Identify the principle parts of the nervous system Describe the cells that make up the nervous system Describe what starts and stops a nerve impulse (action potential) The role of neurotransmitters Compare the functions of the CNS & PNS Identify the principle parts of the brain
20% of cells in nervous system are neurons and the rest are neuroglial cells – support cells Support & protection Maintenance of surrounding chemical concentrations No impulse generation
Astrocytes – star shaped, largest & most numerous, form tight sheaths around capillaries of the brain Microglia – small, usually stationary –but enlarge and mobilize in degenerating brain tissue for phagocytosis Oligodendroglia – produce myelin sheath that envelopes nerve fibers in the brain and spinal cord Schwann cells – found in nerves only (not in brain or spinal cord); help to form myelin sheath
In PNS these neuroglial cells produce myelin – a fatty insulating material surrounding axon – called the myelin sheath = myelinated neurons Nodes of Ranvier are short uninsulated gaps where the surface of the axon is exposed
1) Energy saving – prevents slow leakage of Na+ in / K+ out (recall: sodium-potassium pump = 3 Na+ out & 2 K+ in) 2) Speeds transmission of impulses – local depolarizing at Nodes of Ranvier is much faster than continuous propagation down an unmyelinated axon = saltatory conduction – action potentials appear to jump from node to node (5 mph vs. 250 mph) 3) Helps damaged or severed axons of the PNS to regenerate – severed end near soma can regrow in sheath channel (weeks-year +)
In CNS these protective sheaths of myelin do not regenerate when the cell dies – this is why spinal cord injuries or MS causes permanent damage
Identify the principle parts of the nervous system Describe the cells that make up the nervous system Describe what starts and stops a nerve impulse (action potential) The role of neurotransmitters Compare the functions of the CNS & PNS Identify the principle parts of the brain
Junctions that allow transfer of information from one neuron: ◦ to another neuron ◦ to an effector cell (muscle) Pre synaptic neuron – transmits signal to synapse Post synaptic neuron – transmits signal away from synapse Synaptic cleft is the fluid filled space separating the pre/post synaptic neurons
Convert an electrical signal (action potential) into a chemical signal (neurotransmitter) 4 step sequence 1)Action potential arrives at axon bulb, Ca channels open and Ca diffuses into axon bulb 2)Ca causes vesicles containing neurotransmitters to fuse with presynaptic membrane and release contents into synaptic cleft 3)Neurotransmitter molecules bind to receptors on post synaptic membrane opening gated Na channels 4)Na diffuses into postsynaptic membrane & starts a new action potential (electrical signal)
> 50 chemicals can function as neurotransmitters! Excitatory – encourage the generation of new impulses in post synaptic neuron Inhibitory – prevents generation of action potentials in post synaptic neuron Some neurotransmitters can do both depending on the receptor that they bind to Many small graded potentials are needed to fire the impulse across the cleft
Prompt removal of neurotransmitter causes signals to stop – 3 ways to remove 1)Taken back up by presynaptic neuron & repackaged 2)Destroyed by enzymes in synaptic cleft 3)Diffuse away from synaptic cleft general circulation and destroyed Convergence – one neuron receives information from several neurons Divergence – action potential will go to several neurons
Neurons may integrate and process thousands of simultaneous incoming stimulatory and inhibitory signals before generating and transmitting their own action potentials Bottom line: individual neurons do not “see, smell or hear” yet combined actions allow us to experience these complex sensations Filter of information – extraneous information expelled during REM sleep
Muscle cells are targets of presynaptic neurons – Skeletal muscle does not process information Neuromuscular junction in large with many points of contact between neuron and muscle cell Threshold is reached in skeletal muscle every time a motor neuron sends a single action potential So the nervous system has absolute control of skeletal muscle!