The Nervous System Nervous Tissue

Neurology The study of the nervous system

Functions of the Nervous System Sensory – Senses changes in the body Integrative – Analyzes sensory information, stores some of it and determines behavior based on the information Motor – Responds to the stimuli by initiating action

Organization of the Nervous System Central Nervous System (CNS) - Composed of the brain and spinal cord Responsible for: i. Incoming sensory Information ii. Thoughts and emotions iii. Memories iv. Generating Nerve Impulses

Organization of the Nervous System Peripheral Nervous System (PNS) – Composed of Cranial Nerves and Spinal Nerves Responsible For: i. Sensory (Afferent) – input ii. Motor (Efferent) – output

Organization of the Nervous System Peripheral Nervous System (continued) iii. Somatic Nervous System (SNS) – voluntary iv. Autonomic Nervous System (ANS) – involuntary 1. Sympathetic vs. Parasympathetic

Sympathetic vs. Parasympathetic 2 branches of the ANS that usually have opposing actions Ex – sympathetic speeds up the heart while parasympathetic slows it down

Organization if the Nervous System PNS CNS SNS ANS sympathetic parasympathetic

Histology of the Nervous System Cells Neuroglia (glia or glial cells) – used for support and protection Astrocytes Oligodentrites Microglia Ependymal Cells Schwann Cells Satellite Cells

Histology of the Nervous System Cells Neurons “nerve cell” Conducts impulses from one part of the body to another

Neurons Cell Body – contains the nucleus and nucleolus, cytoplasm and typical cellular organelles Dendrites – Short thick branched extensions used to receive impulses and send them to the cell body Axon – Single long thin extension that sends impulses to another neuron or tissue Synaptic End Bulb – Structure at the end of an axon that store neurotransmitters

Typical Neuron

Myelin Sheath Fat and protein covering that insulates the axon to increase the speed of a nerve impulse

Myelinated – with myelin Unmyelinated – without myelin Schwann Cells – glial cells wrapped around the axon that form the myelin sheath. Nodes of Ranvier – gaps in the myelin scheath

Gray and White Matter White Matter – contains both myelinated and unmyelinated axons Found in outer sections of the spinal cord and inner portion of the brain Gray Matter – contains only unmyelinated axons Found in an “H” shape in the middle of the spinal cord and the outer covering of the brain

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Action Potentials – “Nerve Impulse” Membrane Potential – difference in the amount of electrical charge between the outside and inside of the plasma membrane Ion Channel – openings in the plasma membrane through which ions flow

Action Potential The sequence of events that decrease and reverse the resting membrane potential then return it to normal

Threshold – a critical level that once reached will cause a nerve impulse Depolarizing Phase – decreasing and reversing a membrane potential (making it more positive on the inside than on the outside) Repolarizing Phase – the membrane potential is returned to its resting state

Sodium-Potassium Pump Na+/K+ pump First, Na ions rush into the cell causing the cell to BECOME MORE POSITIVE ON THE INSIDE; then K ions move out of the cell causing it to return to its resting membrane potential

All or None Principle… As long as the threshold is reached, the channels will open and an action potential will occur along the ENTIRE nerve. A stronger stimulus does not cause a stronger impulse.. (it’s just like the muscle cells guys)

Conduction of a nerve impulse Positive Feedback Mechanism – When the first Na channels open it causes an inflow of Na ions that depolarize the next portion of membrane to threshold, which causes another Na channel to open which causes another depolarization which causes another Na channel to open which causes…

Continuous Conduction Each patch of the axon depolarizes and causes the next patch to depolarize. Found on unmyelinated axons and is relatively slow.

Saltatory Conduction Na/K channels only open at the nodes of Ranvier, then the current travels under the myelin sheath to the next node. Only in myelinated axons, faster than continuous

Speed of Impulse… Myelination: Diameter of Nerve: Temperature Myelination = faster Diameter of Nerve: Wider = faster Temperature Warmer = faster

Synapses – space between neuron Synaptic Cleft – tiny space filled with interstitial fluid Ca ions from the interstitial fluid flow into the synaptic end bulb causing the neuron to release neurotransmitters.

Common Neurotransmitters

Acetylcholine (ACh) Used at neuromuscular junctions

Glutamate and Aspartate Amino Acids that have an excitatory effect on the CNS

GABA and Glycine Inhibitory Neurotransmitters Anti-anxiety drugs often enhance these neurotransmitters

Norepinephrine Functions in awakening, dreaming and mood

Dopamine (DA) Active during emotional response and addictive behavior Cocaine blocks the re-uptake of dopamine allowing it to stay in the synaptic cleft longer and prolonging pleasurable feelings

Endorphins Natural Pain Killers Endorphins interact with the opiate receptors in the brain to reduce our perception of pain and act similarly to drugs such as morphine and codeine. In contrast to the opiate drugs, however, activation of the opiate receptors by the body's endorphins does not lead to addiction or dependence.

Nitric Acid Functions in learning and memory

NERVOUS SYSTEM DISORDERS

Multiple Sclerosis (MS) Progressive destruction of the myelin sheath on the axons.

Epilepsy Seizure disorder

Guillain-Barre Syndrome Macrophages “eat” the myelin sheath on the axons and cause sudden paralysis Patients usually recover at least partially