The Nervous System

Functions of the Nervous System Figure 7.1

Functions of the Nervous System Sensory input—gathering information To monitor changes occurring inside and outside the body Changes = stimuli Integration To process and interpret sensory input and decide if action is needed

Functions of the Nervous System Motor output A response to integrated stimuli The response activates muscles or glands

Organization of the Nervous System Figure 7.2

Structural Classification of the Nervous System Central nervous system (CNS) Brain Spinal cord Peripheral nervous system (PNS) Nerves outside the brain and spinal cord Spinal nerves Cranial nerves

Functional Classification of the Peripheral Nervous System Sensory (afferent) division Nerve fibers that carry information to the central nervous system Motor (efferent) division Nerve fibers that carry impulses away from the central nervous system

Functional Classification of the Peripheral Nervous System Motor (efferent) division (continued) Two subdivisions Somatic nervous system = voluntary Autonomic nervous system = involuntary

Neuron Classification Figure 7.6

Figure 7.4

Neurons = nerve cells Cells specialized to transmit messages

Nervous Tissue: Neurons Cell body Nucleus Large nucleolus Processes outside the cell body Dendrites—conduct impulses toward the cell body Axons—conduct impulses away from the cell body

Figure 7.4

Nervous Tissue: Neurons Figure 7.5

Nervous Tissue: 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 Neurilemma – exposed layer of cell membrane of Schwann cells

Nervous Tissue: Neurons Axons end in axonal terminals Most cells average about 10,000 axon terminals Axon terminals contain vesicles with neurotransmitters (chemicals that excite or inhibit neurons or effector cells) Axon terminals are separated from the next neuron by a gap Synaptic cleft—gap between adjacent neurons Synapse—junction between nerves

Neuron Cell Body Location Most neuron cell bodies are found in the central nervous system White matter – condensed areas of myelinated fibers Gray matter—areas of unmyelinated fibers Ganglia—collections of cell bodies outside the central nervous system

Figure 7.8a Structural Classification of Neurons Multipolar neurons—many extensions from the cell body

Structural Classification of Neurons Bipolar neurons—one axon and one dendrite Figure 7.8b

Structural Classification of Neurons Unipolar neurons—have a short single process leaving the cell body Figure 7.8c

Nerve Impulse The principle way neurons communicate is by generating and propagating ACTION POTENTIALS (Aps). Only cells with excitable membranes (like muscle cells and neurons) can generate APs. An AP is a brief reversal of membrane potential. In neurons, an AP is called a NERVE IMPULSE and only axons can generate one. An AP involves the movement of Na+ and K+ ions moving in and out of the neuron, resulting in changes in POLARITY.

All or None Response If the action potential (nerve impulse) starts, it is propagated over the entire axon There are NO partial impulses. The impulse happens completely or not at all. Impulses travel faster when fibers have a myelin sheath.

Transmission of a Signal at Synapses 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

Transmission of a Signal at Synapses 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

Transmission of a Signal at Synapses 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

Transmission of a Signal at Synapses 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

Nerve Impulses 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 The exchange of ions initiates an action potential in the neuron

Nerve Impulses 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

Nerve Impulses Repolarization Potassium ions rush out of the neuron after sodium ions rush in, which repolarizes the membrane The sodium-potassium pump, using ATP, restores the original configuration

Nerve Impulses Figure 7.9a–b

Nerve Impulses Figure 7.9c–d

Nerve Impulses Figure 7.9e–f