The Nervous System

The master controlling and communicating system of the body. Vital in maintaining homeostasis Two divisions: – Central Nervous System (CNS) = brain and spinal cord – Peripheral Nervous System (PNS) = nerves that extend from CNS

PNS Structures Sensory / Afferent Division – Nerves that convey impulses to the CNS from sensory receptors in the body. Motor / Efferent Division – Nerves that convey impulses from the CNS to organs, muscles, and glands. Two divisions of motor / efferent nerves: Somatic nervous system – voluntary control Ex: skeletal muscle contraction Autonomic nervous system (ANS) – automatic or involuntary regulation Ex: cardiac muscles

Basic Divisions of the Nervous System Figure 12.2

THREE BASIC FUNCTIONS OF THE NERVOUS SYSTEM Sensory - gathers info Integrative - information is brought together Motor - responds to signals, homeostasis

Neurons

Neurons – nerve cells Parts: Cell body – metabolic center Dendrites – convey signals toward the cell body Axons – convey signals away from the cell body Myelin sheath – enclose axon, increase transmission rate Nodes of Ranvier – gaps between myelin

White vs Grey Matter Myelinated (white matter) – myelinated axons Unmyelinated (grey matter) - unmyelinated

Types of Nerves Sensory Neurons - conduct impulses into the brain or spinal cord Motor Neurons - carry impulses to muscles of glands Interneurons- contain both sensory and motor nerves

Neurons Classified by Function: Sensory vs. Motor Neurons Figure 12.11

Neuroglial Cells – “nerve glue” - support cells for the neurons 1. Microglial Cells: scattered throughout, digest debris or bacteria Microglial cells respond to immunological alarms

Neuroglial Cells 2. Oligodendrocytes: Produce Myelin sheath around axons of neurons in CNS

Neuroglial Cells 3. Astrocytes: connect blood vessels to neurons I connect to blood vessels

Neuroglial Cells 4. Ependymal Cells: form a protective membrane (Blood-brain barrier), allow diffusion

5. Schwann cells: form the insulating myelin sheath around neurons in PNS Practice with neuroglia coloring!

Supporting Cells - NEUROGLIA

Label

Interesting Facts about the Neuron Longevity – can live and function for a lifetime Do not divide – fetal neurons lose their ability to undergo mitosis; neural stem cells are an exception High metabolic rate – require abundant oxygen and glucose The nerve fibers of newborns are unmyelinated - this causes their responses to stimuli to be course and sometimes involve the whole body. Try surprising a baby!

Cell Membrane Potential

Membrane of a resting, or inactive, neuron is polarized – meaning it is internally more negative (more negative ions) and there are more positive ions outside.

During stimulation, often by a neurotransmitter, the sodium channel will open, allowing sodium ions to flow into the cell. This will change the polarity of the neuron locally, an event called depolarization. Locally the inside is now more positive and the outside less positive. This is called a graded potential.

If stimulus is strong enough (threshold reached; threshold = minimum stimulus needed for response) and enough Na + ions enter cell, the graded potential activates the neuron to begin a long-distance signal called an action potential (or nerve impulse)

Action potential propagates along the entire length of the axon, making it an All-or-none response.

After the sodium influx, the membrane becomes impermeable to sodium and permeable to potassium, causing K+ ions to diffuse into cell. This loss of positive ions leads to the membrane becoming polarized, at rest, in a process called repolarization. Until repolarization, cell cannot conduct another impulse (this time is called the refractory period)

After repolarization, the neuron’s initial concentrations of ions are restored by the sodium-potassium pump.

Nerve Impulses Animations of Nerve Impulses hill.com/sites/ /student_ view0/chapter14/animation __the_nerve_impulse.html animations/actionpotential.swf

The Synapse Junction between two communicating neurons Neurons do not touch– synapse To complete a signal, a neurotransmitter is released across synaptic cleft to reach next neuron

Structure of a Synapses

Events at the Synapse 1.Arriving action potential depolarizes the synaptic knob and presynaptic membrane 2.Calcium ions enter cytoplasm of knob 3.Neurotransmitter released through diffusion and exocytosis of neurotransmitter vesicles 4.Neurotransmitter goes across synapse and binds to receptors on post-synaptic membrane 5.Sodium channels open on new neurons 6.Neurotransmitter is broken down This ends depolarization

Neurotransmitters Excitatory - increase membrane permeability, increases chance for threshold to be achieved Inhibitory - decrease membrane permeability, decrease chance for threshold to be achieved

Examples of Neurotransmitters Acetylcholine - stimulates muscle contraction Catecholamines – Epinephrine and Norepinephrine (fight-or-flight response) & Dopamine (sense of feeling good, low levels = depression) Serotonin (happiness, sleepiness, metabolism) GABA (gamma-Aminobutyric acid) – chief inhibitor

Common Neurotransmitter Disorders Disorder Neurotransmitter Alzheimer’s Depression Epilepsy Huntington’s Insomnia Mania Parkinson’s Schizophrenia SIDS Deficient ACh Def serotonin/norepinephrine Excess GABA Deficient GABA Deficient serotonin Excess norepinephrine Deficient dopamine Def. GABA / excess dopamine Excess dopamine

Reflexes Reflexes – rapid, predictable, involuntary responses to stimuli Reflex arc – neural pathways that involve both CNS and PNS on which reflexes occur Somatic reflexes – stimulate skeletal muscles (hot pan) Autonomic reflexes – regulate smooth muscles, heart, glands (secretion of saliva, dilation of pupils)