The Nervous System

• The nervous system controls all other organ systems and maintains balance within them.
• The nervous system sends messages to and from the brain and spinal cord to all parts of the body.
• Cell's ability to respond to the environment evolved over billions of years

Organization of Nervous Systems

• Nervous systems show diverse patterns of organization.
• Nerve nets are a pattern of organization.
• Hydra (cnidarian) uses nerve nets
• Sea stars (echinoderm) use radial nerves and nerve rings

Cephalization

• With Cephalization comes more complex nervous systems.
• Planarian (Flatworm), Leech (Annelid), Insect (Arthropod), Chiton and Squid (Mollusks), and Salamanders (Chordate) are examples of Nervous systems

Basic Nervous System Division

• Central Nervous System ("CNS") occupies cranium and vertebral column.
• The Brain is part of the CNS
• The Spinal cord is part of the CNS
• Peripheral Nervous System ("PNS")
• Cranial nerves are a part of the PNS
• Spinal nerves are part of the the PNS

Nervous Tissue Cell Types

• Nervous tissue consists of two types of cells.
• Neurons, excitable nerve cells, transmit electrical signals.
• Supporting cells, called neuroglia or glia, means "nerve glue".

CNS Brain Sections

• The Brain consists of four sections
• Cerebrum (cerebral hemisphere)
• Diencephalons
• Brain stem
• Cerebellum

Cerebrum

• The Cerebrum is the largest section of the brain
• The Cerebrum is composed of regions designed for motor movement, sensations, auditory processing, and vision.

PNS Neuroglia

• Schwann cells surround all axons of neurons in the PNS and create myelin sheath around the PNS axons.
• Satellite cells surround the neuron cell body.

Myelin

• Myelin is a Lipoprotein, increases speed of conduction, found in large axons.
• Myelin functions as insulation
• Myelin prevents leakage of electric current.
• Layers of myelin have spaces (nodes of Ranvier) between cells.
• Impulses "jump" from node to node, where present
• "Unmyelinated" axons are generally smaller and slower.

Myelin in the PNS and CNS

• In multiple sclerosis (MS), patches of myelin are destroyed in the brain and spinal cord

Gray and White Matter of the CNS

• Gray matter is gray-colored, where neuron cell bodies are clustered.
• White matter is white-colored, and consists of millions of axons running between different parts of the CNS, bundled in "tracts".
• Tracts are in the CNS, whereas nerves are in the PNS
• White matter is white because of the myelin sheath.

Neuron Structure

• The cell body contains the nucleus and cytoplasm; cell bodies are found in clusters.
• CNS clusters are called nuclei.
• PNS clusters are called ganglia (reside outside of the CNS)
• Dendrites are short, receive nerve impulses for the neuron.
• Axons are long, send nerve impulses away from the cell body.

Neuron Structure Illustration

• Dendrite is part of the neuron, and receives signals
• Axon Terminals send the signals
• Sensory neuron with Schwann and satellite cells.
• Myelin Sheath covers the Axon

Neurons

• Neurons structural classification is based on the number of processes coming off of the cell body.
• These include Bipolar, Unipolar, and Multipolar

Additional Neuron Facts

• Neurons can live for a lifetime (over 100 years).
• Neurons cannot divide, with limited exceptions.
• Cannot replace themselves
• High metabolic rate.
• Neurons require continuous oxygen and glucose.
• Neurons die within a few minutes without oxygen.

Neuron Types

• There are three types of neurons (function/direction).
• Afferent or sensory nerves carry sensory information from the environment or inside the body to the CNS for interpretation.
• Efferent or motor nerves send impulses from the CNS to PNS to allow for movement or action.
• Interneurons are interpretive neurons between afferent and efferent nerves in the CNS.

Reflex Arcs

• Receptor -> Sensory Neurons -> Interneurons -> Motor Neurons -> Effectors

Terminology

• Input: sensory = sensory input.
• Receptors monitor changes inside and outside of the body.
• Changes are called “stimuli” (singular, stimulus).
• Information is sent by "afferent" nerves.
• Integration.
• Information is processed.
• Decision is made about what should be done.
• Output: motor = motor output.
• Effector organs (muscles or glands) are activated.
• Effected by "efferent" nerves.

Synapses

• Synapses are junctions between neurons.
• Information passes from one neuron to another through them, usually chemically
• Synapses are always unidirectional
• Presynaptic (toward synapse) vs postsynaptic (away from synapse): most neurons function as both
• Synaptic cleft is a tiny gap involved between neurons.
• Neurons can synapse with Neurons, Muscles, and Glands

Synapses Simplified

• Information is passed between neurons by chemicals.
• The signals can be be excitatory or inhibitory
• Along the axons, the information is passed electrically.

Central Nervous System

• The Central Nervous System is composed of the spinal cord and brain
• Meninges comprise a membrane to protect the brain and spinal cord.

Spinal Cord Details

• The spinal cord is a continuous slender structure from the brain
• Descends into the vertebral canal and ends around the level of the first or second lumbar vertebrae
• The spinal cord carries sensory information to and from the brain and participates in reflexes (unpredictable, automatic response to stimuli).

Peripheral Nervous System

• The Peripheral Nervous System (PNS) includes nerves that branch off the CNS
• There are two types of peripheral nerves:
• Cranial nerves
• Spinal nerves

Nerve Types

• Cranial nerves emerge directly from the brain in 12 pairs, bringing information from the sense organs to the brain, controlling muscles, and connecting to glands or internal organs such as the heart and lungs.
• Spinal nerves are peripheral nerves originating from the spinal cord, in 31 pairs.