The Nervous System – Lesson 8

1. Simple Reflex Arc

• A simple reflex arc consists of three neurons:
  • Sensory neuron: Receives stimuli from the external environment and transmits nerve impulses to the central nervous system.
  • Interneuron: Located in the spinal cord or brain, receives nerve impulses from the sensory neuron and transmits them to the motor neuron.
  • Motor neuron: Transmits nerve impulses from the central nervous system to the responding organ.

2. Cerebrospinal Fluid and its Compartments

• Cerebrospinal fluid (CSF) is a clear fluid that protects the brain and spinal cord from injury, provides nutrients to the nervous tissue, and removes waste products.
• CSF is produced by the choroid plexuses of the ventricles and circulates within the brain and spinal cord cavities:
  • Ventricles: Hollow cavities within the brain that contain CSF.
    • Lateral Ventricles (1, 2): Located in each cerebral hemisphere.
    • Third Ventricle: Located between the thalami.
    • Fourth Ventricle: Located behind the pons and in front of the cerebellum.
  • Subarachnoid Space: The space between the arachnoid mater and dura mater, containing CSF.
  • Cerebellomedullary Cistern: A small cavity located behind the pons, containing CSF.

3. Brain Regions and their Functions

Cerebrum

• Cerebral Cortex: The outermost layer of gray matter covering the cerebral hemispheres.
  • Precentral Gyrus: Motor cortex, responsible for voluntary movements.
  • Postcentral Gyrus: Sensory cortex, responsible for sensory perception.
  • Superior Temporal Gyrus: Auditory cortex.
  • Occipital Lobe: Visual cortex.
  • Middle Frontal Gyrus: Writing center.
  • Orbitofrontal Cortex: Olfactory cortex.
• White Matter: The layer of white matter beneath the cortex, consisting of the axons of neurons, connecting different brain regions.
• Basal Ganglia: Regions of gray matter within the white matter, involved in motor control, sensation, thought, and language.
  • Caudate Nucleus: Regulates muscle strength.
  • Putamen: Regulates muscle strength.
  • Thalamus: Controls vegetative functions, serves as the subcortical center for hearing and sight.
  • Mammillary Bodies: Regulates hormone production and electrolyte balance.
  • Internal Capsule: Transmits information from the cortex to the thalamus and hypothalamus.
  • Corpus Callosum: Transmits information between the left and right cerebral hemispheres.
  • Fornix: Transmits information from the front to the back.

Diencephalon

• Thalamus: The subcortical center for hearing and sight, controls vegetative functions.
  • Dorsal Thalamus: Contains structures involved in sensation and motor functions, including the corpus callosum, fornix, septum pellucidum, and lateral ventricles.
  • Ventral Thalamus: Contains structures involved in hormone regulation and vegetative functions, including the hypothalamus, pituitary gland, mammillary bodies, and oculomotor nerve (III).
  • Posterior Thalamus: Contains structures involved in vision and hearing, including the superior colliculus, inferior colliculus, medial geniculate body, and lateral geniculate body.
• Hypothalamus: Regulates hormone production and vegetative functions, including the hypothalamus, pituitary gland, mammillary bodies, and oculomotor nerve (III).
• Pituitary Gland: Regulates hormone production.
• Pineal Gland: Inhibits sex hormone production.

Hindbrain

• Midbrain: Contains nuclei that control eye movement and some reflexes like pupillary reflex.
• Pons: Relays information between brain regions, controls movement, balance, and respiration.
• Cerebellum: Coordinates movement, maintains balance.

4. Meninges

• Meninges are the membranes that envelop the brain and spinal cord, composed of three layers:
  • Dura Mater: The outermost layer, thick, non-elastic, protecting the brain and spinal cord from injury.
  • Arachnoid Mater: The middle layer, thin, elastic, containing cerebrospinal fluid.
  • Pia Mater: The innermost layer, thin, rich in blood vessels, supplying nutrients to the brain and spinal cord.

5. Cerebral Arteries

• Internal Carotid Arteries: Supply blood to the brain, face, and neck.
  • Anterior Cerebral Artery: Originates from the internal carotid artery, supplying blood to the medial surface of the cerebral hemispheres.
  • Middle Cerebral Artery: Originates from the internal carotid artery, supplying blood to the lateral surface of the cerebral hemispheres.
• Basilar Artery: Located at the base of the brain, connecting with the internal carotid artery and vertebral arteries, supplying blood to the posterior brain.
  • Posterior Cerebral Artery: Originates from the basilar artery, supplying blood to the occipital lobe, and inferior surface of the temporal lobe.
• Middle Meningeal Artery: Originates from the maxillary artery, supplying blood to the lateral surface of the cerebral hemispheres. When damaged, it can cause an epidural hematoma.
• Circle of Willis: A network of arteries surrounding the hypothalamus, ensuring continuous blood flow to the brain, preventing stroke.

6. Peripheral Nervous System

Cranial Nerves

• Olfactory Nerve (I): Sense of smell.
• Optic Nerve (II): Sense of sight.
• Oculomotor Nerve (III): Controls 5 extraocular muscles, constricts pupil.
• Trochlear Nerve (IV): Controls the superior oblique muscle.
• Trigeminal Nerve (V): Facial sensation, controls muscles of mastication.
• Abducent Nerve (VI): Controls the lateral rectus muscle.
• Facial Nerve (VII): Controls facial muscles, salivary glands (submandibular, sublingual), and lacrimal glands.
• Intermediate Nerve (VII’): Sense of taste on the anterior 2/3 of the tongue, controls parotid gland and lacrimal gland secretions.
• Vestibulocochlear Nerve (VIII): Sense of hearing and balance.
• Glossopharyngeal Nerve (IX): Sense of taste on the posterior 1/3 of the tongue, controls parotid gland and salivary glands (submandibular, sublingual).
• Vagus Nerve (X): Largest parasympathetic nerve, controlling most internal organs.
• Accessory Nerve (XI): Controls trapezius and sternocleidomastoid muscles.
• Hypoglossal Nerve (XII): Controls tongue muscles.

Spinal Nerves

• Control organs in the trunk and limbs.

7. Nerve Plexuses

• Nerve plexuses are complex networks formed by the combination of spinal nerves.
  • Cervical Plexus: Controls sensation and movement in the neck, shoulder, and upper limb.
  • Brachial Plexus: Controls sensation and movement in the upper limb.
  • Lumbosacral Plexus: Controls sensation and movement in the lower limb.
  • Visceral Plexus: Controls internal organs.

8. Definitions

• Reflex Arc: The pathway for the transmission of nerve impulses from a sensory organ to a responding organ.
• Optic Chiasm: The crossing point of the optic nerves.
• Circle of Willis: A network of arteries surrounding the hypothalamus, ensuring continuous blood flow to the brain, preventing stroke.
• Arachnoid Granulations: Small projections on the arachnoid mater, helping to absorb CSF.
• Interventricular Foramen: The opening connecting the third and fourth ventricles, allowing CSF to circulate.
• Cerebral Aqueduct: The canal that carries CSF from the third to the fourth ventricle.
• Cerebral Recesses: Small cavities within the brain, containing CSF.
• Tela Choroidea: A thin membrane covering the ventricles, containing blood vessels, producing CSF.
• Falx Cerebri: The dura mater fold separating the two cerebral hemispheres.
• Falx Cerebelli: The dura mater fold separating the two cerebellar hemispheres.

Conclusion

The nervous system is a complex system that ensures the body’s life functions, controls both conscious and involuntary activities, and helps humans adapt to the environment. Understanding the nervous system will enhance your knowledge of the human body and help you protect your own health.