Central and Peripheral Nervous System: Study Guide Notes

Central Nervous System (CNS)

Divisions and Development of the Brain

The brain is divided into several regions, each with distinct locations and functions. Understanding its development from the neural tube is essential for grasping CNS structure.

• Primary Brain Vesicles: Prosencephalon (forebrain), Mesencephalon (midbrain), Rhombencephalon (hindbrain).

• Secondary Brain Vesicles: Telencephalon, Diencephalon, Mesencephalon, Metencephalon, Myelencephalon.

• Developmental Progression:

  • Telencephalon → Cerebrum

  • Diencephalon → Diencephalon (thalamus, hypothalamus, pineal gland)

  • Mesencephalon → Midbrain

  • Metencephalon → Pons and Cerebellum

  • Myelencephalon → Medulla oblongata

Spinal Cord: Begins at the foramen magnum and ends at the level of L1-L2 vertebrae.

Gray and White Matter Organization

Gray and white matter are organized differently in the brain and spinal cord.

• Cerebral Hemispheres: Gray matter forms the cortex (outer layer) and basal nuclei; white matter lies beneath.

• Spinal Cord: Gray matter is central (H-shaped), white matter surrounds it.

Anatomical Landmarks of the Brain

• Precentral Gyrus: Primary motor cortex, controls voluntary movements.

• Postcentral Gyrus: Primary somatosensory cortex, processes sensory input.

• Central Sulcus: Separates frontal and parietal lobes.

• Longitudinal Fissure: Divides left and right cerebral hemispheres.

• Lateral Fissure: Separates temporal lobe from frontal and parietal lobes.

Lobes of the Cerebral Cortex

• Frontal Lobe: Motor function, problem-solving, speech production (Broca's area).

• Parietal Lobe: Sensory processing, spatial orientation.

• Temporal Lobe: Auditory processing, memory, language comprehension (Wernicke's area).

• Occipital Lobe: Visual processing.

Functional Areas of the Cortex

• Primary Motor Cortex: Located in precentral gyrus; initiates voluntary movement.

• Premotor Cortex: Plans movements.

• Primary Somatosensory Cortex: Located in postcentral gyrus; receives sensory information.

• Primary Visual Cortex: Located in occipital lobe; processes visual input.

• Primary Auditory Cortex: Located in temporal lobe; processes auditory input.

• Broca’s Area: Speech production; damage causes expressive aphasia.

• Wernicke’s Area: Language comprehension; damage causes receptive aphasia.

Basal Nuclei and Limbic System

• Basal Nuclei: Caudate nucleus, putamen, globus pallidus; regulate movement.

• Limbic System: Includes hippocampus, amygdala, cingulate gyrus; involved in emotion and memory.

Diencephalon Structures and Functions

• Thalamus: Relay station for sensory information.

• Hypothalamus: Controls homeostasis, endocrine functions.

• Pineal Gland: Secretes melatonin, regulates circadian rhythms.

Brainstem Subdivisions

• Midbrain: Visual and auditory reflexes.

• Pons: Relays information, regulates breathing.

• Medulla Oblongata: Controls vital functions (heart rate, respiration).

Reticular Formation

• Function: Regulates arousal, consciousness, and sleep-wake cycles.

Protection of the Brain

• Meninges: Three layers—dura mater (outer), arachnoid mater (middle), pia mater (inner).

• Ventricles: Four ventricles (lateral, third, fourth); contain cerebrospinal fluid (CSF).

• Blood-Brain Barrier (BBB): Formed by endothelial cells, astrocytes; protects brain from toxins.

CNS and Homeostasis

• Hypothalamus: Coordinates temperature, hunger, thirst, and endocrine functions.

• Pineal Gland & Reticular Formation: Light levels influence melatonin secretion and sleep-wake cycles.

Higher Mental Functions

• Prefrontal Cortex: Responsible for reasoning, planning, personality.

• Memory Consolidation: Hippocampus is primary structure.

• Long-Term Potentiation (LTP): Strengthening of synapses, crucial for memory formation.

Spinal Cord Structure

• Regions: Cervical, thoracic, lumbar, sacral.

• Gray Matter: Dorsal (sensory), ventral (motor), lateral horns.

• Tracts: Ascending (sensory), descending (motor).

• Dorsal Root: Sensory input.

• Ventral Root: Motor output.

• Cauda Equina: Bundle of spinal nerves below L2.

CNS in Sensation

• Dorsal Column Medial Lemniscus: Carries touch, vibration, proprioception.

• Anterolateral System: Carries pain, temperature.

• Sensory Homunculus: Map of body regions in somatosensory cortex.

CNS in Voluntary Movement

• Corticospinal Tract: Main pathway for voluntary motor control.

• Motor Homunculus: Map of body regions in motor cortex.

• Basal Ganglia & Cerebellum: Modulate movement, coordination.

Peripheral Nervous System (PNS)

Overview of PNS

The PNS consists of cranial and spinal nerves, which connect the CNS to the rest of the body.

• Cranial Nerves: 12 pairs, emerge from the brain.

• Spinal Nerves: 31 pairs, emerge from the spinal cord; named by vertebral level.

Cranial Nerves

Each cranial nerve has specific functions and carries sensory, motor, or mixed information.

Number	Name	Type	Function
I	Olfactory	Sensory	Smell
II	Optic	Sensory	Vision
III	Oculomotor	Motor	Eye movement
IV	Trochlear	Motor	Eye movement
V	Trigeminal	Mixed	Facial sensation, chewing
VI	Abducens	Motor	Eye movement
VII	Facial	Mixed	Facial expression, taste
VIII	Vestibulocochlear	Sensory	Hearing, balance
IX	Glossopharyngeal	Mixed	Taste, swallowing
X	Vagus	Mixed	Visceral sensation, parasympathetic control
XI	Accessory	Motor	Head movement
XII	Hypoglossal	Motor	Tongue movement

Spinal Nerves

• Naming Scheme: Cervical (C1–C8), Thoracic (T1–T12), Lumbar (L1–L5), Sacral (S1–S5), Coccygeal (Co1).

Reflexes

Reflexes are rapid, involuntary responses to stimuli, mediated by reflex arcs.

• Reflex Arc Components: Receptor, sensory neuron, integration center, motor neuron, effector.

• Simple Stretch Reflex: Muscle stretch → sensory neuron → spinal cord → motor neuron → muscle contraction (e.g., knee-jerk reflex).

• Withdrawal Reflex: Painful stimulus → withdrawal of limb.

• Crossed-Extension Reflex: Withdrawal reflex on one side, extension on the opposite side for balance.

Autonomic Nervous System (ANS)

The ANS regulates involuntary functions and is divided into sympathetic and parasympathetic divisions.

• Targets: Smooth muscle, cardiac muscle, glands.

• Sympathetic vs. Parasympathetic:

  • Sympathetic: Cell bodies in thoracolumbar region; short preganglionic, long postganglionic axons; neurotransmitters: acetylcholine (preganglionic), norepinephrine (postganglionic); "fight or flight" functions.

  • Parasympathetic: Cell bodies in craniosacral region; long preganglionic, short postganglionic axons; neurotransmitter: acetylcholine; "rest and digest" functions.

Special Senses

Special senses include taste and hearing, each with unique structures and pathways.

• Taste Sensation: Taste buds on tongue; activated by chemicals in food.

• Five Foundational Tastes: Sweet, salty, sour, bitter, umami.

• Taste Pathway: Taste receptors → cranial nerves VII, IX, X → medulla → thalamus → gustatory cortex.

• Auditory System: Structures: cochlea, hair cells; activated by sound vibrations.

• Auditory Pathway: Cochlea → vestibulocochlear nerve (VIII) → brainstem → thalamus → auditory cortex.

Example:

Damage to Broca's area results in expressive aphasia, where speech production is impaired but comprehension remains intact.

Additional info: LTP (long-term potentiation) is a cellular mechanism underlying learning and memory, involving increased synaptic strength after repeated stimulation.