anatomy & physiology of the neuron anatomy & physiology 2013-2014


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How does the human body facilitate communication between nearly 100 trillion cells ?

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

How many neurons?


1 Trillion?

Anatomy of the Neuron

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


• 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


•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.

Action Potential

•Electrochemical message that can stimulate or inhibit another neuron

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


Parasympathetic Shock?