Nervous System and Tissue Overview

Nervous System Organization

• The nervous system's main function is to coordinate activities to maintain homeostasis.
• The central nervous system (CNS) includes the brain and spinal cord.
• The peripheral nervous system (PNS) includes nerves and ganglia.
• The nervous system consists of sensory and motor components.
• The nervous system has different divisions that cooperate with each other.
• Sensory components collect information from receptors and relay to CNS.
• Motor components transfer information from CNS to target organs (like muscles or glands).

Nervous Tissue

• Nervous tissue is made up of neurons and neuroglia.
• Neurons respond to stimuli and transmit electrical signals.
• Neuroglia includes cells that support neurons, like oligodendrocytes, ependymal cells, microglia, and astrocytes.

Neuroglia (Support Cells of the Nervous System)

• Neuroglia are the support staff of the nervous system.
• There are typically ten times more neuroglia than neurons.
• Neuroglia provide neurons with physical and regulatory support and communicate with them on how to carry out their functions.
• Oligodendrocytes create the myelin sheath, that surrounds axons in the CNS.
• Ependymal cells line cavities of CNS and produce cerebrospinal fluid.
• Microglia are the immune cells that act like macrophages to remove debris, etc.
• Astrocytes regulate the environment around neurons by maintaining the blood-brain barrier and help repair damaged tissue.

PNS Neuroglia

• Satellite cells surround neuron cell bodies in the ganglia and provide support.
• Schwann cells surround axons and produce myelin sheath in the PNS.

Myelin

• Myelin is a protective covering around axons that increases electrical signal transmission speed.
• Myelin is created by Oligodendrocytes in the CNS and Schwann cells in the PNS.
• It increases the speed of signals.
• Myelin minimizes ion leakage.
• Myelin thickness can be hundreds of times thicker than the cell membrane.

Neurons

• Neurons are the functional units of the nervous system.
• Neurons have unique characteristics including excitability (respond to stimuli).
• Neurons conduct (transmit) electrical signals.
• Neurons secrete signaling molecules.
• They have an extreme longevity.
• Neurons cannot divide.
• They have a high metabolic rate, and their function depends on glucose and oxygen.

Generic Neuron Structure

• Dendrites are thin, branched extensions radiating outward from the cell body.

• The neurosoma (cell body) contains the nucleus and organelles.

• The axon is a long extension that transmits signals from the cell body to ending branches.

• The axon hillock is the origin point of the axon.

• Axons vary in length from micrometers to meters.

• Extensive cytoskeleton extends into both the axon and dendrites.

• Terminal arborization are multiple branches at the end of an axon.

• These branches form synapses with the target cells, using synaptic vesicles to release signaling molecules.

• Some neurons do not have extensive axons, but they use varicosities instead.

• Neurons have an actual size that corresponds to certain proportions, for example, axons around a meter in length, dendrites that spread out in the classroom, or neurosoma the size of a tennis ball.

Neuron Classification

• Sensory (afferent) neurons detect and transmit information from sensory receptors to the CNS about stimuli. (e.g. temp, light, smell, body position).
• Interneurons completely reside within the CNS, to integrate and connect sensory and motor pathways.
• Motor (efferent) neurons carry information away from the CNS to the effector (such as muscle and glands)

Neuron Structure Variations

• Neurons vary in structure based on function and location in the body.

The Central Nervous System

• The brain is 1350 ccs and 3 lbs of processing power and contains 20 billion neurons.
• Its key functions are for complex responses and utilization of 100% of its processing power.
• Different parts of the brain (like the frontal or temporal lobe) have various functions.

Cranial Meninges

• Cranial meninges are connective tissue coverings that protect and anchor the brain in the cranial cavity.
• Layers include dura mater, arachnoid mater, and pia mater.
• Spaces between the layers contain cerebrospinal fluid (CSF).

Cerebrospinal Fluid (CSF)

• CSF is a clear, colorless fluid in the CNS's ventricles and canals.
• It's a filtrate from plasma/blood, with glucose, oxygen, ions, and other substances.
• CSF functions include cushioning/shock absorption, chemical protection, and exchange of nutrients/wastes.

Ventricles of the Brain

• Ventricles are fluid-filled chambers within the brain.
• They're numbered and create flow pathways for CSF.
• CSF flows from ventricles to spinal cord then into the subarachnoid space.

CSF Formation & Pathways of Flow

• CSF is created in the lateral ventricles by the choroid plexus.
• Ependymal cells produce CSF. Flow through different ventricles and into subarachnoid space
• CSF is absorbed into the venous sinuses.

Blood-Brain Barrier (BBB)

• The BBB is the least permeable capillaries in the body.
• It maintains a stable environment for the brain.
• It shields the brain from foreign substances, hormones, and other neurotransmitters in the bloodstream moving through the body.
• Lipid soluble molecules pass through.