Physiology of the Nervous System

Physiology of the Nervous System

# I. Anatomical Division of the Nervous System

1. Central Nervous System (Neuron cell body):

• Brain

• Spinal cord

2. Peripheral Nervous System (Axons):

• 12 pairs of cranial nerves: connect the brain to sensory and motor organs in the head and neck.

• 31 pairs of spinal nerves: connect the spinal cord to sensory and motor organs in the trunk and limbs.

# II. Brain

1. Forebrain:

• Cerebrum:

• Cerebral cortex: the control center for higher mental functions such as thinking, language, memory, etc.

• Basal ganglia: involved in the control of automatic movements and the coordination of complex movements.

• Diencephalon:

• Thalamus: sensory center, receives information from sensory organs and relays it to the cerebral cortex.

• Hypothalamus: control center for physiological activities such as thermoregulation, sleep, hunger, thirst, etc.

2. Midbrain:

• Mesencephalon: involved in the control of visual and auditory reflexes and coordinated movements.

3. Hindbrain:

• Rhombencephalon:

• Surrounds the fourth ventricle, including:

• Anteriorly: Medulla oblongata, Pons.

• Posteriorly: Cerebellum.

# III. Brainstem

• Medulla oblongata: control center for basic physiological activities such as breathing, circulation, etc.

• Pons: involved in the control of coordinated movements, sleep, and activities related to hearing.

• Mesencephalon: already mentioned above.

# IV. Basal Ganglia (5)

• Caudate nucleus: controls complex and refined movements.

• Globus pallidus: participates in learning, memory, and movement control.

• Putamen: involved in movement control and sensory-related activities.

• Substantia nigra: participates in movement control and cognitive activities.

• Hypothalamus: already mentioned above.

# V. Diencephalon

• Part of the brain buried between the two cerebral hemispheres.

• Includes: Thalamus and hypothalamus, separated by the hypothalamic sulcus.

• Surrounding the third ventricle.

# VI. Spinal Cord Grey Matter (3)

• Anterior horn: motor control center.

• Posterior horn: center for receiving sensory information from the body.

• Lateral horn: center for controlling autonomic nervous system activities.

# VII. Physiological Division of the Nervous System

1. Motor nervous system: controls the motor activities of the body.

2. Sensory nervous system: receives information from the environment and the body, relays it to the brain for processing.

3. Autonomic nervous system: controls involuntary activities of the body such as breathing, circulation, digestion, etc.

# VIII. Sensory Nervous System

1. Transmits information in the form of action potentials.

2. 5 Sensory receptors:

• Mechanoreceptors: sense mechanical pressure.

• Thermoreceptors: sense temperature changes (hot and cold).

• Nociceptors: sense tissue damage.

• Photoreceptors: sense light entering the retina.

• Chemoreceptors: sense taste, smell, arterial oxygen and CO2 levels, osmolarity of body fluids.

3. General characteristics of receptors:

• Cause changes in the opening and closing state of ion channels when stimulated, leading to changes in cell membrane potential (receptor potential).

4. When receptors are stimulated by stimuli:

• They generate a stimulus potential due to changes in membrane permeability to ions.

5. Components of the sensory nervous system:

• Receptive apparatus: Receptors located at the end of sensory nerve fibers (skin, mucous membranes or special sensory areas).

• Afferent nerve fiber: transmits signals to the center.

• Neuron 1: Sensory neuron, located in the sensory ganglion, transmits signals to the center.

• Neuron 2: Located in the posterior horn of the spinal cord or the brainstem, receives and processes information from neuron 1.

• Neuron 3: Located in the thalamus, is the central processing center before transmitting to the cerebral cortex.

• Neuron 4:

• Cerebellum: unconscious sensation.

• Cerebral cortex: conscious sensation.

6. Types of sensations:

• Somatic sensations:

• Superficial sensation: light touch, temperature, pain.

• Deep sensation: deep touch, two-point discrimination, vibration.

• Special sensory sensations: vision, hearing, smell, taste.

7. Receptors in sensory pathways:

• Touch: Merkel’s discs, Meissner’s corpuscles, Ruffini corpuscles, Pacinian corpuscles.

• Temperature:

• Hot: senses 30-45 degrees Celsius.

• Cold: senses 10-40 degrees Celsius.

• Adapts in the range of 20-40 degrees Celsius.

• Pain:

• Heat pain occurs at temperatures below 15 degrees Celsius and above 45 degrees Celsius.

• Two types of pain: cutaneous pain and visceral pain.

• Chemical substances that stimulate pain receptors: bradykinin, serotonin, histamine, K+ ions, acid, acetylcholine, proteolytic enzymes.

8. Deep sensation pathways:

• Unconscious: controlled by the cerebellum.

• Muscle tension.

• Balance, control automatic movements.

• Conscious: controlled by the cerebral cortex.

• Awareness of the position of each part during movement.

• Feeling of discrimination when touching.

9. Sensory pathways:

• Light touch, pain, temperature: Spinothalamic tract.

• Deep touch, vibration: Gracile and cuneate fasciculi (lateral column).

• Proprioception (muscle, tendon, joint): Spinothalamic tract.

10. Somatosensory area (parietal lobe):

• Sensation: touch, hot, cold.

• Somatosensory areas I, II: 1B, 2B, 3B.

• Associative sensory areas: 5B, 7B.

# IX. Vision

1. Physiology:

• Converging lens system: focuses light on the retina.

• Light-sensing system: retina contains photoreceptor cells.

2. Visual pathways:

• Order:

• Optic nerve.

• Optic chiasm.

• Optic tract.

• Lateral geniculate nucleus in the thalamus.

• Geniculocalcarine tract.

• Occipital lobe.

3. Visual area on the cerebral cortex:

• Occipital lobe:

• Normal vision: 17B.

• Visual perception: 18B, 19B.

# X. Hearing

1. Auditory pathways:

• Cochlea: receives sound.

• Cochlear nucleus in the brainstem: receives and processes sound signals.

• Medial geniculate nucleus in the thalamus: receives and relays sound signals to the cerebral cortex.

• Temporal lobe: auditory processing center.

2. Auditory area:

• Temporal lobe:

• Normal hearing: 41B, 42B.

• Auditory perception: 22B.

# XI. Taste

1. Taste pathways:

• Anterior 2/3 of the tongue: Trigeminal nerve (V), Facial nerve (VII).

• Posterior 1/4 of the tongue: Glossopharyngeal nerve (IX).

• Base of the tongue: Vagus nerve (X).

• Travels from:

• Nucleus of the solitary tract.

• Thalamus.

• Postcentral gyrus and insular cortex.

2. Taste area:

• Parietal lobe:

• Near the tongue sensory area of the somatosensory area I.

• Senses taste, texture, temperature, pain, smell.

# XII. Smell

1. Olfactory pathways:

• Olfactory nerve -> olfactory bulb.

• Internal olfactory area: anterior hypothalamic nucleus (licking, salivation…).

• External olfactory area: prepiriform cortex, piriform cortex, amygdaloid nucleus -> limbic system (vomiting).

• Thalamus -> cerebral cortex: smell awareness and discrimination.

2. Olfactory area:

• Internal olfactory area: anterior hypothalamic nucleus (licking, salivation…).

• External olfactory area: prepiriform cortex, piriform cortex, amygdaloid nucleus -> limbic system (vomiting).

• Olfactory area: olfactory cortex in the frontal lobe.

# XIII. Electroencephalography (EEG)

• Recording: potentials generated by neurons through the surface of the skull.

• Nature: membrane potential of neuronal cells.

• Two responsible ions: Na+, K+.

# XIV. EEG Diagnosis

• Sleep disorders (PSG).

• Coma.

• Brain development.

• Brain tumor (CT and MRI are more advantageous).

• Epilepsy (most important).

# XV. Sleep

• 2 stages:

• Rapid eye movement (REM) sleep: when dreams occur, the brain inhibits the skeletal muscle system.

• Non-rapid eye movement (non-REM) sleep: 4 stages: 1, 2, 3, 4.

• 3, 4: slow-wave sleep.

# XVI. Motor Nervous System

1. Components:

• Central nervous system: anterior horn of the spinal cord and the brain.

• Motor nerve: centrifugal.

• Neuromuscular junction.

• Skeletal muscle.

2. When a patient has symptoms of weakness or paralysis, the problem can occur in any of the 4 parts above.

3. Central nervous system components involved in motor control (5):

• Anterior horn of the spinal cord.

• Motor areas of the cerebral cortex.

• Brainstem.

• Cerebellum.

• Basal ganglia.

4. Motor organization of the spinal cord:

• Motor neurons: located in the anterior horn of the spinal cord, form nerve fibers that follow the anterior root to control muscles.

• Interneurons: receive, process, and transmit sensory signals or signals from the brain to motor neurons. They perform an integrative function.

5. 5 components of a spinal reflex arc:

• Receptive apparatus: skin, tendon, mucous membranes.

• Afferent fiber: sensory fiber entering the spinal cord through the posterior root.

• Spinal cord grey matter (central nervous system).

• Efferent fiber: motor fiber exiting from the anterior root of the spinal cord.

• Effector organ: muscle, gland.

6. 2 types of spinal reflex arcs:

• Monosynaptic spinal reflex arc (2 neurons): short latency (0.5-0.9 ms), receptors in muscle spindles or Golgi tendon organs, stretch reflex.

• Polysynaptic spinal reflex arc (3 or more neurons): long latency (3-5 ms), receptors in skin, extrafusal muscles, flexor reflex, crossed extensor reflex.

7. Stretch reflex:

• Muscle, joint, and ligament fibers have sensory receptors that send postural information and instantaneous muscle contraction activity to the spinal cord.

• Muscle spindles are located within muscle fibers.

• Golgi tendon organs are located in the junction of muscle fibers and tendons.

8. Operation of the stretch reflex:

• Extrafusal muscle fibers are stretched -> muscle spindles are stretched accordingly -> signals are transmitted to the spinal cord -> signals are transmitted to alpha neurons (anterior horn) -> causes a reflex contraction of that muscle.

• Extrafusal muscle fibers are shortened -> the opposite phenomenon occurs -> alpha neurons are not stimulated -> muscle relaxes.

9. The stretch reflex helps to:

• Stabilize muscle length.

• Prevent excessive stretching.

10. Flexor reflex, crossed extensor reflex:

• Noxious heat stimulation -> signals travel along afferent fibers to the posterior horn of the spinal cord -> interneurons -> signals travel along efferent fibers to muscles -> reflex to avoid noxious heat stimuli.

11. Motor cortex:

• Located in the frontal lobe, controls muscles on the opposite side, the more refined the movement, the larger the area.

• Primary motor area: 4B.

• Premotor area: 6B.

• Supplementary motor area: 8B.

12. Primary motor area:

• 4B: origin of the corticospinal and corticobulbar tracts, voluntary movement of the opposite half of the body, the more sophisticated the movement, the wider the area (face and hand), stimulation causes simple movements.

13. Premotor area:

• 6B: voluntary movement according to afferent signals (visual, auditory, somatic sensations), coordinated complex movements.

14. Supplementary motor area:

• 8B: stimulation must be stronger, bilateral muscle contraction, movement: posture, shaping different parts of the body, for example: head-eye position.

15. Supplementary motor area and premotor area:

• Program for refined and skillful movement.

• Arranges movement in terms of time, uses motor memory.

16. Motor pathways:

• Pathways from the brain down to the spinal cord.

• Pyramidal system (cortical, spinal).

• Extrapyramidal system.

17. Pyramidal system:

• Lateral corticospinal tract (90%): crosses at the brainstem.

• Anterior corticospinal tract (10%): crosses at the corresponding segment of the spinal cord.

• Voluntary motor impulses.

• Contralateral paralysis.

18. Extrapyramidal system:

• Motor pathways that descend from the brain that are not part of the corticospinal tract (originating from the basal ganglia, brainstem).

• Rubrospinal tract, reticulospinal tract, tectospinal tract, vestibulospinal tract, pontine tract.

• Involuntary movement.

• Regulates muscle tone.

• Controls balance reflexes, posture, and adjustment of posture.

# XVII. Autonomic Nervous System

1. Overview:

• Distributed to most internal organs.

• Regulates involuntary activities, internal environment of the body.

• 3 parts: sympathetic, parasympathetic, enteric.

2. Sympathetic nervous system:

• Cooperates in the body’s response to stress.

3. Parasympathetic nervous system:

• Regulates homeostasis.

4. Enteric nervous system:

• Regulates gastrointestinal activities.

5. Autonomic reflex arc – sympathetic:

• Preganglionic.

• Postganglionic.

• 8% of spinal nerves (arrector pili muscles, blood vessels of skin and bones, sweat glands).

6. Autonomic reflex arc – parasympathetic:

• Preganglionic.

• Postganglionic (target organ).

7. Neurotransmitter in preganglionic fibers:

• Acetylcholine.

• Receptor: nicotinic (similar to neuromuscular junction).

8. Neurotransmitter in postganglionic parasympathetic fibers:

• Acetylcholine.

• Muscarinic receptor.

9. Neurotransmitter in postganglionic sympathetic fibers:

• Norepinephrine.

• Alpha 1-2, Beta 1-3 receptors.

• Except for sweat glands: acetylcholine.

10. Alpha receptors:

• Smooth muscle of peripheral blood vessels.

• Blood vessels of internal organs.

11. Beta 1 receptors:

• Sinoatrial node.

• Atrioventricular node.

• Purkinje system.

12. Beta 2 receptors:

• Coronary arteries.

• Skeletal muscle.

• Bronchioles.

• Smooth muscle of the gastrointestinal tract, gallbladder, uterus, bladder.

• Liver.

13. Muscarinic receptors:

• Arrector pili muscles.

• Blood vessels of skeletal muscle.

• Sweat glands.

14. Target organs of the autonomic nervous system:

• Termination: smooth muscle, cardiac muscle.

• Structure: varicosities (containing neurotransmitter storage granules), grooves.

15. Sympathetic stimulation:

• Exercise, fear.

16. Parasympathetic stimulation:

• After a full meal, reading, relaxation.

17. Functions of the sympathetic nervous system:

• Fight-or-flight response: widespread activation of many target cells.

18. Circulation of the sympathetic nervous system:

• Increased contractility, increased cardiac output, increased blood pressure.

19. Respiration of the sympathetic nervous system:

• Increased respiratory rate, bronchodilation.

20. Liver of the sympathetic nervous system:

• Glycogenolysis, increased blood glucose.

21. Adipose tissue of the sympathetic nervous system:

• Lipidolysis, increased blood free fatty acids.

22. Body temperature of the sympathetic nervous system:

• Sweating: acetylcholine.

• Erector pili muscle contraction and vasoconstriction of skin (norepinephrine).

23. Digestion of the sympathetic nervous system:

• Decreased digestive activity.

24. Functions of the parasympathetic nervous system:

• Constriction of the ciliary muscle (near vision accommodation).

• Increased secretion of lacrimal, salivary, gastric, pancreatic, and bile glands.

• Increased motility and absorption of the gastrointestinal tract.

• Increased contraction of the bladder wall, relaxation of the sphincter.

• Decreased heart rate, decreased heart contractility.

• Vasodilation of blood vessels to the skin and internal organs.

• Slight bronchoconstriction.

• No increase in basal metabolism.