Post on 19-Jan-2016
The Neuron Doctrine
• Santiago Ramon y Cajal (1887): Neurons are the functional unit of the nervous system • Neurons are discrete, metabolic
units • Neurons are electrochemically
excitable cells
Anatomy of the Neuron• Soma• The cell body of the neuron; contains the nucleus and other
organelles necessary for survival of the cell• Dendrite• Branches of the neuron that receive chemical messages
(neurotransmitters) from another neuron• Axon• Long “arm(s)” of the neuron that propagate electrical messages
(action potentials) from within the neuron • Axon hillock• The site of the soma where the axon stems from and most often
the site of action potential origination
Neuroglia
• Example: Astrocytes, Oligodendrocytes•Functions:• Surround neurons and hold
them in place• Supply nutrients and oxygen• Insulate one neuron from
another• Destroy pathogens and
remove dead neurons
Types of Neurons
• Sensory (afferent)• Transmits signals from sensory receptors to the spinal
cord/brain.
•Motor (efferent)• Transmit signals from the brain/spinal cord to muscle
fibers, resulting in muscle contractions, and affect glands.
• Interneurons (association)•Connect neurons to other neurons within the same region
of the brain or spinal cord.
The Action Potential
• Electrochemical changes that occur along the axon •Voltage change caused by
migration of sodium and potassium ions across the cell membrane of the axon•Action potentials may be
excitatory (EPSP’s) or inhibitory (IPSP’s) of the next neuron
Schwann Cells & Myelin
• The axons of many neurons are wrapped in glial tissue called Schwann cells• Schwann cells produce a fatty
tissue called myelin that wraps around regions of the axon• The spaces between the
wrapped layers of myelin are called Nodes of Ranvier
Saltatory Conduction
• The myelin sheaths insulate the axon, preventing excessive leakage of K+ ions• Depolarization at one Node of
Ranvier is sufficient to propagate the action potential at an adjacent node.• As less gated channels need to
be opened and closed along the axon, the effective speed of the action potential is greater
Neurotransmitters•Neurotransmitters (NT) are
endogenous chemicals that transmit signals from a neuron to a target cell across a synaptic cleft• Synthesis of the NT can take place
in the cell body, in the axon, or in the axon terminal.• Storage of the NT in storage
granules or vesicles in the axon terminal.
Major Neurotransmitters
•Acetylcholine (ACH)•Stimulates motor neurons• Inhibits cardiac contraction
•Norepinephrine• Increases cardiac contraction rate & strength
Synaptic Transmission
•Calcium enters the axon terminal during an action potential, releasing the neurotransmitter (NT) into the synaptic cleft.• The NT binds to and activates a receptor in
the postsynaptic membrane.• The neurotransmitter is either destroyed
enzymatically, or taken back into the terminal from which it came, where it can be reused, or degraded and removed.
The Nervous System•The nervous system is the part of an animal’s body that coordinates voluntary and involuntary actions of the animal and transmits signals between the different parts of its body.
Central v. Peripheral Divisions
Central (CNS)o Form: Brain, Spinal Cord, Optic
Nerveo Form: Protected by bone and/or
blood-brain barriero Function: Integrates messages
received from extremities, sensory organs and internal organs
Peripheral (PNS):Form: Nerves and Ganglia
outside of CNS. Includes 10 of 12 cranial nerves
Form: Not protected by bone or blood-brain barrier
Function: Relays messages between CNS and extremities.
Autonomic v. Somatic Divisions of PNS
• Autonomic Divison (ANS)o Function: Involuntary control
over heart rate, breathing, perspiration, salivation, pupillary dilation and digestion
o Form: Afferent (sensory) & Efferent (motor) neurons
o Subdivided in Sympathetic and Parasympathetic Divisions
• Somatic Division (SoNS)Function: Voluntary control of
body via efferent motor neuronsSoNS also encompasses reflex
arcs, which do NOT travel to brain but instead rely on association neurons
Form: Comprised of three types of nervesSpinalCranialAssociation
Somatic Division (SoNS)
• The SoNS is responsible for controlling voluntary movements, using efferent (motor) neurons, and reflex arcs, using association (inter-) neurons.• Three types of nerves:• Spinal: Innervate much of the body, and connect through the
spinal column to the spinal cord. (letter-number designations according to the vertebra through which they connect to the spinal column)• Cranial: Innervate the head, and connect directly to the brain
(especially the brainstem). (Roman Numerals 1-12 + descriptive names).• Association: Connects other neurons (not “projection” neurons)
Somatic Reflex Arcs• A reflex arc is a neural pathway that controls action
reflexes (ex: patellar reaction) by synapsing in the spinal cord (not the brain).• This allows for faster response time. Sensory
information is still relayed to your brain, as the reflex action occurs.
Autonomic Division (ANS)•The ANS is responsible for controlling involuntary movements, such as heart rate, breathing, perspiration, salivation, pupillary dilation,digestion.•It is composed of afferent (sensory) and efferent (motor) neurons.•The ANS is further subdivided two divisions:•The Sympathetic Division (“Fight or Flight”)•The Parasympathetic Division (“Feed & Breed”)
Sympathetic and Parasympathetic Divisions of the ANS
• Sympathetic (Fight or Flight)• Elevates blood pressure via
vasoconstriction• Increases respiratory volume via
dilation of bronchioles• Inhibition of peristalsis• Dilation of pupil
• Parasympathetic (Feed & Breed)• Decreased blood pressure via
vasodilation• Decreased respiratory rate• Increased digestion, urination and
defecation