Microanatomy of a Neuron

Key Structures of a Neuron

The neuron is the fundamental unit of the nervous system, specialized for the transmission of electrical and chemical signals.

• Soma (cell body): Contains the nucleus and organelles; responsible for metabolic activities.

• Dendrite: Branch-like extensions that receive signals from other neurons.

• Axon: Long projection that transmits impulses away from the cell body.

• Axon hillock: Region where the axon originates from the soma; site of action potential initiation.

• Axon terminal/terminal bud: End of the axon where neurotransmitters are released.

Example: Sensory neurons in the skin transmit touch information to the spinal cord via their axons.

Neuroglial (Glial) Cells/Myelin Sheath

Types and Functions

Neuroglial cells support and protect neurons. Myelin sheath increases the speed of nerve impulse conduction.

• Oligodendrocytes: Form myelin in the central nervous system (CNS).

• Schwann cells: Form myelin in the peripheral nervous system (PNS).

• Node of Ranvier: Gaps between myelin segments; facilitate saltatory conduction.

Additional info: Myelinated axons conduct impulses faster than unmyelinated axons.

Spinal Cord

Gray Matter and White Matter

The spinal cord is organized into gray and white matter, each with distinct functions.

• Gray matter:

  • Central canal: Contains cerebrospinal fluid (CSF).

  • Posterior horn: Contains sensory neurons.

  • Lateral horn: Contains autonomic neurons (in thoracic region).

  • Anterior horn: Contains motor neurons.

• White matter:

  • Spinal nerve (know total number): 31 pairs in humans.

  • Dorsal root ganglion: Contains cell bodies of sensory neurons.

  • Dorsal root: Sensory input to spinal cord.

  • Ventral root: Motor output from spinal cord.

  • Ascending tracts: Sensory information to brain.

  • Descending tracts: Motor commands from brain.

Example: The dorsal root transmits touch and pain signals to the CNS.

Plexuses: Definition and Major Plexuses

What is a Plexus?

A plexus is a network of intersecting nerves. Plexuses allow for redistribution of nerve fibers to different body regions.

• Cervical plexus: Serves the neck and diaphragm.

• Brachial plexus: Serves the shoulder and upper limb.

• Lumbar plexus: Serves the lower back and anterior thigh.

• Sacral plexus: Serves the pelvis, buttocks, and lower limb.

Additional info: Damage to the brachial plexus can result in loss of arm function.

Structures of the Brain

Major Brain Regions

The brain is divided into several major regions, each with specialized functions.

• Cerebrum: Responsible for higher brain functions such as thought and action.

• Cerebral hemispheres: Left and right halves of the cerebrum.

• Lobes of the cerebrum:

  • Frontal

  • Parietal

  • Temporal

  • Occipital

• Corpus callosum: Connects the two cerebral hemispheres.

Example: The occipital lobe processes visual information.

Structures of the Brain, cont'd

Additional Brain Structures

Other important brain structures include those involved in sensory processing, hormone regulation, and basic life functions.

• Thalamus: Relay station for sensory information.

• Hypothalamus: Regulates homeostasis and endocrine functions.

• Pituitary gland: Master endocrine gland.

• Mammillary bodies: Involved in memory.

• Medulla oblongata: Controls vital autonomic functions.

• Cerebellum: Coordinates movement and balance.

• Arbor vitae: Tree-like white matter in cerebellum.

• Meninges & spaces: Protective membranes and fluid-filled spaces around the brain and spinal cord.

  • Dura mater

  • Arachnoid mater

  • Pia mater

  • Subarachnoid space

  • Epidural space (spinal cord only)

• Ventricles: Fluid-filled cavities in the brain.

  • Lateral ventricles

  • Third ventricle

  • Fourth ventricle

  • Cerebral aqueduct

• Central canal: Runs through spinal cord; contains CSF.

Additional info: The meninges protect the CNS from injury and infection.

Reflex Arc

Reflex Terminology and Pathways

A reflex arc is the neural pathway that mediates a reflex action, allowing for rapid, involuntary responses to stimuli.

• Ipsilateral response: Reflex occurs on the same side as the stimulus.

• Contralateral response: Reflex occurs on the opposite side of the stimulus.

Components of a Reflex Arc

• Receptor

• Sensory neuron

• Integration center

• Motor neuron

• Effector

Example: The knee-jerk (patellar) reflex is a classic example of a simple reflex arc.

Classification of Reflexes

• Somatic reflex: Involves skeletal muscles (e.g., knee-jerk reflex).

• Flexor (withdrawal) reflex: Protects body from harm (e.g., pulling hand away from hot object).

• Crossed extensor reflex: Maintains balance during withdrawal.

• Autonomic reflex: Involves smooth muscle, cardiac muscle, or glands (e.g., pupil constriction).

Summary Table: Major Brain Structures and Functions

Key Equation: Nerve Impulse Conduction

The speed of nerve impulse conduction is affected by myelination and axon diameter.