Neurons and Synapses Types of Neurons Sensory Motor

Neurons and Synapses Types of Neurons Sensory Motor Interneurons 1

Sensory Neurons z INPUT From sensory organs to the brain and spinal cord. Drawing shows a somatosensory neuron Brain Sensory Neuron Spinal Cord Vision, hearing, taste and smell nerves are cranial, not spinal 2

Motor Neurons z OUTPUT From the brain and spinal cord To the muscles and glands. Sensory Neuron Brain Spinal Cord Motor Neuron 3

Interneurons z Interneurons carry information between other neurons only found in the brain and spinal cord. Brain Sensory Neuron Spinal Cord Motor Neuron 4

Structures of a neuron 5

The cell body z Contains the cell’s Nucleus y Round, centrally located structure y Contains DNA y Controls protein manufacturing y Directs metabolism y No role in neural signaling 6

Dendrites z Information collectors z Receive inputs from neighboring neurons z Inputs may number in thousands z If enough inputs the cell’s AXON may generate an output 7

Dendritic Growth z Mature neurons generally can’t divide z But new dendrites can grow z Provides room for more connections to other neurons z New connections are basis for learning 8

Axon z The cell’s output structure z One axon per cell, 2 distinct parts ytubelike structure branches at end that connect to dendrites of other cells 9

Myelin sheath z White fatty casing on axon z Acts as an electrical insulator z Not present on all cells z When present increases the speed of neural signals down the axon. Myelin Sheath 10

How neurons communicate z Neurons communicate by means of an electrical signal called the Action Potential z Action Potentials are based on movements of ions between the outside and inside of the cell z When an Action Potential occurs a molecular message is sent to neighboring neurons 11

Ion concentrations Outside of Cell K+ Na+ Cl- Cell Membrane in resting state K+ Na+ Cl- Inside of Cell A 12

The Cell Membrane is Semi. Permeable K+ Na+ Cl- Outside of Cell Membrane at rest K+ Na+ Cl- - 70 mv A- Inside of Cell Potassium (K+) can pass through to equalize its concentration Sodium and Chlorine cannot pass through Result - inside is negative relative to outside 13

Resting Potential z At rest the inside of the cell is at -70 microvolts z With inputs to dendrites inside becomes more positive z if resting potential rises above threshold an action potential starts to travel from cell body down the axon z Figure shows resting axon being approached by an AP 14

Depolarization ahead of AP z AP opens cell membrane to allow sodium (NA+) in z inside of cell rapidly becomes more positive than outside z this depolarization travels down the axon as leading edge of the AP 15

Repolarization follows z After depolarization potassium (K+) moves out restoring the inside to a negative voltage z This is called repolarization z The rapid depolarization and repolarization produce a pattern called a spike discharge 16

Finally, Hyperpolarization z Repolarization leads to a voltage below the resting potential, called hyperpolarization z Now neuron cannot produce a new action potential z This is the refractory period 17

Neuron to Neuron z Axons branch out and end near dendrites of neighboring cells z Axon terminals are the tips of the axon’s branches z A gap separates the axon terminals from dendrites z Gap is the Synapse Dendrite Axon Cell Body 18

Synapse z axon terminals contain small storage sacs called synaptic vesicles Sending Neuron Axon Terminal Synapse y vesicles contain neurotransmitter molecules 19

Neurotransmitter Release z Action Potential causes vesicle to open z Neurotransmitter released into synapse z Locks onto receptor molecule in postsynaptic membrane 20

Locks and Keys z Neurotransmitter molecules have specific shapes z Receptor molecules have binding sites z When NT binds to receptor, ions enter positive ions (NA+ ) depolarize the neuron negative ions (CL-) hyperpolarize 21

Some Drugs work on receptors z Some drugs are shaped like neurotransmitters z Antagonists : fit the receptor but poorly and block the NT y e. g. beta blockers z Agonists : fit receptor well and act like the NT y e. g. nicotine. 22

Summary z 3 types of neurons z The cell membrane z Ion movements z Action potentials z Synapse z Neurotransmitters z Receptors and ions z Agonists and antagonists 23