ANATOMY & PHYSIOLOGY OF THE NEURON

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? 320 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

Neuroglia Functions: Example: Astrocytes, Oligodendrocytes 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.

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) Form: Brain, Spinal Cord, Optic Nerve Form: Protected by bone and/or blood-brain barrier 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) Function: Involuntary control over heart rate, breathing, perspiration, salivation, pupillary dilation and digestion Form: Afferent (sensory) & Efferent (motor) neurons Subdivided in Sympathetic and Parasympathetic Divisions Somatic Division (SoNS) Function: Voluntary control of body via efferent motor neurons SoNS also encompasses reflex arcs, which do NOT travel to brain but instead rely on association neurons Form: Comprised of three types of nerves Spinal Cranial Association

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

Parasympathetic Shock?