Central Nervous System: Brain and Spinal Cord

Main Regions of the Adult Brain

The adult brain is divided into four main regions, each with distinct structures and functions.

• Cerebral Hemispheres: The largest part, responsible for higher brain functions.

• Diencephalon: Contains structures such as the thalamus and hypothalamus.

• Brain Stem: Includes the midbrain, pons, and medulla oblongata; controls vital functions.

• Cerebellum: Coordinates movement and balance.

Gray and White Matter Organization

• Cerebrum: Has an outer cortex of gray matter and islands of gray matter (nuclei) within white matter.

• Brainstem: Contains regions of gray matter (nuclei) buried within white matter.

• Spinal Cord: Central gray matter surrounded by outer white matter.

Major Fissures and Lobes of the Cerebrum

• Longitudinal Fissure: Separates left and right cerebral hemispheres.

• Transverse Cerebral Fissure: Separates cerebrum from cerebellum.

• Frontal Lobe: Motor areas; voluntary movement.

• Parietal Lobe: Somatosensory areas; touch, pressure, pain.

• Occipital Lobe: Vision.

• Temporal Lobe: Auditory processing.

• Insula: Language, taste, visceral sensation integration.

Cerebral Cortex Functions

The cerebral cortex is the site of conscious mind activities, including:

• Awareness

• Sensory perception

• Voluntary motor initiation

• Communication

• Memory storage

• Understanding

Functional Areas of the Cortex

• Primary Motor Cortex (precentral gyrus): Controls precise, skilled skeletal muscle movements.

• Primary Somatosensory Cortex (postcentral gyrus): Receives sensory information from skin and proprioceptors.

Somatotopy: The mapping of body regions onto the cortex.

Language Areas

• Broca’s Area: Motor language area; directs muscles for speech production. Located in left frontal lobe.

• Wernicke’s Area: Language comprehension. Located in left parietal and temporal lobes.

Basal Ganglia

• Includes caudate, putamen, globus pallidus, and substantia nigra.

• Regulates voluntary movement intensity, initiation, and termination.

Parkinson’s Disease: Degeneration of substantia nigra; symptoms include tremors, slow movement.

Huntington’s Disease: Degeneration of caudate and putamen; symptoms include uncontrolled limb movements.

Diencephalon

• Thalamus: Relay station for sensory and motor signals, learning, and memory.

• Hypothalamus: Main visceral control center; regulates homeostasis.

Brainstem

• Composed of midbrain, pons, and medulla oblongata.

• Medulla Oblongata contains vital centers:

  • Cardiovascular center: Regulates heart rate and blood pressure.

  • Respiratory centers: Control breathing rate and depth.

  • Other centers: Vomiting, hiccupping, swallowing, coughing, sneezing.

Cerebellum

• Coordinates precise, smooth, and balanced movements.

• Processes input from cortex, brainstem, and sensory receptors.

Limbic System

• Emotional brain; includes cingulate gyrus (emotion expression, pain interpretation) and amygdala (fear, danger assessment).

• Hippocampus: Memory consolidation.

Protection of the Brain

• Cerebrospinal Fluid (CSF): Provides buoyancy, protection, and nourishment.

• Blood-Brain Barrier: Tight junctions prevent passage of many substances; allows nutrients and fat-soluble substances (e.g., alcohol, nicotine), but blocks toxins, proteins, and most drugs.

Spinal Cord Structure and Function

• Ends at L1 or L2 vertebra in adults.

• 31 pairs of spinal nerves.

• Gray matter regions: dorsal horn (interneurons), ventral horn (somatic motor neurons), lateral horn, gray commissure.

• Sensory neuron cell bodies: dorsal root ganglion.

Spinal Cord Injuries and Disorders

• Paraplegia: Loss of function in lower limbs (injury between T1 and L1).

• Quadriplegia: Loss of function in all limbs (injury in cervical region).

• Amyotrophic Lateral Sclerosis (ALS): Destruction of ventral horn motor neurons; symptoms include loss of speech, swallowing, and breathing ability.

Spinal Cord Tracts

• Decussation: Pathways cross from one side of CNS to the other.

• Relay: Consist of two or three neurons in sequence.

• Somatotopy: Spatial correspondence between CNS and body regions.

• Symmetry: Pathways are paired (right and left).

Ascending pathways: Sensory information to the brain. Descending pathways: Motor information from the brain.

Peripheral Nervous System (PNS)

Receptor Types by Stimulus

• Mechanoreceptors: Touch, pressure, vibration, stretch.

• Thermoreceptors: Temperature.

• Photoreceptors: Light (e.g., retina).

• Chemoreceptors: Chemicals (e.g., smell, taste).

• Nociceptors: Pain-causing stimuli.

Receptor Types by Location

• Exteroceptors: Respond to external stimuli.

• Interoceptors (Visceroceptors): Respond to internal stimuli (viscera, blood vessels).

• Proprioceptors: Respond to stretch in muscles, tendons, joints; inform brain of movement.

Special vs. General Sense Receptors

• Special senses: Localized, complex receptors (e.g., vision, hearing).

• General senses: Widespread, simple receptors (e.g., touch, pain).

Referred Pain

Pain perceived at a location other than the site of the painful stimulus, due to shared nerve pathways (e.g., left arm pain during heart attack).

Nerve Classifications

• Mixed nerves: Both sensory and motor fibers.

• Sensory (afferent) nerves: Carry impulses to CNS.

• Motor (efferent) nerves: Carry impulses from CNS.

Somatic and Visceral Nerve Fibers

• Somatic afferent: Sensory from skeletal muscle to brain.

• Somatic efferent: Motor from brain to skeletal muscle.

• Visceral afferent: Sensory from organs to brain.

• Visceral efferent: Motor from brain to organs.

Nerve Regeneration

• PNS: Can regenerate if cell body and neurolemma are intact; Schwann cells aid regeneration.

• CNS: Cannot regenerate due to inhibitory proteins from oligodendrocytes and scar tissue from astrocytes.

Spinal Roots and Rami

• Roots: Medial, form spinal nerves; purely sensory (dorsal) or motor (ventral).

• Rami: Lateral branches; carry both sensory and motor fibers.

• Injury to root: Sensory or motor deficit only. Injury to ramus: Both sensory and motor deficit.

Nerve Plexuses

• Networks of crisscrossing nerve fibers formed by ventral rami (except T2–T12).

• Each limb muscle innervated by more than one spinal nerve; damage to one does not cause complete loss of function.

Major Nerves from Plexuses

Reflex Arcs

• Receptor: Detects stimulus.

• Sensory neuron: Transmits afferent impulse.

• Integration center: Synapse(s) in CNS.

• Motor neuron: Conducts efferent impulse.

• Effector: Muscle or gland responding.

Proprioceptors

• Muscle spindles: Sense muscle length.

• Golgi tendon organs: Sense muscle tension.

Types of Reflexes

• Stretch reflex: Maintains muscle length; muscle contracts in response to stretch.

• Reciprocal inhibition: Antagonist muscles inhibited during stretch reflex.

• Flexor (withdrawal) reflex: Withdrawal from painful stimulus.

• Crossed extensor reflex: Maintains balance during withdrawal; opposite limb extends.

Autonomic Nervous System (ANS)

Overview

The ANS is the involuntary division of the nervous system, controlling smooth muscle, cardiac muscle, and glands.

Somatic vs. Autonomic Nervous System

Divisions of the ANS

• Sympathetic: "Fight or flight"; increases heart rate, dilates pupils, redirects blood to muscles, increases glucose release.

• Parasympathetic: "Rest and digest"; decreases heart rate, increases GI activity, constricts pupils.

Dual innervation: Most organs receive input from both divisions.

Origin and Fiber Lengths

• Sympathetic: Thoracolumbar origin; short preganglionic, long postganglionic fibers.

• Parasympathetic: Craniosacral origin; long preganglionic, short postganglionic fibers.

Cranial Nerves with Parasympathetic Fibers

• Oculomotor (III): Pupils constrict, lens bulges.

• Facial (VII): Stimulates glands (nasal, lacrimal, salivary).

• Glossopharyngeal (IX): Parotid salivary glands.

• Vagus (X): Thoracic and abdominal viscera.

Autonomic Reflexes

• Involve two consecutive motor neurons and visceral sensory receptors.

Neurotransmitters and Receptors

• Both divisions: Preganglionic axons release ACh (always stimulatory).

• Parasympathetic postganglionic: Release ACh (effect varies).

• Sympathetic postganglionic: Most release norepinephrine (NE).

Cholinergic Receptors

• Nicotinic: On all postganglionic neurons, adrenal medulla, skeletal muscle; always stimulatory.

• Muscarinic: On all parasympathetic target organs; effect can be excitatory or inhibitory depending on organ.

Adrenergic Receptors

• Alpha 1: Constricts blood vessels of skin and viscera.

• Alpha 2: Inhibits presynaptic NE release.

• Beta 1: Increases heart rate and contractility.

• Beta 2: Dilates bronchioles and blood vessels to skeletal muscle.

Sympathetic vs. Parasympathetic Effects on Organs

Organs with Only Sympathetic Innervation

• Adrenal medulla

• Sweat glands

• Arrector pili muscles

• Kidneys

• Most blood vessels

Duration and Spread of Effects

• Sympathetic: Longer-lasting, more widespread (due to hormone release from adrenal medulla).

• Parasympathetic: Shorter duration (ACh broken down quickly).

Additional info: For equations or more advanced neurophysiology, see chapters on membrane potentials and synaptic transmission.