Chapter 10 Nervous System I Cell Types of Neural Tissue neurons neuroglial cells 10-2
Divisions of the Nervous System Central Nervous System brain spinal cord Peripheral Nervous System peripheral nerves cranial nerves spinal nerves 10-1
Divisions of Peripheral Nervous System Sensory Division picks up sensory information and delivers it to the CNS Motor Division carries information to muscles and glands Divisions of the Motor Division Somatic – carries information to skeletal muscle Autonomic – carries information to smooth muscle, cardiac muscle, and glands 10-4
Functions of Nervous System Sensory Function sensory receptors gather information information is carried to the CNS Integrative Function sensory information used to create sensations memory thoughts decisions Motor Function decisions are acted upon impulses are carried to effectors 10-6
Myelination of Axons White Matter contains myelinated axons Gray Matter contains unmyelinated structures cell bodies, dendrites 10-8
Classification of Neurons Bipolar two processes eyes, ears, nose Unipolar one process ganglia Multipolar many processes most neurons of CNS 10-9
Classification of Neurons Sensory Neurons afferent carry impulse to CNS most are unipolar some are bipolar Interneurons link neurons multipolar in CNS Motor Neurons multipolar carry impulses away from CNS carry impulses to effectors 10-10
Types of Neuroglial Cells Schwann Cells peripheral nervous system myelinating cell Oligodendrocytes CNS myelinating cell Astrocytes CNS scar tissue mop up excess ions, etc induce synapse formation connect neurons to blood vessels Microglia CNS phagocytic cell Ependyma CNS ciliated line central canal of spinal cord line ventricles of brain 10-11
Resting Membrane Potential inside is negative relative to the outside polarized membrane due to distribution of ions Na + /K + pump 10-14
Local Potential Changes occur on membranes of dendrites and cell bodies caused by various stimuli chemicals temperature changes mechanical forces if membrane potential becomes more negative, it has hyperpolarized if membrane potential becomes more positive, it has depolarized graded summation can lead to threshold stimulus that starts an action potential 10-16
Resting Membrane Potential Charged ions that carry electric charges across the membranes are sodium, chloride, potassium, and calcium. Ion pumps and channels generate resting and action potentials. Sodium/potassium pump uses energy to move the ions against their concentration gradient. K + leaks out, because the membrane is more permeable to K +, and Na/K pump keeps the concentration of K + inside the cell higher than that outside.
This causes an excess of negative charges inside the cell. There is a higher concentration of Na + outside the membrane and higher K + concentration inside. The Na + / K + pumps, three sodium ions out for every two potassium ions it pumps in. When voltage-gated channels open and close the concentration of ions change, causing a change in membrane potential.
Action Potential Sodium gates open in response to threshold potential at the axon hillock. The channels are depolarized to their threshold (5/10 m V more positive). Sodium rushes In; causing the rising phase or spike of an action potential. Repolarizing the membrane is due to rapid closing of the sodium channels. Potassium gates open more slowly and stay open longer, repolarizing and hyperpolarizing the cell. All gated channels close. The cell returns to resting potential.
Potential Changes at rest membrane is polarized sodium channels open and membrane depolarizes potassium leaves cytoplasm and membrane repolarizes threshold stimulus reached 10-15
Action Potentials nerve impulse occur on axons all-or-none self-regenerating, because it spreads to adjacent regions of the membrane. Summation- several subthreshold potential changes that combine to reach threshold. refractory period absolute - time when threshold stimulus does not start another action potential ( sodium gates are open) relative – time when stronger threshold stimulus can start another action potential ( reestablishing its resting potential). 10-17
In unmyelinated fibers, the axon potential spreads smoothly along the axon. In myelinated fibers, action potential jump across the myelin segments to nodes of Ranvier. The hopping from node to node is much faster than non-myelinated conduction. Additionally, ion channels are clustered at the nodes. AP’s travel faster in large-diameter axons than small-diameter axons.
The Synapse Nerve impulses pass from neuron to neuron at synapses 10-21
Synaptic Transmission Neurotransmitters are released when impulse reaches synaptic knob 10-22
Chemical synapse is the most common type of synapse. Acetylcholine is the neurotransmitter used by neurons innervating skeletal muscle. It is released by exocytosis when the vesicle fuses with membrane. Depolarization of the axon terminal causes Ca+ to enter the cell triggering fusion of the vesicles.
Synaptic Potentials EPSP excitatory postsynaptic potential graded depolarizes membrane of postsynaptic neuron action potential of postsynaptic neuron becomes more likely IPSP inhibitory postsynaptic potential graded hyperpolarizes membrane of postsynaptic neuron action potential of postsynaptic neuron becomes less likely 10-23
Summation of EPSPs and IPSPs EPSPs and IPSPs are added together in a process called summation More EPSPs lead to greater probability of action potential 10-24
Impulse Processing Neuronal Pools groups of interneurons that make synaptic connections with each other interneurons work together to perform a common function each pool receives input from other neurons each pool generates output to other neurons 10-26
Convergence neuron receives input from several neurons incoming impulses represent information from different types of sensory receptors allows nervous system to collect, process, and respond to information makes it possible for a neuron to sum impulses from different sources 10-27
Divergence one neuron sends impulses to several neurons can amplify an impulse impulse from a single neuron in CNS may be amplified to activate enough motor units needed for muscle contraction 10-28
Clinical Application Multiple Sclerosis Symptoms blurred vision numb legs or arms can lead to paralysis Causes myelin destroyed in various parts of CNS hard scars (scleroses) form nerve impulses blocked muscles do not receive innervation may be related to a virus Treatments no cure bone marrow transplant interferon (anti-viral drug) hormones 10-29