Chapter 10 Nervous System I

Divisions of the Nervous System Central Nervous System Brain and spinal cord All sensations have to be relayed here to be acted on Muscle & gland stimulation Control center for the entire system

Divisions of the Nervous System Peripheral Nervous System Connection between and CNS & receptors, muscles, and glands Nerves Cranial nerves Spinal nerves

Divisions of Peripheral Nervous System Sensory Division Afferent System Picks up sensory information and delivers it to the CNS Motor Division Efferent System Carries information to muscles and glands

Divisions of Peripheral Nervous System Divisions of the Motor Division Somatic (SNS) Carries information to skeletal muscle Voluntary Autonomic (ANS) Carries information to smooth muscle, cardiac muscle, and glands Involuntary

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 Most rapid means of maintaining homeostasis in your body

Nervous System Cytology Two Types of Cells Neurons Conduct nerve impulses from one part of the body to another Information Processing Units Neuroglial cells

Neuron Structure

Neuron Structure Dendrites Cell Body Large nucleus surrounded by granular cytoplasm Dendrites Thick branched divisions of cell body Bring nerve impulses toward the cell body

Neuron Structure Axon Usually a single, longer process that conducts nerve impulses from the cell body Terminates at another neuron, muscle or gland May be up to a meter

Neuron Structure Axon Axon terminal End of an axon with many branching fibers End expands to form synaptic end bulb Nerve Fiber Common name for an axon and its myelin sheath

Myelination of Axons Myelin sheath Formed by neuroglial cell Phospholipid segment that wraps around axon Provides protection for axon Increases speed of impulse along axon

Myelination of Axons Myelin sheath Schwann cells Form myelin sheath Found ONLY in peripheral nervous system Assist in repair of damaged axons, provides tube for axon or dendrite to grow

Myelination of Axons Myelin sheath Production begins during 1st year of life Amount increases from birth to maturity This is why adults react quicker to certain stimuli

Myelination of Axons Myelin sheath Nodes of Ranvier Segments on axon that are not myelinated Gaps in sheath

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

Classification of Neurons – Structural Differences Bipolar Two processes One axon, one dendrite Eyes, ears, nose Unipolar One process that branches in two Ganglia Specialized masses of nerve tissue outside brain & spinal cord

Classification of Neurons – Structural Differences Multipolar Many processes Only one axon Most neurons of CNS

Classification of Neurons – Functional Differences Sensory Neurons Afferent Carry impulse to CNS Dendrites act as sensory receptors Most are unipolar Some are bipolar Motor Neurons multipolar carry impulses away from CNS Carry impulses to effectors (muscles or glands) Interneurons Link neurons Multipolar In CNS

Classification of Neurons – Functional Differences

Types of Neuroglial Cells in the PNS Schwann Cells Produce myelin found on peripheral myelinated neurons Speed neurotransmission Satellite Cells Support ganglia in PNS

Types of Neuroglial Cells in the CNS Astrocytes CNS Scar tissue in CNS Regulate ion concentrations (K+) Induce synapse formation Connect neurons to blood vessels Oligodendrocytes Form myelin in CNS Myelinating cell

Types of Neuroglial Cells in the CNS Microglia CNS Phagocytic cell Proliferate w/ CNS injury Ependyma CNS Cuboidal or columnar Ciliated Line central canal of spinal cord Line ventricles of brain

The Synapse Junction between two neurons Also called synaptic clefts Essential in homeostasis because of the ability to transmit some impulses while inhibiting others Brain disease & many psychiatric disorders result from bad synaptic communication

The Synapse 2 Types Electrical & Chemical Most in CNS are chemical Function Neuron secretes neurotransmitters across synaptic cleft Post-synaptic neuron has receptors to match transmitter When they match, impulse continues

Resting Membrane Potential Inside is negative relative to the outside Polarized membrane Due to distribution of ions Unequal distribution of Na+ and K+ ions K+ 28x greater inside Na+ 14x greater outside Na+/K+ pump

Sodium/Potassium Pump Fights osmosis Transports 3 Na+ out & 2 K+ into a resting neuron Active Process

Local Potential Changes Caused by various stimuli Temperature changes Light Pressure Environmental changes affect the membrane potential by opening a gated ion channel Allows Na+ to diffuse in & K+ to diffuse out

Local Potential Changes If membrane potential becomes more negative, it has hyperpolarized If membrane potential becomes less negative, it has depolarized Summation can lead to threshold stimulus that starts an action potential Multiple impulses often needed to reach threshold stimulus

Local Potential Changes

Action Potentials At rest membrane is polarized Threshold stimulus reached Na+ channels open and membrane depolarizes Na+ enters cell K+ leaves cytoplasm and membrane repolarizes

Action Potentials

Action Potentials

All-or-None Response If a neuron responds at all, it responds completely A nerve impulse is conducted whenever a stimulus of threshold intensity or above is applied to an axon All impulses carried on an axon are the same strength

Refractory Period Absolute – Time when threshold stimulus does not start another action potential Relative – Time when stronger threshold stimulus can start another action potential Under normal conditions each fiber may conduct 10-500 impulses per second Larger neurons conduct up to 2500 per second

Impulse Conduction Nerve cell membrane maintains resting potential by diffusion of Na+ and K+ down their concentration gradients as the cell pumps them up the gradients Neurons receive stimulation, causing local potentials, which may sum to reach threshold Sodium channels in a local region of the membrane open Sodium ions diffuse inward, depolarizing the membrane

Impulse Conduction Potassium channels in the membrane open Potassium ions diffuse outward, repolarizing the membrane The resulting action potential causes an electric current that stimulates adjacent portions of the membrane Series of action potentials occurs sequentially along the length of the axon as a nerve impulse

Saltatory Conduction

Saltatory Conduction Impulse along myelinated fiber Sheath inhibits movement of ions Nodes of Ranvier allow generation of action potentials and conduction Ionic current flows through extra-cellular fluid & triggers impulse at next node Impulse mechanism is the same, BUT impulse skips from one node to the next.

Saltatory Conduction Valuable to Homeostasis Speed of impulse greatly increased Low ATP expenditure by Na-K pump due to little exposed membrane

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