Chapter 48 ~ Nervous System

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Nervous System OverviewNervous System Overview Sensory Input Integration Motor Output-signal conducted from processing

center to effector cells

Signals Conducted by Nerves-extensions of nerve cells

Nervous System Composition:Neurons and Glia (supporting cells)

Neurons communicate information via electrical and chemical signals

Both Divisions of the Nervous System InvolvedBoth Divisions of the Nervous System Involved

1. Central nervous system (CNS)~ brain and spinal cord; Integration

2. Peripheral nervous system (PNS)~ sensory (input) and motor neurons (output)

Effector cells~ muscle or gland cells

Nerves~ bundles of neurons wrapped in connective tissue

Neuron structureNeuron structure Neuron- structural and functional unit

– Cell body- nucelus and organelles

– Dendrites- signals to cell body. Short, numerous

– Axons- away from cell body. Long, Myelin sheath- supporting, insulating layer produced by Schwann Cells Schwann cells-PNS support cells; surround axons Axon hillock-Hillock-axon extends from here Synaptic terminals~ neurotransmitter releaser

Synapse- gap / neuron junction

3 Classes of neurons3 Classes of neurons

1. Sensory neuron: receive & convey from sensory environment information to spinal cord

2.Interneurons: information integration; located in CNS. Synapse only with other neurons.

3. Motor neurons: convey impulses from CNS to effector cell. (muscle or gland)

Neurons Grouped into Nerve CircuitNeurons Grouped into Nerve Circuit

The Reflex Arc– Simplest : – Knee-Jerk Reflex (Patellar

Reflex)– Stretch receptor– simple response; sensory

to spinal cord to motor neurons—knee contracts

Neural SignalingNeural Signaling Signal transduction depends on voltages across neuron plasma

membranes.– Membrane Potential: voltage differences across the plasma membrane).

Net negative charge of about -70mV

Ions Intracellular ( -) ; K+ principal cation Large organic ions- anions Extracellular (less negative) Na+- principal cation Cl- main anion. Ion channels- ungated, gated; all selective

K+ diffuses out (Na+ in); large anions cannot follow….selective permeability of the plasma membrane

Creating & Maintaing the Membrane PotentialCreating & Maintaing the Membrane Potential

Na + - K + Pumps --pump against their conc. gradients

ATP

K+ pumped back in

Na+ pumped back out

Changes in membrane potential key to neural transmission

Changes in membrane potential key to neural transmission

Only neurons and muscle cells can change their membrane potentials in response to stimuli – Excitable Cells– Sensory neurons-environmental stimuli– Interneurons stimuli transmitted via other neurons

– Resting Potential: M.P. of excitable cell at rest.– Change due to flow of ions as gated ion channels open.– stimuli cause ion channels to open

Stimuli that open K+ channels HYPERPOLARIZE the neuron

Stimuli that open NA+ channels DEPOLARIZE the neuron

Graded Potentials –these voltage changes Graded Potentials –these voltage changes

1- Hyperpolarization (outflow of K+); increase in electrical gradient; cell becomes more negative

2- Depolarization (inflow of Na+); reduction in electrical gradient; cell becomes less negative

MylenationMylenation

Electrical insulation—lipid is poor conductor

– Increasing speed of nerve impulse propagation

Multiple Sclerosis: myelin sheaths deteriorated-los of coordination

Normal Membrane PotentialNormal Membrane Potential

Resting Potential: Resting Neuron -70 mV Cytoplasm is negatively charged relative to cell

interior

Resting potentialResting potential~ the membrane potential of the unexcited nerve. – A change in voltage MAY result in an

electrical impulse.

When the Threshold potential is reached, usually sl. More positive (-50 to -55 mV)….

The action potential is triggered….

– The rapid change in membrane potential in an excitable cell

– b/c stimulus triggered the selective opening and closing of voltage-gated ion channels

Action Potential-All Or None change in the Membrane Potential Phases

Action Potential-All Or None change in the Membrane Potential Phases

1. Resting stage •both channels closed

2-Depolarization: •a stimulus opens some Na+ channel gates

Na+ influx reverses membrane polarity.

Threshold reached. (cell interior sl. positive)

Action potential generated .

3-Repolarization •Na+ channels close. K+ channels open; K+ leaves

cell returns to resting potential—then ..

4-Undershoot •K+ channels still open-temporarily HYPERPOLAR.

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The Action PotentialThe Action Potential

Followed by a Refractory period~ insensitive to stimulus.

Amplitude not affected by stimuli Intensity

Action Potentials are self-propagatingAction Potentials are self-propagating

Action Potential regenerated along axon membrane begins at Axon Hillock “Travel” of the action potential is self-propagating One direction only.

Nodes of Ranvier-action potential jumps from one node to the next– Gaps, ion sensitive channels concentrated here, extracellular fluid

contact here

Forward direction only

Action potential speed:Action potential speed:1) Axon diameter (larger = faster; 100m/sec)

2) Saltatory Conduction:

– Mylenation

– Nodes of Ranvier (concentration of ion channels in gaps of the myelin).

– A.P. “jumps” from node to node. 120m/sec

Chemical or Electrical Communication between cells occurs at synapses

Chemical or Electrical Communication between cells occurs at synapses Synapse-tiny gap

Presynaptic cell: transmitting cell Postsynaptic cell: receiving cell

1) Electrical Synapses-via gap junctions; no delay or less in signal strength; less common; fish tail-swim away quickly from predator

2) Chemical Synapses: synaptic cleft separates pre and post-synaptic cells.

Not electrically coupled

Synaptic communicationSynaptic communication Synaptic cleft: separation gap Synaptic vesicles: neurotransmitter

releasers

When an Action Potential arrives at synaptic terminal of presynaptic cell

Causes Ca++ influx; Synaptic vesicles fuse with presynaptic membrane and release…. Neurotransmitter

Neurotransmitters quickly degraded

Neurotransmitter maydo one of the followingNeurotransmitter maydo one of the following

1. Excite the membrane by depolarization

Or

2. Inhibit the postsynaptic cells by hyperpolarization

Types of NeurotransmittersTypes of Neurotransmitters Acetylcholine (most common)

– may be excitatory or inhibitatory– skeletal muscle

Biogenic amines (derived from amino acids)•norepinephrine , epinephrine•dopamine •serotonin (from tryptophan)

Amino acids– GABA—most abundant inhibitory transmitter in brain

Neuropeptides (short chains of amino acids)•endorphin-natural analgesics for the brain

Gaseous Signals of the Nervous SystemGaseous Signals of the Nervous System

NO (nitric oxide)—blood vessel dilation.

– Acetylcholine stimulates blood vessel walls to release NO; neighboring smooth muscles relax & dilate heart’s blood vessels.

– Nitroglycerine is converted to NO—similar response

Nervous system organization tends to corrolate with body symmetry

Nervous system organization tends to corrolate with body symmetry

Vertebrate PNSVertebrate PNS

Cranial nerves (brain origin)

Spinal nerves (spine origin)

Sensory division Motor division

•somatic system voluntary, conscious control •autonomic system √parasympathetic

conservation of energy √sympathetic

increase energy consumption

The Vertebrate BrainThe Vertebrate Brain Forebrain

•cerebrum~memory, learning, emotion•cerebral cortex~sensory and motor nerve cell bodies •corpus callosum~connects left and right hemispheres •thalamus; hypothalamus

Midbrain •inferior (auditory) and superior (visual) colliculi

Hindbrain •cerebellum~coordination of movement •medulla oblongata/ pons~autonomic, homeostatic functions