Chapter 48 ~ Nervous System

Nervous systems

Effector cells~ muscle or gland cells

Nerves~ bundles of neurons wrapped in connective tissue

Central nervous system (CNS)~ brain and spinal cord

Peripheral nervous system (PNS)~ sensory and motor neurons

Structural Unit of Nervous System

Neuron~ structural and functional unit

Cell body~ nucelus and organelles

Dendrites~ impulses from tips to neuron

Axons~ impulses toward tips

Myelin sheath~ supporting, insulating layer

Schwann cells~PNS support cells

Synaptic terminals~ neurotransmitter releaser

Synapse~ neuron junction

Simple Nerve Circuit

Sensory neuron: convey information to spinal cord

Interneurons: information integration

Motor neurons: convey signals to effector cell (muscle or gland)

Reflex: simple response; sensory to motor neurons

Ganglion (ganglia): cluster of nerve cell bodies in the PNS

Supporting cells/glia: nonconductiong cell that provides support, insulation, and protection

Neural signaling, I

Membrane potential (voltage differences across the plasma membrane)

Intracellular/extracellular ionic concentration difference

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

Net negative charge of about -70mV

Neural signaling, II

Excitable cells~ cells that can change membrane potentials (neurons, muscle)

Resting potential~ the unexcited state of excitable cells

Gated ion channels (open/close response to stimuli): photoreceptors; vibrations in air (sound receptors); chemical (neurotransmitters) & voltage (membrane potential changes)

Graded Potentials (depend on strength of stimulus) :

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

The Resting Potential

Neural signaling, III

Threshold potential: if stimulus reaches a certain voltage (-50 to -55 mV)….

The action potential is triggered….

Voltage-gated ion channels (Na+; K+)

1- Resting state •both channels closed

2- Threshold •a stimulus opens some Na+ channels

3- Depolarization •action potential generated •Na+ channels open; cell becomes positive (K+ channels closed)

4- Repolarization •Na+ channels close, K+ channels open; K+ leaves •cell becomes negative

5- Undershoot •both gates close, but K+ channel is slow; resting state restored

Refractory period ~ insensitive to depolarization due to closing of Na+ gates

The Action Potential

Neural signaling, IV

“Travel” of the action potential is self-propagating

Regeneration of “new” action potentials only after refractory period

Forward direction only

Action potential speed :

1-Axon diameter (larger = faster; 100m/sec)

2-Nodes of Ranvier (concentration of ion channels) ; saltatory conduction; 150m/sec

Synaptic communication

Presynaptic cell: transmitting cell

Postsynaptic cell: receiving cell

Synaptic cleft: separation gap

Synaptic vesicles: neurotransmitter releasers

Ca+ influx: caused by action potential; vesicles fuse with presynaptic membrane and release….

Neurotransmitter

Signal transmission

Neurotransmitters

Acetylcholine (most common) •skeletal muscle

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

Amino acids

Neuropeptides (short chains of amino acids) •endorphin

Vertebrate 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 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