Chapter 10 Nervous System I Cell Types of Neural Tissue neurons neuroglial cells

Composed of some blood vessels and connective tissue but mostly neural tissue (2 cell types) neurons neuroglia Neurons transmit information as nerve impulses along nerve fibers Nerves are bundles of nerve fibers

Neuroglia surround, support, and nourish neurons and might even send and receive messages Synapses are spaces between neurons Neurotransmitters are biological messengers Central nervous system (CNS) consists of the brain and spinal cord

Peripheral nervous system (PNS) consists of the peripheral nerves that connect the CNS to other body parts CNS and PNS provide 3 general functions: sensory, integrative, and motor

Sensory receptors at the ends of peripheral neurons gather information: Info  nerve impulse  CNS  integration  decision made  motor neurons  muscles or glands (effectors)

Divisions of the Nervous System Central Nervous System brain spinal cord Peripheral Nervous System nerves cranial nerves spinal nerves

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

Divisions Nervous System

Functions of Nervous System Sensory Function sensory receptors gather information information is carried to the CNS Motor Function decisions are acted upon impulses are carried to effectors Integrative Function sensory information used to create sensations memory thoughts decisions

Neuron Structure

Mature neurons generally do not reproduce All neurons have a cell body and nerve fibers Cell body contains: granular cytoplasm, mitochondria, lysosomes, a Golgi apparatus, microtubules Neurofibrils (fine threads) extends into and supports fibers

Chromatophilic substance (nissl bodies) made of rough endoplasmic reticulum is scattered in cytoplasm Cytoplasmic inclusions include glycogen, lipids, and pigments Large nucleus near the center with a nucleolus Nerve fibers, dendrites and axons extend from the cell body

Dendrites are usually branched and communicate with other neurons Axons carry nerve impulses away from the cell body Axons also convey biochemicals produced in the cell body (axonal transport) Schwann cells wind around axons in layers called myelin

Myelin has higher lipids than other surface membranes and forms a myelin sheath on the outside of axons A neurilemmal sheath made of Schwann cells that have most of the cytoplasm and nuclei forms outside the myelin sheath Nodes of Ranvier are gaps in myelin sheath between Schwann cells

Myelinated fibers appear white (white matter in brain and spinal cord) Small axons don’t have myelin (unmyelinated) and appears gray (gray matter in brain and spinal cord)

Myelination of Axons White Matter contains myelinated axons Gray Matter contains unmyelinated structures cell bodies, dendrites

Classification of Neurons – Structural Differences Bipolar two processes eyes, ears, nose Unipolar one process ganglia Multipolar many processes most neurons of CNS

Neurons Vary in size and shape May differ in length and size of axons and dendrites Vary in the number of processes by which they communicate with other neurons Vary in function

3 types classified by structure: bipolar, unipolar, multipolar (Table 10.1) 3 types classified by function: sensory, interneuron, motor (Table 10.1)

Classification of Neurons – Functional Differences 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

Spinal Cord Injuries

Types of Neuroglial Cells in the PNS Schwann Cells produce myelin found on peripheral myelinated neurons speed neurotransmission Satellite Cells support clusters of neuron cell bodies (ganglia)

Neuroglia Guide neurons in the embryo to their positions and may stimulate them to specialize Produce growth factors that nourish neurons Remove ions and neurotransmitters that build up in between neurons, allowing continued information transmission

Some may communicate with neurons Schwann cells are the neuroglia of the PNS CNS neuroglia: astrocytes, oligodendrocytes, microglia, and ependyma

Neuroglia form more than half the volume of the brain Most brain tumors are neuroglia that multiply too often

Types of Neuroglial Cells in the CNS 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 Oligodendrocytes CNS myelinating cell

Types of Neuroglial Cells

Regeneration of A Nerve Axon

Mature neurons do not reproduce Injury to a neuron cell body usually kills it Damaged axons in a peripheral nerve may regenerate (3-4mm /day), but may end up in the wrong place so full function often does not return

Neuroglial cells assist in regeneration (nerve growth factors) Damaged axons in the CNS are unable to produce myelin and regeneration is unlikely

The Synapse Nerve impulses pass from neuron to neuron at synapses

Synaptic Transmission Neurotransmitters are released when impulse reaches synaptic knob

Cell Membrane Potential A cell membrane is polarized (electrically charged) due to unequal distribution of ions. The inside is negative with respect to the outside.

Distribution of Ions K+ are the major ions inside the cell (intracellular) Na+ are the major extracellular ions Channels in the membrane allow movement in and out Chemical and electrical factors affect the opening and closing of these gatelike channels

Resting nerve cell is not being stimulated to send an impulse K+ pass through resting cell membranes much easier than Na+ Ca2+ less able to cross a resting cell membrane than K+ or Na+

Resting Membrane Potential inside is negative relative to the outside polarized membrane due to distribution of ions Na+/K+ pump

Active transport keeps a greater concentration of K+ inside the cell and a greater concentration of Na+ outside the cell Cytoplasm contains anions: (PO4-2), (SO4-2), and proteins that can’t diffuse through the cell membrane

Na+ and K+ follow the laws of diffusion (high to low) more K+ diffuses out than Na+ can move in more + charges leave the cell than enter outside of the cell has a positive charge and the inside has a negative charge  

Difference in electrical charge = potential difference (measured in volts) Difference between inside and outside = -70 millivolts Resting potential = separation of charge Action potential = work it may do to send a nerve impulse

Local Potential Changes caused by various stimuli temperature changes light pressure environmental changes affect the membrane potential by opening a gated ion channel

Local Potential Changes if membrane potential becomes more negative, it has hyperpolarized if membrane potential becomes less negative, it has depolarized graded summation can lead to threshold stimulus that starts an action potential

Local Potential Changes

Nerve cells can respond to changes in their surroundings Changes affect the resting potential of the membrane Hyperpolarized = membrane potential becomes more negative than resting potential

Depolarized = membrane potential becomes more positive than resting potential (Na+ move in) Threshold potential = strong enough depolarization to conduct a nerve impulse (action potential)

Neuron Structure

Action Potentials at rest membrane is polarized threshold stimulus reached sodium channels open and membrane depolarizes potassium leaves cytoplasm and membrane repolarizes

Action Potentials Reached when the membrane potential becomes positive When the threshold potential is reached Na+ move into the cell causing the membrane potential to become more positive (as high as +30 mV)  depolarization

At the same time K channels open and allow K+ to move out causing the inside to become negative again  repolarization Rapid sequence of depolarization and repolarization causes an electric current to move down the nerve fiber  nerve impulse

Action Potentials

Action Potentials

All-or None Response If a nerve fiber responds, it responds completely All impulses are the same strength Greater intensity of stimulation produces more impulses per second

Refractory Period Short time after a nerve impulse that a threshold stimulus will not trigger another impulse Limits the rate of nerve conduction about one /millisecond

Impulse Conduction

Impulse conduction Unmyelinated nerve fiber conducts an impulse over the entire surface Myelinated fibers conduct impulses at only the nodes of ranvier myelin insulates and prevents the flow of ions through the membrane

myelin made of lipids that are insoluble to water soluble substances action potentials jump from node to node (saltatory conduction) Myelinated axons conduct impulses much faster than unmyelinated axons

The larger the diameter of the fiber the faster the impulse is conducted Ex: skeletal muscle motor fiber = 120m /s unmyelinated sensory fiber = 0.5m/s

Saltatory Conduction

The Synapse Junction between two cells Synaptic cleft is a space between two cells Synaptic knobs are at the end of axons contain vesicles that contain neurotransmitters Neurotransmitters (NT) are released when a nerve impulse reaches the end of the axon – they diffuse across the synaptic cleft

Synaptic Potentials Some NT depolarize membranes causing an action potential  excitatory postsynaptic potential (EPSP) Some NT hyperpolarize membranes inhibiting an action potential  inhibitory postsynaptic potential (IPSP) EPSPs and IPSPs are summed at the trigger zone (axon area close to cell body)  decision making part of the axon

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

Neurotransmitters

Neurotransmitters

Table 10.05

Neurotransmitters At least 30 different types Neurons release 1-3 kinds Synthesized in synaptic knobs and stored in synaptic vesicles

Ca2+ cause NT to be released Enzymes decompose some NT to keep the impulse short Reuptake transports some NT back into synaptic knobs Removal of NT prevents continuous stimulation

Synaptic Transmission Neurotransmitters are released when impulse reaches synaptic knob

Neuropeptides Act as NT or neuromodulators Alter a neurons response to a NT or block its release Enkephalins – bind to opiate receptors in brain and relieve pain Beta endorphin – pain reliever

Substance P – transmits pain impulses into the spinal cord and then to the brain enkephalins and endorphins may relieve pain by inhibiting the release of substance P

Impulse Processing Neuronal Pools Groups of neurons in the CNS Each receive impulses from afferent nerve fibers Each input fiber divides and branches Impulses are conducted away from CNS on efferent output fibers

Facilitation If incoming impulses do not reach threshold the neuron becomes more excitable to incoming stimulation  

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

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

Clinical Application Drug Addiction occurs because of the complex interaction of neurons, drugs, and individual behaviors understanding how neurotransmitters fit receptors can help explain the actions of certain drugs drugs have different mechanisms of action several questions remain about the biological effects of addiction, such as why some individuals become addicted and others do not