Biology Nervous System Overview

The Nervous System

• Consists of central (CNS) and peripheral (PNS) systems
• The CNS includes the brain and spinal cord
• The PNS extends throughout the body

The Spinal Cord

• Encased in the vertebral column
• Protected by three membranes: dura mater, arachnoid mater, and pia mater
• Connected to the peripheral nervous system via spinal nerves

Spinal Nerves

• Consist of afferent (sensory) and efferent (motor) fibers
• Afferent fibers carry impulses from sensory receptors to the CNS
• Efferent fibers carry impulses to organs (muscles and glands)

Nerve Fibers

• Bundles of nerve fibers form a nerve
• Enclosed by connective tissue layers called endoneurium
• Schwann cells are located under the endoneurium
• Myelinated fibers (wrapped by Schwann cells several times) have faster conduction speed
• Myelination creates regular interruptions called nodes of Ranvier

Nerve Cells (Neurons)

• Consist of a cell body, dendrites, and an axon
• Dendrites receive stimuli
• Axons conduct electrical impulses
• Terminal buttons at the end of the axon form the first part of a synapse
• Terminal buttons contain calcium channels and vesicles with neurotransmitters

Synapses

• Allow communication between neurons
• Terminal buttons release neurotransmitters via exocytosis
• Neurotransmitters bind to receptors on the postsynaptic cell membrane
• This converts the chemical signal back to an electrical impulse

Spinal Ganglion (Dorsal Root Ganglion)

• Thickening in the spinal nerve that houses cell bodies of sensory neurons
• Neurons in the spinal ganglion are pseudo-unipolar (axon is divided into two branches)

Myelinated vs. Unmyelinated Nerve Fibers

• Myelinated fibers have faster conduction speed due to the presence of Schwann cells forming a myelin sheath
• Unmyelinated fibers lack myelination, resulting in slower conduction speed

Sensory Receptors in the Skin

• Located in various layers of the skin
• Can be rapidly adapting or slowly adapting
• Meissner's corpuscles: rapidly adapting mechanoreceptors, respond to changes in pressure
• Merkel cells: slowly adapting mechanoreceptors, respond to both dynamic and static stimuli
• Ruffini corpuscles: slowly adapting mechanoreceptors, respond to tissue stretch

Spinal Cord: Gray and White Matter

• Gray matter: contains cell bodies of neurons
• White matter: contains axons of neurons
• Sensory impulses are transmitted through both gray and white matter in the spinal cord

Reflexes

• Sensory impulses can be processed directly in the spinal cord
• Sensory fibers can transmit impulses directly or indirectly to motor nerve fibers, causing muscle contraction or relaxation
• Example: Burning yourself triggers a reflex where sensory receptors send impulses to interneurons, which then activate motor neurons to contract muscles.