The Nervous System

1. The Nervous System Chapter 48

2. 2 components • Central Nervous System (CNS): brain and spinal cord • Peripheral Nervous System (PNS): all nerves outside the CNS

3. The Neuron • Consists of a cell body with a nucleus and other organelles and 2 types of extensions: dendrites and axons • Dendrites: sensory; they receive incoming messages from other cells and carry the electrical signal to the cell body. Each neuron can have 100s! • Axon: only one per neuron; can be several feet long (in large mammals). Transmits an impulse from the cell body outward to another cell. • See figure

4. Structure of a vertebrate neuron (fig 48.4 in book)

5. The Neuron • Synapse: the space between 2 neurons • Neurotransmitters: chemical messengers sent across the synapse from one neuron to another

6. 3 types of neurons (see fig 48.5 in book) • Sensory neurons: (PNS) receive an initial stimulus from a sense organ, such as eyes, ears or another neuron • Motor neurons: (PNS) stimulate effectors (muscle cells or endocrine gland cells); for example – a motor neuron can stimulate a digestive gland to release a digestive enzyme or to stimulate a muscle to contract • Interneurons: (CNS) in the spinal cord and brain; receive sensory stimuli and transfer the information directly to a motor neuron or to the brain for processing

8. Membrane Potential (fig 48.6 in book) • Describes the difference in electrical charge across a cell membrane • Resting potential is the membrane potential of a neuron at rest; typically -60 to -80mV (millivolts) potential; the negative = the inside of the neuron is negative to the outside • Figure 48.6 

9. Membrane Potential

10. Nerve Impulses • Change in the membrane potential of a neuron give rise to nerve impulses • A stimulus affects the membrane’s permeability to ions, which begins an action potential • Steps in a nerve impulse: 1) Dendrites of a neuron receive stimulus = depolarization 2) Action Potential – signal sent along axon; neurotransmitters sent 3) Repolarization; refractory period 4) Resting Potential restored

11. Action Potential • An all or nothing response • If a stimulus is great enough to excite an neuron, we say the cell “reaches its threshold” and an action potential will occur

12. Steps in an Action Potential • Depolarization: the stimulus opens Na+ channels in the membrane, allowing Na+ to rush inside the cell, changing the membrane potential to a positive value (from negative at rest) = more + inside, - outside • Repolarization: the membrane potential is restored when K+ channels open and K+ leaves the cell. The charge is restored to resting potential but the ions are on the wrong sides of the membrane • Refractory period: Na-K pump reestablishes the original ion distribution – Na is pumped out of the cell, K is pumped in = ACTIVE TRANSPORT; during this period, a neuron cannot send another impulse

14. Action Potential is like a domino effect down the entire neuron

15. Chemical Synapses • When the nerve impulse reaches the end of one neuron (axon bulb), neurotransmitters are released into the synapse, then they bind to receptors on the plasma membrane of the dendrites of the postsynaptic cell (fig 48.15 in book)  • The neurotransmitters then trigger an action potential in the postsynaptic neuron

16. A chemical synapse

17. Common neurotransmitters • Acetylcholine – excitatory transmitter between neuron and muscle cell (botulism, botox – muscles lose function) • Epinephrine – neurotransmitter and hormone – fight or flight response • Norepinephrine – neurotransmitter and hormone – fight or flight response • Dopamine – linked to behavior, cognition, and voluntary movement (among others) • Serotonin – linked to mood, aggression

18. Glial Cells • Protect the axon • Make myelin sheath • Support the neuron, but are NOT neuronal cells • Supply oxygen and nutrients to neuron • Remove dead neurons • Hold neurons in place • Insulate neuron from other neurons • Provide clearance for the release of neurotransmitters • Suppress repair in the CNS and promote repair in PNS (is there hope to use this to fix the CNS? MAYBE!)

19. Types of Glial Cells • Astrocytes – provide support (structural and otherwise) for neurons • Oligodendrocytes – wrap up to 50 axons and make myelin sheath for neuron in CNS • Schwann Cells – wrap around only 1 axon to make the myelin sheath for neurons in the PNS; multiple Schwann cells are on each axon

20. Schwann cell Myelin Sheath Node of Ranvier

21. Myelin Sheath • 80% lipid, 20% protein • Presence of myelin sheath causes signal to move faster, enhances electric current • A nerve impulse can jump between nodes of Ranvier(areas of the axon that are not covered by myelin sheath), speeding up nerve impulse conduction – called saltatory conduction • Without myelin, nodes are not present and electrical impulse decreases and is eliminated = Multiple Sclerosis involves the destruction of myelin sheath by one’s OWN body!