Nervous System Notes

1. Divisions of the nervous system (Fig. 11.2 p. 241N; Fig. 44.14 p. 1011C)
   1. memory, learning, language, perception, and response all reside in the nervous system
   2. Types of neurons
      1. sensory - carry stimuli from the internal and external environment
      2. motor - carry signals to effectors - cells that cause things to happen
      3. interneurons - link neurons together - found mostly in brain and spinal cord
2. Neural Anatomy (Fig. 11.3 p. 242N)
   1. dendrites and axon
   2. myelin sheath
      1. insulates the neuron
      2. fatty covering formed by Schwann cells
      3. impulse jumps from one node to the next; faster than non-myelinated
      4. Multiple Sclerosis
         1. destruction of myelin sheath
         2. loss of signals
         3. double vision, slurred speech, poor motor control, no fine motor control, partial paralysis
   3. neurilemma
      1. thin membrane surrounding axon
      2. functions in regeneration of neuron
      3. brain and spinal cord have no neurilemma therefore damage is permanent
3. Reflex Arc (Fig. 11.5 p. 244N)
   1. some stimuli are too dangerous to wait for brain to interpret and respond; e.g., touch a burner
   2. sensory receptor - spinal cord - interneuron - motor neuron - contract muscles (or other appropriate response) - feel sensation fractions of a second later
   3. too much time for brain to judge intensity of pain, weigh alternatives, choose a course of action, effect a response
   4. common reflexes - blink, sneeze, cough, laugh when tickled, curling foot when step on a pebble
4. Electrochemical Impulse
   1. Membrane Potential (p. 246N; 997C)
      1. Na+/K+ pump creates huge gradient
      2. positive charge outside cell
      3. K+ leaks out along the diffusion gradient but flow back in along the charge gradient
      4. Na+ flows in along both concentration and charge gradients
      5. resting potential should be -80mV (outside is assigned a potential of 0) but because of Na+ leaking in, it is slightly less at -70mV (called resting potential)
   2. Voltage-gated Channels
      1. Na+ channels open quickly
         1. Na+ enters neuron
         2. potential becomes +40 mV (called action potential)
         3. depolarization opens more gates - positive feedback
      2. Na+ close when potential becomes positive so no more Na+ enters
      3. K+ gates open after Na+ gates and K+ diffuses out
      4. Problem: original polarity now restored but ions are on wrong side of membrane. Na+/K+ pump corrects this.
      5. note: gates are concentrated at nodes of Ranvier so that the extracellular fluid is in contact with the axon only at the nodes. This is why the impulse "jumps" from node to node.
   3. Propagation of the impulse (Fig. 11.10 and 11.11 p. 250N)
      1. the signal cannot proceed in both directions because of the refractory period
   4. Threshold and All-or-None
      1. stimulus must have a certain minimum intensity to cause a neuron to fire - this is the threshold of the neuron
      2. smaller, or weaker, stimuli do not provoke a response
      3. the stimulus causes channels to open and there must be enough of them opened to depolarize the membrane
      4. increasing a stimulus above threshold does not result in a larger response - this is all-or-nothing.
      5. If all stimuli above threshold cause a neuron to fire, how do we detect different intensities of stimuli?
         1. temporal summation - frequency of stimulation - a neuron fires more or less often. A warm object sends less frequent impulses to the brain
         2. spatial summation - area of stimulation - more neurons fire
         3. different thresholds - not all neurons have the same threshold. A warm object may trigger only a few neurons while a hot object provides a stimulus above the threshold of more neurons, causing them to fire
5. The Synapse
   1. synapses are small spaces between neurons, between neurons and effectors, or between neurons and sensory receptors
   2. neurotransmitters are chemicals that bind to receptors on the post-synaptic membrane and trigger the opening of Na+ gates, "transmitting" the signal across the space
   3. neurotransmitters are contained in vesicles in the cytosol near the presynaptic membrane. Depolarization of the membrane causes the neurotransmitters to be released into the synapse
   4. enzymes degrade the neurotransmitter quickly (e.g., acetylcholine and cholinesterase). Why is this important? Note: some insecticides block the action of cholinesterase; the insect heart contracts and cannot relax (poor little guy)
   5. the presence of synaptic vesicles only in the presynaptic membrane and the presence of neurotransmitter receptors only on the postsynaptic membrane ensures that an impulse can only be propagated in one direction
   6. note: neurotransmitters may have inhibitory effects on some neurons by opening K+ gates and causing hyper-polarization. This means that the resting potential is lower (more negative) and that more Na+ gates would have to open before the neuron can reach threshold and the membrane be depolarized
6. Neurotransmitters
   1. acetylcholine
      1. excites skeletal muscles
      2. can inhibit or excite at other locations
      3. low acetylcholine has been linked to Alzheimer's disease
   2. norepinephrine - excite and inhibit
   3. dopamine
      1. mostly excites but also inhibits some neurons
      2. in the brain it affects mood, sleep, attention, and learning
      3. low dopamine has been linked to Parkinson's disease while high dopamine has been linked to schizophrenia
   4. serotonin
      1. mostly inhibitory effects
      2. LSD binds to serotonin receptors and blocks the inhibitory effect which leads to hallucinations (acid trip)
   5. endorphins
      1. so-called "natural pain killers" released in the brain
      2. block neurotransmitter receptor sites in the brain so neurons relaying messages from sensory neurons don't fire
      3. heroine, codeine, and morphine are chemically similar to endorphins and have the same effect