Chapter 7: The Nervous System The Communication System of the Body
Nervous System Master controlling and communicating system of the body
The Nervous System : communication A. Neurons = masses of nerve cells that transmit information Two processes: 1. Dendrites – shorter, more numerous, receive information 2. Axons - send information
Central Nervous System (CNS): brain and spinal cord. Peripheral Nervous System (PNS): nerves of the body -- Includes 31 pairs of spinal nerves -- And 12 pairs of cranial nerves
Three Basic Functions of Nervous System Sensory function: gathers info Integrative function: bring the info together Motor function: responds to signals
Neuroglial cells (neuroglia) – supportive tissue of the nervous system (more numerous than neurons) 5 Types of Cells: Microglial cells Oligodendrocytes Astrocytes Ependymal cells Schwann cells
Microglial cell – digest debris or bacteria Respond to immunological alarms
Oligodendrocytes – makes the myelin sheath that provides insulation to the axon
Astrocytes – connect blood vessels to neuron
Ependymal cell- forms a membrane
Schwaan cells – forms the insulating myelin sheath around a neuron
Neuron (nerve cell) structure: Axon – long section, transmits impulses Dendrite – extend from the cell body; receive information Neurofibril – fibers within the impulse Chromatophilic substance (rough ER) – transport system Myelin – insulation surrounding axons Nodes of Ranvier – gaps in the insulation
*Myelinated vs. Unmyelinated Myelinated (white matter) – myelinated axons Unmyelinated (grey matter) - unmyelinated
Facts about Neurons Longevity: can live and function for a lifetime Do not divide: fetal neurons lose their ability to undergo mitosis; neural stem cells are an exception High metabolic rate: require abundant amounts of oxygen and glucose
Classification of Neurons Functional Sensory – receives information from senses Motor – sends information to muscles, glands, organs Interneuron – relays information Structural Bipolar Unipolar Multipolar
Cell Membrane Potential Resting Potential / Threshold Potential / Action Potential Nerve Impulse = weak electric current
2. Threshold stimulus is received 3. Sodium channels open 1. Neuron membrane maintains resting potential 2. Threshold stimulus is received 3. Sodium channels open 4. Sodium ions diffuse inward, depolarizing the membrane 5. Potassium channels open 6. Potassium ions diffuse outward, repolarizing the membrane 7. The resulting action potential causes a local bioelectric current that stimulates adjacent portions of the membrane. 8. Wave of action potentials travel the length of the axon as a nerve impulse
Nerve Impulse Speed of an impulse is proportionate to the DIAMETER of the AXON. Greater diameter = faster speed **Myelinated Axons conduct faster than unmyelinated ones**
The Synapse A: Neuron (axon) B: Neuron (dendrite) Mitochondria Vesicle Receptor Synapse Calcium Channel Releases neurotransmitter Re-uptake
Synapse - junction between two communicating neurons Nerve pathway - nerve impulse travels from neuron to neuron Dendrite → cell body → along axon -> synapse (gap) → dendrite
To complete the signal, a NEUROTRANSMITTER is released in the gap to signal the next neuron. They can be two types: Excitatory: increase membrane permeability, increases chance for threshold to be achieved Inhibitory: decrease membrane permeability, decrease chance for threshold to be achieved
Types of neurotransmitters in Humans Acetylcholine - stimulates muscle contraction Monoamines - Norepinephrine & Dopamine (sense of feeling good, low levels = depression) Serotonin (sleepiness) and mood Endorphins = reduction of pain, good mood
Nerves – bundles of axons common to a section of the body Types of Nerves: Sensory: conduct impulses into the brain and spinal cord Motor: carry impulses to muscles or gland Mixed: contains both sensory and motor
Nerve Pathways Reflex Arc: simple pathway, only includes a few neurons (involuntary, instantaneous) Reflex behavior Knee-jerk reflex: maintains uprightness Withdrawal reflex: avoids painful stimuli