Exercise 8

Tract

A bundle of axons located within the central nervous system (CNS) that carries information up or down the spinal cord or between regions of the brain.

Nerve

A bundle of axons located in the peripheral nervous system (PNS), surrounded by connective tissue layers.

Ganglia

Clusters of neuron cell bodies located in the PNS.

Nuclei

Clusters of neuron cell bodies located in the CNS.

Plexus

A network of intersecting nerves or nerve fibers in the PNS.

Soma (cell body)

The main part of a neuron containing the nucleus and most organelles; responsible for metabolism and processing information.Neuronal nucleus

Neuronal nucleus

The nucleus inside the soma that contains the neuron's genetic material.

Axon hillock

The cone-shaped region of the soma where the axon begins; the site where action potentials are initiated.

Dendrite

Branching extensions of a neuron that receive signals from other neurons.

Axon

A long, singular extension that carries electrical impulses away from the soma toward other neurons or effectors.Axolemma

Axolemma

The plasma membrane of the axon.

Axon terminal

The endpoint of an axon where neurotransmitters are released into the synaptic cleft.

Schwann cell

A PNS glial cell that forms the myelin sheath around axons.

Neurilemma

The outermost layer of a Schwann cell that surrounds the myelin sheath; important for nerve regeneration.

Myelin sheath

A lipid-rich insulating layer around axons that speeds up electrical conduction.

Nodes of Ranvier

Gaps between Schwann cells where the axon membrane is exposed; enables saltatory conduction.

Unipolar neuron

A neuron with one process that splits into a peripheral and central branch; common in sensory neurons.

Bipolar neuron

A neuron with one dendrite and one axon; found in sensory organs such as the retina.

Multipolar neuron

A neuron with multiple dendrites and a single axon; the most common neuron type in the CNS.

Sensory (afferent) neurons

Neurons that carry information from sensory receptors toward the CNS.

Motor (efferent) neurons

Neurons that carry signals from the CNS to muscles or glands.

Interneurons

Neurons located entirely in the CNS that connect sensory and motor pathways.

Endoneurium

Connective tissue surrounding individual axons.

Perineurium

Connective tissue surrounding bundles of axons called fascicles.

Fascicle

A bundle of axons within a nerve.

Epineurium

The outermost connective tissue layer surrounding an entire nerve.

Dura mater

The tough, outermost meningeal layer protecting the spinal cord.

Epidural space

The space between the dura mater and vertebral column; contains fat and blood vessels.

Arachnoid mater

The middle, web-like meningeal layer.

Subarachnoid space

The space between the arachnoid mater and pia mater; filled with cerebrospinal fluid (CSF).

Pia mater

The delicate, innermost meningeal layer that adheres directly to the spinal cord.Cervical enlargement –

Cervical enlargement

A widened area of the spinal cord where nerves supplying the upper limbs emerge.

Lumbar enlargement

A widened area of the spinal cord where nerves supplying the lower limbs emerge.

Filum terminale

A thin extension of pia mater anchoring the spinal cord to the coccyx.

Denticulate ligaments

Lateral extensions of pia mater that stabilize the spinal cord.

Conus medullaris

The tapered, lower end of the spinal cord.

Cauda equina

A bundle of spinal nerves extending beyond the conus medullaris.

Spinal nerve

A mixed nerve formed by the union of dorsal and ventral roots.

Anterior median fissure

A deep groove on the anterior side of the spinal cord.

Posterior median sulcus

A shallow groove on the posterior side of the spinal cord.

Central canal

A small canal in the center of the spinal cord containing CSF.

Anterior white column

The front portion of white matter containing ascending and descending tracts.

Posterior white column

The back portion of white matter containing ascending tracts.

Lateral white column

The side portion of white matter containing ascending and descending tracts.

Anterior gray horn

Contains motor neuron cell bodies.

Posterior gray horn

Contains sensory neuron cell bodies and interneurons.

Lateral gray horn

Contains autonomic motor neuron cell bodies (thoracic/lumbar regions).

Gray commissure

A bridge of gray matter connecting the two sides of the spinal cord; surrounds the central canal.

Anterior white commissure

A band of white matter where axons cross from one side of the spinal cord to the other.

Dorsal root

Contains sensory axons entering the spinal cord.

Dorsal root ganglion

A cluster of sensory neuron cell bodies on the dorsal root.

Ventral root

Contains motor axons leaving the spinal cord.

Dorsal ramus

Supplies the skin and muscles of the back.

Ventral ramus

Supplies the limbs and anterior/lateral trunk.

Cervical plexus

Network of nerves from C1–C4 that innervates the neck and diaphragm.

Phrenic nerve

Major nerve of the cervical plexus; controls the diaphragm.

Brachial plexus

Network of nerves from C5–T1 that innervates the shoulders and upper limbs.

Axillary nerve

Innervates the deltoid and teres minor.

Musculocutaneous nerve

Innervates anterior arm flexor muscles.

Median nerve

Innervates most forearm flexors and some hand muscles.

Ulnar nerve

Innervates some forearm flexors and most intrinsic hand muscles.

Radial nerve

Innervates extensor muscles of the arm and forearm.

Lumbar plexus

Network of nerves from L1–L4 that innervates the anterior and medial thigh.

Femoral nerve

Innervates the quadriceps and anterior thigh.

Obturator nerve

Innervates the medial thigh/adductor muscles.

Sacral plexus

Network of nerves from L4–S4 that innervates the pelvis, buttocks, and lower limbs.

Sciatic nerve

The largest nerve in the body; splits into the tibial and common fibular nerves.

Tibial nerve

Supplies posterior leg and foot muscles.

Common fibular nerve

Supplies anterior and lateral leg muscles.

What is the structural difference between the CNS and PNS?

• CNS (Central Nervous System): Includes the brain and spinal cord. Contains tracts (axon bundles) and nuclei (cell body clusters).

• PNS (Peripheral Nervous System): Includes nerves and ganglia outside the CNS. Contains nerves (axon bundles) and ganglia (cell body clusters).

How do you identify sensory vs motor roots in the spinal cord?

• Dorsal root

  • Contains sensory (afferent) axons.

  • Has a dorsal root ganglion (bulge) — this is the easiest identifier.

• Ventral root

  • Contains motor (efferent) axons.

  • No ganglion present.

Which part of the neuron initiates action potentials?

The axon hillock (also called the trigger zone).
It integrates incoming signals and fires an action potential if threshold is met.

What is the functional significance of myelination?

• Myelin increases conduction speed down the axon.

• Allows saltatory conduction, where impulses jump between Nodes of Ranvier.

• Myelinated axons transmit signals much faster than unmyelinated ones.

What is the difference between a tract and a nerve?

• Tract: Bundle of axons in the CNS.

• Nerve: Bundle of axons in the PNS, wrapped in connective tissue layers (endo/peri/epineurium).

What is found in the dorsal root ganglion?

Cell bodies of sensory (afferent) neurons.
These are mostly unipolar neurons.

Which nerves arise from the brachial plexus?

The major terminal branches are:

• Axillary nerve

• Musculocutaneous nerve

• Median nerve

• Ulnar nerve

• Radial nerve

What is the function of the phrenic nerve?

Innervates the diaphragm

Essential for breathing (inspiration)

Origin: C3–C5 (“C3, 4, 5 keep the diaphragm alive”).

Where are autonomic neurons located in the spinal cord?

• In the lateral gray horn

  • Present only in thoracic and upper lumbar regions (T1–L2).
    This region contains sympathetic autonomic motor neurons.

What forms the cauda equina and where is it located?

• Formed by spinal nerve roots extending downward from the conus medullaris.

• Located in the lumbar cistern of the vertebral canal (around L2–S5).

• It looks like a horse’s tail.