 Neurons = nerve cells  The major function of nerve cells is to transmit messages (nerve impulses) from one part of the body to another. ◦ Major regions of neurons  Cell body —nucleus and metabolic center of the cell  Processes —fibers that extend from the cell body

◦ Major regions of neurons  Cell body —nucleus and metabolic center of the cell  Processes —fibers that extend from the cell body Dendrites and Axons

 Some Functions: 1. supporting cells act as phaogcytes 2. protect and myelinate 3. act as a selective barrier between the capillary blood supply and neurons See the next 5 slides for details of support cells

 Astrocytes ◦ Abundant, star-shaped cells ◦ Brace neurons ◦ Form barrier between capillaries and neurons ◦ Control the chemical environment of the brain

 Microglia ◦ Spiderlike phagocytes ◦ Dispose of debris

 Ependymal cells ◦ Line cavities of the brain and spinal cord ◦ Circulate cerebrospinal fluid

 Oligodendrocytes ◦ Wrap around nerve fibers in the central nervous system ◦ Produce myelin sheaths

 Satellite cells ◦ Protect neuron cell bodies  Schwann cells ◦ Form myelin sheath in the peripheral nervous system

 Central nervous system (CNS) ◦ Brain ◦ Spinal cord  Peripheral nervous system (PNS) ◦ Nerves outside the brain and spinal cord  Spinal nerves  Cranial nerves

Figure 7.6

 Nuclei- collections of cell bodies inside the central nervous system  Ganglion—collection of cell bodies outside the central nervous system  Tract – cranial and spinal nerves ( nerve process in CNS)  Nerves – nerve processes outside the CNS (in the PNS) Terms for collections of nerve cell bodies and collections of nerve fibers

 Sensory (afferent) neurons ◦ Carry impulses from the sensory receptors to the CNS  Cutaneous sense organs  Proprioceptors—detect stretch or tension  Motor (efferent) neurons ◦ Carry impulses from the central nervous system to viscera, muscles, or glands  Interneurons (association neurons) ◦ Found in neural pathways in the central nervous system ◦ Connect sensory and motor neurons

 Axonal terminals contain vesicles with neurotransmitters-chemical released by axon terminals  Axonal terminals are separated from the next neuron by a gap ◦ Synaptic cleft—gap between adjacent neurons ◦ Synapse—junction or point of close contact between neurons

 Myelin sheath—whitish, fatty material covering axons  Schwann cells—produce myelin sheaths in jelly roll–like fashion  Nodes of Ranvier—gaps in myelin sheath along the axon Multipolar neuron have several processes extending from its cell body All motor neurons and interneurons!

 Neurons have only ONE axon that carries impulses away from the nerve cell body toward the synapse

 Bipolar neurons—one axon and one dendrite  Found in eyes and nose as receptor cells Figure 7.8b

 Unipolar neurons—have a short single process leaving the cell body  Sensory neurons found in PNS ganglia Figure 7.8c

Figure 7.5 Axons in the PNS are myelinated by special supporting cells called Schwann cells, which may wrap themselves tightly around the axon in jelly roll fashion so that when the process is completed, a tight core of plasma membrane material called myelin sheath encompasses the axon. The Schwann cell nucleus and the bulk of its cytoplasm end up just beneath the outermost portion of the plamsa membrane. The part of the schwann cell which is external to the myelin sheath, is referred to as the neurilemma.

Stimulus at distal end of neuron Skin Spinal cord (in cross section) Interneuron Receptor Effector Sensory neuron Motor neuron Integration center (a)

 Resting neuron ◦ The plasma membrane at rest is polarized ◦ Fewer positive ions are inside the cell than outside the cell  Depolarization ◦ A stimulus depolarizes the neuron’s membrane ◦ A depolarized membrane allows sodium (Na+) to flow inside the membrane ◦ Reversal of the resting potential owing to an influx of sodium ions  Depolarization initiates an action potential in the neuron----BUT only if the stimulus is greater than the threshold intensity

Figure 7.9a–b

 Action potential ◦ If the action potential (nerve impulse) starts, it is propagated over the entire axon ◦ Impulses travel faster when fibers have a myelin sheath ◦ Transmission of the depolarized wave along the neural membrane

Figure 7.9c–d

 Repolarization ◦ Potassium ions rush out of the neuron after sodium ions rush in, which repolarizes the membrane ◦ Period during which potassium ions are diffusing out of the neuron sodium-potassium pump, mechanism that restores the resting membrane voltage and intracellular ionic concentrations using ATP

Figure 7.9e–f

 Impulses are able to cross the synapse to another nerve ◦ Neurotransmitter is released from a nerve’s axon terminal ◦ The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter ◦ An action potential is started in the dendrite

Figure 7.10 Axon terminal Vesicles Synaptic cleft Action potential arrives Synapse Axon of transmitting neuron Receiving neuron Neurotrans- mitter is re- leased into synaptic cleft Neurotrans- mitter binds to receptor on receiving neuron’s membrane Vesicle fuses with plasma membrane Synaptic cleft Neurotransmitter molecules Ion channels Receiving neuron Transmitting neuron Receptor Neurotransmitter Na + Neurotransmitter broken down and released Ion channel opensIon channel closes

Figure 7.10, step 1 Axon terminal Vesicles Synaptic cleft Action potential arrives Synapse Axon of transmitting neuron Receiving neuron

Figure 7.10, step 2 Axon terminal Vesicles Synaptic cleft Action potential arrives Synapse Axon of transmitting neuron Receiving neuron Vesicle fuses with plasma membrane Synaptic cleft Ion channels Receiving neuron Transmitting neuron

Figure 7.10, step 3 Axon terminal Vesicles Synaptic cleft Action potential arrives Synapse Axon of transmitting neuron Receiving neuron Neurotrans- mitter is re- leased into synaptic cleft Vesicle fuses with plasma membrane Synaptic cleft Neurotransmitter molecules Ion channels Receiving neuron Transmitting neuron

Figure 7.10, step 4 Axon terminal Vesicles Synaptic cleft Action potential arrives Synapse Axon of transmitting neuron Receiving neuron Neurotrans- mitter is re- leased into synaptic cleft Neurotrans- mitter binds to receptor on receiving neuron’s membrane Vesicle fuses with plasma membrane Synaptic cleft Neurotransmitter molecules Ion channels Receiving neuron Transmitting neuron

Figure 7.10, step 5 Axon terminal Vesicles Synaptic cleft Action potential arrives Synapse Axon of transmitting neuron Receiving neuron Neurotrans- mitter is re- leased into synaptic cleft Neurotrans- mitter binds to receptor on receiving neuron’s membrane Vesicle fuses with plasma membrane Synaptic cleft Neurotransmitter molecules Ion channels Receiving neuron Transmitting neuron Receptor Neurotransmitter Na + Ion channel opens

Figure 7.10, step 6 Axon terminal Vesicles Synaptic cleft Action potential arrives Synapse Axon of transmitting neuron Receiving neuron Neurotrans- mitter is re- leased into synaptic cleft Neurotrans- mitter binds to receptor on receiving neuron’s membrane Vesicle fuses with plasma membrane Synaptic cleft Neurotransmitter molecules Ion channels Receiving neuron Transmitting neuron Receptor Neurotransmitter Na + Neurotransmitter broken down and released Ion channel opensIon channel closes

Figure 7.10, step 7 Axon terminal Vesicles Synaptic cleft Action potential arrives Synapse Axon of transmitting neuron Receiving neuron Neurotrans- mitter is re- leased into synaptic cleft Neurotrans- mitter binds to receptor on receiving neuron’s membrane Vesicle fuses with plasma membrane Synaptic cleft Neurotransmitter molecules Ion channels Receiving neuron Transmitting neuron Receptor Neurotransmitter Na + Neurotransmitter broken down and released Ion channel opensIon channel closes

Figure 7.23 Endoneurium surrounds each fiber Groups of fibers are bound into fascicles by perineurium Fascicles are bound together by epineurium The connective tissue wrappings help insulate the nerve.

 A bundle of neuron fibers or processes that extends to and/or from the CNS and visceral organs or structures of the body periphery such as skeletal muscle, glands, and skin

 Mixed nerves ◦ Carry both sensory and motor fibers  Sensory (afferent) nerves ◦ Carry impulses toward the CNS  Motor (efferent) nerves ◦ Carry impulses away from the CNS