Introduction to the Nervous System

The nervous system is responsible for integrating, processing, and coordinating sensory data and motor commands
The central nervous system (CNS) consists of the brain and spinal cord
The peripheral nervous system (PNS) includes all neural tissue outside the CNS

Division of the Nervous System

CNS (Central Nervous System):
- Acts as the center of perception
- Integrates sensory information from various sources, including visual, auditory, tactile, olfactory, and gustatory systems.
- Initiates and coordinatesmotor activity encompasses the physical actions and movements that are initiated and controlled by the central nervous system. This includes all voluntary movements, such as walking, running, and writing, as well as involuntary reflexes, such as blinking or withdrawing a hand from a hot surface. Motor activity is critical for interacting with the environment and performing daily tasks, and it is intricately linked with sensory input, which provides the necessary information for these actions to occur smoothly and effectively.
PNS (Peripheral Nervous System):
- Extends from the CNS
- Includes 12 cranial nerves (emerging from the brain)
- Includes 31 pairs of spinal nerves (emerging from the spinal cord)
- Conveys neural impulses to the CNS (input from sensory receptors)
- Conveys neural impulses from the CNS to muscles and glands (output to muscles and glands)

Spinal Nerve Categorization

Spinal nerves are categorized based on location
- Cervical: 8 pairs
- Thoracic: 12 pairs
- Lumbar: 5 pairs
- Sacral: 5 pairs
- Coccygeal: 1 pair

Organization of Spinal Nerves

Spinal nerves have a dorsal root (sensory fibers) and a ventral root (motor fibers)
Rami communicans connect spinal nerves to the sympathetic trunk and ganglia

Organization of Nervous Tissue (CNS vs PNS)

CNS: Collections of neurons are called nuclei
- Examples of nuclei include the cerebral cortex, basal nuclei, and brain stem nuclei
PNS: Collections of neurons are called ganglia
- Examples of ganglia include dorsal root ganglia and autonomic ganglia
- Sensory and motor neurons are also organized in nerves
Tracts are collections of axons in the CNS
Nerves are collections of axons in the PNS

Nervous Tissue Components

Neurons: The functional units of the nervous system
- Comprised of a cell body, dendrites, and axon
Neuroglia (glial cells): Support cells that surround and support the neurons
- Examples include astrocytes, oligodendrocytes, and microglia
  - Astrocytes: Aid in metabolism exchange
  - Oligodendrocytes: Form myelin sheaths around axons in the CNS
  - Microglia: Involved in immune defense

Axon Description

Axon: A single efferent process
- Arises from the cone-shaped portion of the cell body
- Conducts impulses away from the cell body

Reaction to Injury

Neurons are incapable of cell division/replacement
Axons can regenerate if severed

Nervous Tissue Sensory Receptors

The nervous system contains sensory receptors in human skin, including
- Free nerve endings (pain, temperature, and touch)
- Merkel disks, also known as Merkel cells, are specialized mechanoreceptors found in the skin, particularly within the basal epidermal layer. These disks are responsible for detecting light touch and pressure, playing a critical role in our ability to perceive tactile stimuli. They are particularly concentrated in areas of the skin that are sensitive to touch, such as the fingertips and lips. Merkel disks consist of both the Merkel cells themselves and their associated nerve endings, forming a complex structure that enables the transduction of mechanical signals into electrical impulses that can be interpreted by the nervous system. This process is vital for tasks such as reading Braille or sensing the texture of objects. Additionally, Merkel cells are thought to have a role in the regeneration of skin and may contribute to the healing process following injury. (touch)
- Krause end bulbs are sensory receptors located in the skin and mucous membranes that primarily respond to light touch and cold temperatures, helping to perceive tactile sensations. They contribute to the complex experience of touch and thermal sensation.
- The root hair plexus consists of a network of nerve endings that surround hair follicles, allowing for the detection of even the slightest movements of hairs, which is essential for sensing touch in hairy skin.
- Meissner corpuscles, found in the upper dermis, are specialized for detecting light touch and vibrations, playing a key role in the ability to discern textures and fine details through the skin.
- Pacinian corpuscles are large, onion-shaped sensors located deeper in the dermis and subcutaneous tissues; they respond primarily to deep pressure and vibrations, enabling the perception of heavier touch stimuli.
- Ruffini endings are spindle-shaped mechanoreceptors found in the deeper skin layers; they are sensitive to skin stretch and contribute to our awareness of the position and movement of fingers and joints.

Somatic Nervous System vs Autonomic Nervous System

Somatic Nervous System: Controls voluntary skeletal muscle contractions and conscious/unconscious sensations from the head, body, and extremities
Autonomic Nervous System: Automatically regulates smooth muscle, cardiac muscle, glands, and adipose tissue (e.g., digestion, heart rate)

Autonomic Nervous System

Sympathetic Division: Prepares the body for emergency situations
Parasympathetic Division: Promotes normal functions and energy conservation

Dental Application: The jaw jerk reflex, also known as the mandibular reflex, is a deep tendon reflex that involves the contraction of the masseter muscle in response to a tap on the chin. This reflex is classified as a monosynaptic reflex arc, meaning it involves a single synapse between the sensory input and motor output. Sensory fibers from the trigeminal nerve (cranial nerve V) detect the stretching of the muscle when the chin is tapped, sending signals to the brainstem, which then sends back a motor response to the masseter muscle, resulting in the closure of the jaw. This reflex is used clinically to assess neurological function, particularly in evaluating the integrity of the trigeminal nerve and the central nervous system pathways it intersects with. It can also be influenced by certain neurological conditions, making it a useful diagnostic tool.