Nervous System Communication

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Neurons Nerve cells Parts of neurons –Cell body –Long extensions (fibers=dendrites or axons) Message = nerve impulse

Animal Nervous Systems Sponges – no nervous system Other animals all have neurons in systems

Nerve Net In all cnidarians Interconnected nerve cells No brain

Ringlike Nervous System In echinoderms Ring with 5 radiating nerves

Ladderlike Nervous System In many Platyhelminthes Some have distinct brain (ganglia)

Annelid Nervous System Segmental ganglia –Ganglia = aggregations of nervous tissue Ventral nerve cord & brain Arthropod Mollusks ( depends level of activity) some have giant fibers

Vertebrate Nervous System Central nervous system –Brain & spinal cord Peripheral nervous system –Nerves to & from CNS

Nerve Cells (Neurons) Cell body –Contains nucleus and organelles –Produces substances necessary for cell to live & function Axon (conducts or passes on impulse) –Long cell extension –May have myelin covering Dendrites (receive impulse) –Usually short cell extensions –No myelin covering – most neurons have multiple dendrites

Sensory Neurons Receive information from sense organ receptors Transmit to the central nervous system Cell bodies of sensory neurons lie near the CNS

Motor Neurons Transmit commands away from CNS to effector (muscle or gland) Each neuron has one long axon Cell bodies on most motor neurons lie in or near CNS

Interneurons Located within brain & spinal cord Integrate information Axons may or may not be myelinated usually axons are shorter than those in PNS

Neuroglia Nerve support cells Provide support, protection, & nutritional stability Schwann cells (special neuroglial cells) –Found around axons –Produce myelin sheath Oligodendrocytes – produce myelin sheath around some CNS axons

Myelin Sheath Insulates axon Nodes of Ranvier-allows impulse to move at a greater speed along axon Uninsulated areas- no myelin sheath

Nerve Impulses Electrical signals transmitted along membranes of nerves

Resting Potential Neuron is electrically charged at rest Outside is positively charged Inside is negatively charged

Sodium-Potassium Pump Proteins embedded within cell membrane Moves sodium to the outside Moves potassium to the inside Maintains resting potential Requires energy (ATP)

Action Potential Nerve impulse is started by a stimulus Stimuli cause movements of ions through membrane Threshold potential –Sufficient stimulation to depolarize membrane Action potential –Rapid reversal of membrane electric potential

Nerve Transmission Action potential at one point depolarizes next area Depolarization moves in self-propagating wave

Saltatory Conduction Nerve impulse jumps & moves faster along myelinated axon

Synapse Area where nerve communicates Transfers message –Another neuron –An effector

Synaptic Cleft Neurons do not touch other neurons or effector cells Nerve impulse must cross gap (electrical signal is changed to a chemical signal)

Neurotransmitters Organic molecules (> 60 different chemicals) Transfer message across synaptic cleft Attach to receptors on target cell

Neuromuscular Junctions Synapse between neuron & skeletal muscle Neurotransmitter is aceytylcholine

Neuron to Neuron Connections Uses many different neurotransmitters (such as dopamine, norepinephrine, serotonin) Some cause different effects –Excitatory synapse – continuation of impulse –Inhibitory synapse – reduce ability to depolarize

Integration of Nerve Impulses Summed impulses determine if postsynaptic neuron will depolarize

Human Nervous System Central nervous system –Brain –Spinal cord Peripheral nervous system –Brings messages to & from CNS –Somatic nervous system – voluntary –Autonomic nervous system - involuntary

Central Nervous System Integrates sensory & motor impulses

Spinal Cord Connects peripheral nervous system with brain Receives information via spinal nerves Includes reflex arcs

CNS Protection - Meninges Layers of membranes –Dura mater –Arachnoid –Pia mater

Peripheral Nervous System Sensory & motor impulses Cranial nerves communicate directly with brain, some are strictly sensory or motor, some are mixed.

Spinal Nerves Sensory & motor nerve fibers 31 pairs and all are mixed nerves Travel directly to spinal cord Nerves are bundled to form mixed nerves

Motor Neuron Systems Somatic nervous system, part of PNS –Voluntary –Movements of skeletal muscles – requires a single motor neuron – Reflex = automatic response to nerve stimulation Autonomic nervous system, part of PNS –Involuntary motor pathways – requires two motor neurons

Divisions of the Autonomic Nervous System Parasympathetic –Acetylcholine is the neurotransmitter –Prevails during periods of inactivity –Housekeeping Sympathetic –Norepinephrine is the neurotransmitter –“fight or flight” –Responds to stress –Prepares body for action Parasympathetic & sympathetic together help to maintain homeostasis

Psychoactive Drugs Affect action of neurotransmitters In specific parts of the brain Some are abused

Drug Addiction Chronic use (or abuse) of psychoactive drugs Person becomes physically dependant Drug use tends to increase due to drug tolerance

End Chapter 32