed randomly through the spinal cord and brain – they are localized in discrete groups of neurons and released in specific pathways

11. Examples of Neurotransmitters • Acetylcholine, Norepinephrine, Dopamine, Serotonin • Each belong to a group called catecholamines which play a role in sleep, motor function, mood and pleasure recognition • Endorphins, Enkaphalins • They inhibit conduction of pain impulses – natural pain killers

12. Neurotransmitters (cont’d) • Nitric Oxide (NO) • Diffuses across the plasma membrane of neurons instead of being released from vesicles • NO is the neurotransmitter that leads to the male arousal response • Sildenafil (Viagra) treats ED by promoting the same response as NO naturally creates

13. Parkinson Disease • Chronic nervous disorder – deficient levels of dopamine in the brain • Symptoms: rigidity and trembling of the head and extremities, a forward tilt of the trunk and a shuffling manner of walking • Famous people with it: Mohammed Ali, Michael J. Fox

14. Central Nervous System • Obviously, it is centrally located • 2 Major Structures: Brain & spinal cord • The brain is protected in the cranial cavity of the skull and the spinal cord is protected by the spinal cavity of the vertebral column • The brain and spinal cord are also covered by protective membranes -- meninges

15. Divisions of the Brain – Fig 9-12 (page 276) • Brainstem • Medulla oblongata • Pons • Midbrain • Cerebellum • Diencephalon • Hypothalamus • Thalamus • Cerebrum

16. Brainstem • The lowest part of the brainstem is the medulla oblongata, above that is the pons and above that is the midbrain • The medulla oblongata is an enlarged extension of the spinal cord, just inside the cranial cavity • It lies directly above the foramen magnum • Consists of gray and white matter but in a different arrangement than the spinal cord

17. Brainstem (Cont’d) • The pons and the midbrain consist of the same pattern of white and gray matter as the medulla oblongata • A reticular formation of gray and white (netlike) and scattered • They all function as two-way conduction paths • Sensory fibers conduct impulses up from the cord to other parts of the brain

18. The Vital Centers • The cardiac, respiratory and vasomotor (the vital centers) are all located in the medulla • Impulses from these centers control heartbeat, respirations and blood vessel diameter