Ch. 12 – Nerve Cells

Organization of the Nervous System (3 Ways to Organize): 1. CNS vs. PNS Central Nervous System = ONLY cells that begin AND end in the brain and spinal cord Peripheral Nervous System = Nerve tissue of the outer regions -originating in brain = cranial nerves -originating in spine= spinal nerves

2. Afferent Division vs. Efferent Division afferent division= all “incoming” sensory pathways efferent division= all “outgoing” motor pathways

3. Somatic (SNS) vs. Autonomic (ANS) coordinates movement of skeletal muscle all voluntary movements are SNS (and some involuntary)

Autonomic: coordinates smooth muscle, cardiac muscle, glands Involuntary control Efferent ANS: Sympathetic responds to immediate threat (“fight or flight”) Parasympathetic coordinates normal resting activity (“rest & repair”) Afferent ANS: called visceral sensory division (viscera=internal organs)

Reflex Arc sensory receptor senses stimulus  afferent neuron carries signal to CNS   interneuron in CNS (integrator) transmits signal to efferent neuron   efferent neuron stimulates the effector (muscle, gland, organ)  effector performs the necessary response DUMB! Many reflexes DO NOT REQUIRE THE BRAIN. (you become aware afterwards)

Nerve cells: 1. Neurons- excitable and conduct impulses (see handout for structure) 2. Glia- support the neurons – make up most of N.S.

Oligodendrocytes- holds nerve fibers together, produces fatty myelin sheath in CNS. Astrocytes- star shape, cover capillaries in brain to form “blood/brain barrier” Ependymal cells- produce cerebrospinal fluid - ciliated Schwann cells- wrap around long nerve fiber in the PNS to form myelin sheath Microglia- destroy microorganisms and debris (“immune cells”)

Myelin – fatty substance that insulates “wiring” (axons) to conduct efficiently.

White matter- nerve tissue containing fatty myelin (axons) Gray matter- nerve tissue without myelin (cell bodies)

Classification of Neurons classified by number of extensions from the cell body: multipolar – many extensions – (motor neuron) bipolar – two extensions – (interneuron) unipolar – one extension – (sensory neuron) pyramidal- pyramid shape cell body (brain neuron)

Nerve Repair: -most glial cells can replace/repair themselves -Neuron repair is EXTREMELY limited (none if cell body is damaged) -stem cell therapy for paralysis is hopeful! (been done in rats)

Nerve Signal Transmission Nerve signals travel by impulses called action potentials – (waves of electrical excitation) requires electrical ­membrane resting potential – charge difference between inside/outside of cell membrane (like a battery +/-) this is a polarized cell  

Electrical resting potential is created by the Na+/K+ pump. pumps Na+ OUT and K+ IN against concentration gradient (requires energy in the form of ATP) KEY!: only 2 K+ come in for every 3 Na+ out This creates cell interior that is negative compared to exterior Cell interior is -70 millivolts compared to outside

Nerve Transmission- 1.The process starts when a neurotransmitter molecule crosses a synapse between neuron- neuron OR when a sensory neuron is stimulated. 2. This signal opens membrane gates allowing Na+ to surge in depolarizing the neuron (making it more positive) 3. if depolarization is strong enough, more Na+ gates open and impulse travels down axon as an action potential (peaks at +30 milivolt) 4. signal travels from dendrite  cell body  axon 5. K+ channels then open behind the action potential allowing K+ out to re-polarize the cell 6. action potential reaches end of axon stimulating neurotransmitter to be released to next neuron OR an effector (muscle, gland, etc.)

Saltatory Conduction- The action potential can “leap” down the axon to the nodes of Ranvier between the Schwann cell thereby allowing it to travel much faster.

Neurotransmitters- Necessary for communication across the gap between neurons and continuation the signal. There are many neurotransmitters depending on location and job in the NS. (see table 12-2) Some inhibit nerve cells rather than excite them Imbalances of these cause problems (i.e. serotonin= depression, Dopamine=motor control, psychosis etc.) http://www.mind.ilstu.edu/flash/synapse_1.swf