The Nervous System

Functions of the Nervous System Nervous System – master controlling and communicating system of the body. Sensory input – gathering information to monitor changes occurring inside and outside the body Changes = stimuli Integration - to process and interpret sensory input and decide if action is needed

Functions of the Nervous System Motor output - A response to integrated stimuli The response activates muscles or glands Nervous system does not act alone to maintain homeostasis. The endocrine system and hormones are a slower method.

Structural Classification of the Nervous System Central nervous system (CNS) – includes the brain and spinal cord. Peripheral nervous system (PNS) – all the nerves outside the brain and spinal cord

Functional Classification of the Peripheral Nervous System Sensory (afferent) division - Nerve fibers that carry information to the central nervous system Figure 7.1 p. 223

Functional Classification of the Peripheral Nervous System Motor (efferent) division - Nerve fibers that carry impulses away from the central nervous system Two subdivisions Somatic nervous system = voluntary; skeletal muscles Autonomic nervous system = involuntary; smooth muscles, cardiac muscle, hormones Figure 7.1 p. 223

Organization of the Nervous System Figure 7.2 p. 224

Nervous Tissue: Support Cells (Neuroglia) Neuroglia – supporting cells in the CNS; means “nerve glue” Astrocytes - Abundant, star-shaped cells; make up half of all nerve tissue. Brace and anchor neurons to capillaries Form a living barrier between capillaries and neurons and play a role in exchange Control the chemical environment of the brain picking up excess ions and recapturing neurotransmitters Figure 7.3a p. 225

Nervous Tissue: Support Cells (CNS) Microglia - Spider-like phagocytes Dispose of debris such as dead brain cells and bacteria Ependymal cells - line cavities of the brain and spinal cord and Circulate cerebrospinal fluid with Cilia Figure 7.3b–c p. 225

Nervous Tissue: Support Cells (CNS) Oligodendrocytes - produce myelin sheath around nerve fibers in the central nervous system Figure 7.3d p. 225

Nervous Tissue: Support Cells (PNS) Satellite cells - Protect neuron cell bodies Schwann cells - Form myelin sheath in the peripheral nervous system Figure 7.3e p. 225

Nervous Tissue: Support Cells Neuroglia cells CAN NOT transmit nerve impulses Neuroglia cells never lose their ability to divide (mitosis) Neurons DO lose the ability to divide (paralyisis)

Nervous Tissue: Neurons Neurons = nerve cells. Cells highly specialized to transmit nerve impulses (messages) from one part of the body to another. Major regions of neurons Cell body – nucleus and metabolic center of the cell Processes – fibers that extend away from the body Axon Hillock – cone-like region that leads to the axon Fig. 7-4 a / b p. 227

Neuron Anatomy Processes -(Extensions) outside the cell body (up to 3 ft) Dendrites – conduct impulses toward the cell body (one or more) Axons – conducts impulses away from the cell body (usually only one) Figure 7.4a p. 227

Axons and Nerve Impulses Axons end in axon terminals (many times in two or more branches) Axon terminals contain vesicles (tiny sacs) with neurotransmitters Axon terminals are separated from the next neuron by a gap (they never touch) Synaptic cleft – gap between adjacent neurons Synapse – junction between nerves http://www.youtube.com/watch?v=i- NgGKSNiNw&feature=related

Schwann cells – produce myelin sheaths in jelly-roll like fashion Nerve Fiber Coverings Schwann cells – produce myelin sheaths in jelly-roll like fashion Nodes of Ranvier – gaps in myelin sheath along the axon You Tube clip later showing this Figure 7.5 p. 228

Neuron Cell Body Location Most are found in the central nervous system Gray matter – cell bodies and unmylenated fibers White matter – mylenated fibers; clusters of cell bodies within the central nervous system Ganglia – collections of cell bodies outside the central nervous system in the PNS

Functional Classification of Neurons Neurons are classified by the direction of the nerve impulse relative to the CNS Sensory (afferent) neurons - Carry impulses from the sensory receptors TO the CNS Cutaneous sense organs such as pain receptors (heat, cold, pressure as pain) Proprioceptors – detect stretch or tension Motor (efferent) neurons - Carry impulses FROM the CNS to the muscles, organs or glands to respond to the stimuli

Functional Classification of Neurons Interneurons (association neurons) - Found in neural pathways in the central nervous system Connect sensory and motor neurons http://www.youtube.com/watch?v=eHgvMKT- 20Q&NR=1

Neuron Classification Figure 7.6 p. 229

Structural Classification of Neurons Multipolar neurons – many extensions from the cell body; most common type of neuron Figure 7.8a p. 231

Structural Classification of Neurons Bipolar neurons – one axon and one dendrite Figure 7.8b p. 231

Structural Classification of Neurons Unipolar neurons – have a short single process leaving the cell body Figure 7.8c p. 231

Neuron Physiology (function) Irritability – ability to respond to stimuli Conductivity – ability to transmit an impulse The plasma membrane at rest is polarized Fewer positive ions are inside the cell than outside the cell (outside of cell is slightly + and inside of cell is slightly - )

Starting a Nerve Impulse Depolarization – a stimulus depolarizes the neuron’s membrane A deploarized membrane allows sodium (Na+) to flow inside the membrane causing it to become more positive The exchange of ions initiates an action potential in the neuron Figure 7.9a–c p. 232

The Action Potential If the action potential (nerve impulse) starts, it is propagated over the entire axon (all or nothing; firing a gun) Potassium ions rush out of the neuron after sodium ions rush in, which repolarizes the membrane (becomes more negative again; K is also a positive ion) The sodium-potassium pump, using ATP, restores the original configuration (3 na+ out and 2 k+ in) https://www.youtube.com/watch?v=fHRC8SlLcH0

Nerve Impulse Propagation The impulse continues to move away from the cell body Impulses travel faster when fibers have a myelin sheath The impulse will “jump” from one node of Ranvier to the next avoiding the mylinated area this is called Saltatory conduction http://www.youtube.com/watch?v=DJe3_3XsBOg (also talks about schwann cells making myelin sheath) Figure 7.9d–f p. 232

Continuation of the Nerve Impulse between Neurons Impulses are able to cross the synapse to another nerve An Action Potential arriving at the axon terminal signals the vesicles to release Neurotransmitter Neurotransmitter vesicles fuse with the plasma membrane and are released into the synaptic cleft by Exocytosis. The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter An action potential is started in the dendrite. Na+ moves in briefly. http://www.youtube.com/watch?v=HXx9qlJetSU&NR=1

How Neurons Communicate at Synapses Figure 7.10 p. 233

Homeostatic Imbalnce Multiple Sclerosis (MS) - Myelin sheaths around nerve fibers are gradually destroyed and become hard. The current becomes short-circuited, and the person loses the ability to control their muscles. This is an autoimmune disease which means the body is destroying itself. http://www.youtube.com/watch?v=qgySDmRRzxY https://www.youtube.com/watch?v=M7O78LvrNSQ Cold and Continuous pressure hinder impulse conduction because they interrupt blood circulation to neurons. Holding ice causes numbness or pressure caused the foot to “fall asleep”. When you return circulation of blood the impulses begin working again and it gives the unpleasant prickly feeling … thought you might want to know!