Nervous System I Chapter 10

Divisions of the Nervous System Central Nervous System (CNS) - brain and spinal cord Peripheral Nervous System (PNS) - nerves

General Functions of the Nervous System Sensory - impulses are sent to the CNS Integration - occurs within CNS - processes incoming sensory input (decision making) Motor - impulses are sent from CNS to effectors (muscles or glands) to cause a response

Cells of Nervous Tissue Neurons - conduct impulses Neuroglia - do not conduct impulses

Neurons-conducting cells with processes (fibers) Parts of a neuron include: Cell body (perikaryon) - contains nucleus, chromatophilic substance/Nissl bodies, no centrioles (no mitosis) Axon - usually one - process which transmits impulse Dendrites - one or more – processes which receive stimuli Impulses typically travel from dendrite end to axon end----ONE WAY

Neuron Classification # of Processes Multipolar - 1 axon, many dendrites Bipolar - 1 axon, 1 dendrite Unipolar - 1 process which splits into 2 branches, the peripheral process and the central process Impulse Direction Sensory - carries impulse toward CNS Motor - carries impulse away from CNS Interneuron - located completely within CNS

Figure 10.07

Neuroglial Cells in PNS Schwann cells - wrap around PNS neuron processes (fibers) in one of two ways: 1. tightly wrap around fibers, forming a myelin sheath (contains CM over CM over CM) -- the outermost layer of wrapping has cytoplasm & nucleus (neurilemma)--outside is endoneurium 2. just neurilemma without tight wrapping (no myelin sheath)

Neuroglial Cells in CNS Oligodendrocytes - they have processes which wrap tightly around CNS neuron processes, forming a myelin sheath, but no neurilemma (therefore, no tube of endoneurium) Astrocytes - large, star-shaped cells which aid in movement of substances between blood and neurons--they also act as “fillers”

Microglia - small, phagocytic cells Ependyma - “epithelial-like” - line cavities in CNS

Regeneration of Neuron Processes In CNS - no regeneration, since there is no endoneurial tube, and no neurilemma In PNS - regeneration MIGHT occur, because the endoneurium’s presence may prevent scar tissue from coming in before the process can regrow If the cell body of a neuron (CNS or PNS) is destroyed, regeneration will never occur.

Myelinated fiber - fiber that has a myelin sheath Unmyelinated fiber - fiber that does not have a myelin sheath Gray matter - nervous tissue containing lots of cell bodies and unmyelinated fibers White matter - nervous tissue containing lots of myelinated fibers

Neuron Potentials A nerve impulse is a chain reaction of permeability changes along a neuron’s membrane. The concentration of ions on either side of the membrane affects its charge distribution (known as its potential).

Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.

Resting Potential (the charge distribution when the neuron is at rest) When a neuron is at rest, it is not conducting an impulse. During that time, the membrane is not very permeable to K+ and is even less permeable to Na+. Active transport pumps move Na+ out and K+ in. Also, there are negatively charged proteins inside the cell. More positive particles move out than in; therefore:

at rest, Na+ on outside and K+ on inside. at rest, the membrane is positive on the outside and negative on the inside. at rest, the membrane is said to be polarized. the difference in charges when the membrane is at rest (resting potential) is -70 mV.

Changes in Potentials When a stimulus encounters a neuron, it responds by changing its permeability to various ions, which affects the potential (charge distribution). If the resting membrane becomes more negative it is hyperpolarized. If the resting membrane becomes more positive, it is depolarized.

Nerve Impulse If the stimulus is strong enough (threshold) it causes Na+ gates to open, and Na+ flows into the cell, depolarizing the membrane. Now, the membrane is - on outside and + on inside. An action potential is reached at +30 mV. Immediately, K+ gates open, and K+ moves out, repolarizing the membrane (+ out and - in). It even hyperpolarizes (less than -70) for a brief moment.

Then the active transport pumps reestablish the resting potential by pumping Na+ out and K+ in. This rapid sequence of Na+ and K+ movements is called the action potential and it triggers a chain reaction of action potentials all down the length of the neuron. This propagation of action potentials is known as a nerve impulse.

Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.

Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.

Refractory Period - time after impulse in which the membrane is not triggered by an ordinary stimulus All-or-None - impulses either conduct completely or not at all (conduction is constant and at maximum strength) Saltatory Conduction - type of impulse conduction along myelinated fibers - the impulse “jumps” from node to node (the nodes of Ranvier are the areas of exposed neuron fiber between myelinated regions) - faster and more efficient because less energy is needed

Sodium-Potassium Pump animation http://www.youtube.com/watch?v=90cj4NX87Yk&NR=1

Speed of Impulse Transmission Thick fibers transmit faster than thin. Myelinated fibers transmit faster than unmyelinated. Fastest - thick and myelinated - ex: motor fibers leading to muscles (120m/sec) Slowest - thin and unmyelinated - ex: sensory fibers in skin (0.5 m/sec)

Synapse A synapse is the junction between the axon of one neuron and the dendrite or cell body of another neuron- the 2 neurons are: 1. Presynaptic fiber - the neuron which sends impulse to synapse ( it releases neurotransmitters into the synaptic cleft) 2. Postsynaptic fiber - the neuron which has receptors for the neurotransmitters

Synaptic Transmission 1. Presynaptic knob of axon contains vesicles filled with neurotransmitters. 2. When impulse reaches knob, Ca++ diffuses into cytoplasm. 3. Vesicles move to membrane and release neurotransmitters into cleft. 4. Neurotransmitters cross cleft and bind to receptors on postsynaptic membrane.

5. If enough neurotransmitters are present, an action potential will occur. 6. Neurotransmitters are either decomposed or they move back into presynaptic knob.

Synapse animation

Types of Neurotransmitters - some are excitatory and some are inhibitory monoamines - ex: epinephrine, dopamine (inhibitory) amino acids - neuropeptides - substance P (pain), endorphins and enkaphalins (natural pain killers since they block the action of substance P)

Synaptic Potentials-change in voltage in localized area of dendrite or cell body Excitatory Postsynaptic Potential - neurotransmitter triggers Na+ gates to open and membrane is depolarized (more likely reach action potential) Inhibitory Postsynaptic Potential - neurotransmitter triggers K+ gates to open and membrane is hyperpolarized (less likely to reach action potential)

Many synaptic knobs contact each neuron…some excitatory, some inhibitory. The summation of their effects (usually occurs at the trigger zone) determines if the action potential occurs or not. Facilitation - if a neuron receives a net excitatory stimulation, but is subthreshold, it is more susceptible to an action potential the NEXT time.

Convergence When a neuron receives impulses from 2 or more other neurons - allows the brain to receive information from different regions of the body and respond in a special way

Divergence When 1 neuron sends impulses to 2 or more other neurons - results in an amplified response

Nerve Classification Origin Cranial Spinal Types of Fibers Sensory - contains only sensory fibers - (afferent) Motor - contains only motor fibers - (efferent) Mixed - contains both sensory and motor fibers Origin Cranial Spinal

Groups of General Nerve Fibers Within Nerves General Somatic Efferent Fibers -carry impulses from CNS to skeletal muscles General Visceral Efferent Fibers-carry impulses from CNS to smooth muscles and glands General Somatic Afferent Fibers-carry impulses from skeletal muscles and skin to CNS General Visceral Afferent Fibers-carry impulses from smooth muscles and glands to CNS

Reflex Arc - simple impulse pathway during a reflex (automatic unconscious response to a stimulus) Receptor - specialized nerve endings stimulated by change Sensory neuron - carries impulse from receptor to CNS Interneuron - connects sensory to motor neuron Motor neuron - carries impulse away from CNS Effector - smooth muscle or gland that responds