List the subdivisions of the nervous system.

The nervous system is broadly divided into two main parts:

• The Central Nervous System (CNS), which includes the brain and spinal cord.
• The Peripheral Nervous System (PNS), which consists of all the nerves outside the CNS. The PNS is further divided into the somatic nervous system and the autonomic nervous system.

$\boxed{\text{The subdivisions of the nervous system are the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).}}$

• Grey matter consists primarily of neuron cell bodies, dendrites, unmyelinated axons, and neuroglia. It is involved in processing information.
• White matter is composed mainly of myelinated axons, which transmit signals rapidly. The myelin sheath gives it its white appearance.
• A nucleus (in the CNS) is a cluster of neuron cell bodies within the central nervous system.
• A ganglion (in the PNS) is a cluster of neuron cell bodies in the peripheral nervous system.
• A tract (in the CNS) is a bundle of axons within the central nervous system that share a common origin, destination, and function.
• A nerve (in the PNS) is a bundle of axons in the peripheral nervous system, often surrounded by connective tissue.

$\boxed{\text{Grey matter: neuron cell bodies and unmyelinated axons (processing). White matter: myelinated axons (signal transmission). Nucleus (CNS): cluster of cell bodies. Ganglion (PNS): cluster of cell bodies. Tract (CNS): bundle of axons. Nerve (PNS): bundle of axons.}}$

• Neurons are the fundamental functional units of the nervous system, specialized cells that transmit electrical and chemical signals. They consist of a cell body, dendrites, and an axon.
• Neuroglia (or glial cells) are non-neuronal cells that provide support, nourishment, insulation, and protection for neurons. They do not transmit nerve impulses.

$\boxed{\text{Neurons are signal-transmitting cells. Neuroglia are supporting cells that protect and nourish neurons.}}$

The major parts of the brain include:

• Cerebrum: The largest part, responsible for higher functions like thought, voluntary movement, language, and perception.
• Diencephalon: Includes the thalamus and hypothalamus, involved in sensory relay and autonomic functions.
• Cerebellum: Located at the back of the brain, coordinating voluntary movements, balance, and posture.
• Brainstem: Connects the cerebrum and cerebellum to the spinal cord, controlling vital functions like breathing, heart rate, and sleep. It consists of the midbrain, pons, and medulla oblongata.

$\boxed{\text{The main parts of the brain are the cerebrum, diencephalon, cerebellum, and brainstem (midbrain, pons, medulla oblongata).}}$

• External features: The spinal cord is a cylindrical structure extending from the brainstem down to the lumbar region. It has cervical and lumbar enlargements, and terminates in the conus medullaris, from which the filum terminale extends. Spinal nerves emerge from segments along its length.
• Internal features: Internally, the spinal cord has a central H-shaped region of grey matter (containing neuron cell bodies) surrounded by white matter (containing myelinated axons). The grey matter is divided into anterior, posterior, and lateral horns, while the white matter is organized into columns (funiculi).

$\boxed{\text{External: cylindrical shape, cervical/lumbar enlargements, conus medullaris, spinal nerves. Internal: H-shaped grey matter (horns) surrounded by white matter (columns).}}$

There are 12 pairs of cranial nerves, typically enumerated with Roman numerals:

1. Olfactory (I): Smell
2. Optic (II): Vision
3. Oculomotor (III): Eye movement, pupil constriction
4. Trochlear (IV): Eye movement
5. Trigeminal (V): Facial sensation, mastication
6. Abducens (VI): Eye movement
7. Facial (VII): Facial expression, taste, salivary/lacrimal glands
8. Vestibulocochlear (VIII): Hearing, balance
9. Glossopharyngeal (IX): Taste, swallowing, salivary glands
10. Vagus (X): Visceral sensation and motor control (heart, lungs, digestive tract)
11. Accessory (XI): Head and shoulder movement
12. Hypoglossal (XII): Tongue movement

$\boxed{\text{The 12 cranial nerves are Olfactory (I), Optic (II), Oculomotor (III), Trochlear (IV), Trigeminal (V), Abducens (VI), Facial (VII), Vestibulocochlear (VIII), Glossopharyngeal (IX), Vagus (X), Accessory (XI), and Hypoglossal (XII).}}$

• Parts of a spinal nerve: Each spinal nerve forms from the union of a dorsal root (carrying sensory information) and a ventral root (carrying motor information) from the spinal cord. After exiting the intervertebral foramen, it branches into a dorsal ramus (supplying the back) and a ventral ramus (supplying the anterior and lateral trunk and limbs).
• Distribution: Spinal nerves are distributed segmentally to specific regions of the body. The ventral rami often form plexuses (e.g., cervical, brachial, lumbar, sacral) that innervate the limbs.
• Dermatome: A dermatome is an area of skin that is mainly supplied by afferent nerve fibers from a single dorsal root of a spinal nerve.

$\boxed{\text{Spinal nerves form from dorsal and ventral roots, branching into dorsal and ventral rami. They distribute segmentally, often forming plexuses. A dermatome is a skin area innervated by a single spinal nerve root.}}$

The central nervous system (brain and spinal cord) is protected by several layers:

• Bones: The skull protects the brain, and the vertebral column protects the spinal cord.
• Meninges: Three layers of connective tissue membranes:
  • Dura mater (outermost, tough layer).
  • Arachnoid mater (middle layer).
  • Pia mater (innermost, delicate layer adhering to the CNS surface).
• Cerebrospinal fluid (CSF): A clear fluid that circulates within and around the CNS, providing cushioning, buoyancy, and nutrient transport.

$\boxed{\text{The CNS is protected by bones (skull, vertebral column), meninges (dura mater, arachnoid mater, pia mater), and cerebrospinal fluid (CSF).}}$