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Nervous System Basics: Neurons Ch. 40

I. Neuron Structure A. Neuron- Basic unit of nervous system 1. AKA Nerve Cell 2.Conduct impulses B. Consists of a cell body, dendrites and an axon

I. Neuron Structure C. Dendrites 1. Branch off the cell body 2. Receives impulses and sends them to the cell body a. “Sensors/Feelers” D. Cell Body (Soma) 1. Contains nucleus 2. Contains other organelles 3. Interprets impulse and sends it to the axon

I. Neuron Structure E. Axon 1. Conducts the impulses 2. AKA “Nerve Fiber” 3. Myelin Sheath- White, fatty tissue layer that covers the axon a. Insulates & aides in relaying impulses faster

I. Neuron Structure E. Types of Neurons 1. Sensory Neurons a. Takes nerve impulses from sensory receptors in the skin, eye, ear, etc to the brain & spinal cord

I. Neuron Structure 2. Motor Neuron a. Takes nerve impulses from the brain & spinal cord to muscles or glands

I. Neuron Structure 3. Interneurons a. Lie in between other neurons b. Ex: Between a sensory neuron & a motor neuron c. Located in spinal cord and brain

II. Relaying an Impulse A. Shoulder Tap 1. Sensors in the skin “feel” the tap. 2. Sensory neurons send an impulse to the brain. 3. The brain interprets the message.

II. Relaying an Impulse A. Shoulder Tap (cont.) 4. The brain sends a response down a motor neuron to neck muscle. 5. The muscles contracting in the neck cause the head to turn.

II. Relaying an Impulse B. A Resting Neuron 1. Potassium ions (K+) and sodium ions (Na+) inside and around the axon membrane are involved in transmitting impulses

II. Relaying an Impulse 2. Na+/K+ Pump a. Uses active transport to move 3 Na+ out for every 2 K+ that goes in. b. This causes the inside of the neuron axon to be slightly more negative than the outside.

II. Relaying an Impulse c. The membrane is said to be polarized. R est = Resting neuron N = Negative charge P eace = “Poloarized”

II. Relaying an Impulse C. Transmitting the Impulse 1. When a stimulus excites an neuron, gates in the axon membrane open and let Na+ move in. a. This causes the inside to change to a slightly positive charge

II. Relaying an Impulse b. This change in charge is called depolarization. c. As depolarization occurs, the Na+/K+ pump works to return the axon to its resting state 1) This is called repolarization 2) See website

II. Relaying an Impulse d. The gates don’t open at the same time, they open as the impulse moves down the axon. 1) Wave-like 2) See website

II. Relaying an Impulse D. Speed of Impulses 1. Myelinated neurons are much faster a. myelinated: 100 m/sec b. nonmeyelinated: 10 m/sec 2. The impulse actually jumps from one node of ranvier to the next

III. Connecting Neurons A. Neurons lie axons to dendrites (end of one to beginning of the next), but they don’t actually touch. 1. Synaptic Cleft- The gap between two neurons

III. Connecting Neurons B. When the impulse reaches the axon terminal (end), neurotransmitters are released into the synaptic cleft. 1. Acetylcholine (muscle) 2. Norepinephrine (neurons)

III. Connecting Neurons C. When the neurotransmitter reaches the dendrites of the next neuron, it signals it to open the Na gates and send the impulse down the axon.

RECAP -What is a neuron? -What are the main parts of a neuron? -What are the types of neurons? -What is the charge of the inside when a neuron is resting

RECAP -What are the steps in transmitting an impulse? -How does the message of the impulse get across the synaptic cleft?