MDCAT Biology: Nervous and Chemical Coordination

Nervous Coordination Introduction

• Nervous coordination involves specialized cells called neurons linked directly or through the Central Nervous System (CNS) to form a network connecting receptors and effectors.
• Receptors: Cells or organs that receive stimuli from the internal or external environment.
• Effectors: Structures that carry out actions or responses based on nervous signals.
• Neurons generate and conduct impulses that travel across synapses to pass signals from receptors to effectors.
• The three elements of nervous system coordination are:
  1. Receptors
  2. Neurons
  3. Effectors

Receptors and Modalities of Sensation

• Receptors can be a single cell, a neuron ending, or a complex receptor organ. They detect environmental changes and are classified as:
  • Chemoreceptors: Detect smells (nose), taste (tongue), blood $CO_2$, oxygen, blood glucose, amino acids, and fatty acids (receptors in the hypothalamus).
  • Mechanoreceptors: Detect stimuli of touch (free nerve endings, expanded tip endings, stray edgiest), pressure, hearing, and equilibrium.
  • Photoreceptors (Electromagnetic receptors): Respond to light stimuli, such as rods and cones in the eyes.
  • Thermoreceptors: Respond to cold and warmth.
  • Nociceptors: Undifferentiated endings that produce the sensation of pain.
• Modality of Sensation: Each principal sensation, such as pain, touch, sight, or sound, is considered a modality.

Sensory Receptors of the Skin

• The skin detects at least five senses: touch, pressure, heat, cold, and pain.
• These sensations are detected by three types of modified sensory neurons (naked nerve endings or specialized cellular capsules).
  1. Naked or Free Nerve Endings: Function as touch and pain receptors (e.g., at the base of hairs). Also produce sensations of itching and tickling.
  2. Meissner's Corpuscles: Encapsulated neuron endings that function as touch receptors. Located in papillae that extend into the ridges of fingertips; they consist of spiral, twisted endings terminating in a knob.
  3. Pacinian Corpuscles: Encapsulated endings located deep in the body to receive deep pressure stimuli. Receptors in limbs detect vibration, and those in joints of terrestrial vertebrates detect ground vibrations.
• Quantity and Distribution:
  • Pain receptors are approximately $27$ times more abundant than cold receptors.
  • Cold receptors are approximately $10$ times more abundant than heat/temperature receptors.
  • Receptors are not distributed evenly (e.g., touch receptors are more numerous in fingertips than in the back).

Neurons and Neuroglia

• Neurons are the chief structural and functional units of the nervous system.
• Neuroglia (Glial cells): Provide nutrition and protection to neurons via the myelin sheath. They make up half of the nervous system in higher animals and humans.
• Functional Types of Mammalian Neurons:
  1. Sensory Neurons
  2. Associative (Intermediate/Relay) Neurons
  3. Motor Neurons
• Structure of a Neuron:
  • Cell Body (Soma): Contains the nucleus and organelles. It is the nutritional center concerned with biosynthesis for growth/maintenance. A mature neuron cannot divide.
  • Nissl's Granules: Groups of ribosomes associated with rough Endoplasmic Reticulum (E.R.) and Golgi apparatus within the cell body.
  • Dendrites/Dendrons: Protoplasmic processes that carry impulses towards the cell body. Large single fibers are called dendrons; smaller fibers are dendrites.
  • Axons: Processes that conduct impulses away from the cell body, sometimes exceeding $1\,m$ in length. Axoplasm contains microtubules, neurofibrils, rough E.R., and mitochondria.

Effectors and the Reflex Arc

• Effectors respond to motor neuron stimulation. Principal effectors include muscles (respond by contracting) and glands (respond by secreting).
• Reflex Action: An involuntary type of action.
• Reflex Arc: The pathway of an impulse during reflex action. The stimulus flow is: Receptors $\rightarrow$ Sensory Neuron $\rightarrow$ Associative (Relay) Neuron $\rightarrow$ Motor Neuron $\rightarrow$ Effectors.
• Reflexes can be categorized as monosynaptic or polysynaptic.

Nerve Impulse and Membrane Potential

• Nerve Impulse: A wave of electrochemical changes traveling along a neuron involving chemical reactions and ion movement across the plasma membrane.
• Electrical Potential: Measure of capacity to do electrical work (stored energy from charge separation).
• Membrane Potential: The electrical potential existing across a cell membrane.
• Resting Membrane Potential: The net difference in charge of a non-conducting neuron, where the outside is more positive than the inside. The potential is approximately $-70\,mV$.
• Factors of Resting Potential:
  • $Na^+$ concentration is $10$ times higher outside than inside.
  • $K^+$ concentration is $20$ times higher inside than outside due to the $Na^+ / K^+$ pumping system.
  • Large negative organic ions (proteins, organic acids) are significantly higher inside.
• Active Membrane Potential: Triggered by a threshold stimulus, the resting potential is replaced by an active potential of $0.05\,V$ ($-50\,mV$ in transcript context). $Na^+$ rushes in, making the inner surface more positive for about $1\,ms$.
• Restoration: The resting potential is restored by $K^+$ moving out after the impulse passes.
• Saltatory Impulse: In myelinated neurons, the impulse jumps between nodes of Ranvier.
• Speed of Impulse: Normal human speed is $100\,m/s$; in myelinated neurons, maximum speed is $120\,m/s$.

Synapse and Neurotransmitters

• Synapse: Microscopic gaps between the axon endings of one neuron and the dendrites of the next. There is no cytoplasmic connection.
• Synaptic Transmission: Messages move across the cleft via chemical messengers. When an impulse reaches a synaptic knob, synaptic vesicles fuse with the presynaptic membrane to release neurotransmitters into the synaptic cleft.
• Neurotransmitters: Chemicals released at axon endings. Examples include:
  • Acetylcholine (main transmitter outside the CNS).
  • Adrenaline.
  • Nor-epinephrine.
  • Serotonin.
  • Dopamine.

The Human Central Nervous System (CNS)

• Protection of the CNS:
  1. Cranium: Skull part protecting the brain.
  2. Vertebral Column: Neural arches protect the spinal cord.
  3. Meninges: Triple layer of protective membranes.
  4. Cerebrospinal Fluid (CSF): Similar to blood plasma, found between meninges, in brain ventricles, and the central canal of the spinal cord. It cushions against jolts.
• The Brain:
  • Weighs about $1.5\,kg$ and is $85\%$ water.
  • Forebrain: Includes the Thalamus, Limbic System, and Cerebrum.
  • Thalamus: Relay center for sensory information (auditory, visual, skin) to the cerebrum and limbic system.
  • Limbic System: Center for unconscious emotional behaviors (love, hate, hunger, fear, rage, thirst, etc.).
    • Hypothalamus: Coordinates nervous and endocrine systems; regulates body temperature, water/salt balance, hunger, and thirst. Controls the pituitary gland.
    • Amygdala: Produces sensations of pleasure, punishment, fear, and rage.
    • Hippocampus: Essential for learning and long-term memory formation.
  • Cerebrum: Largest brain part, divided into two cerebral hemispheres connected by the corpus callosum. The outer cerebral cortex has convolutions to increase surface area.
    • Functions: Memory, intelligence, reasoning, thinking, interpretation of sensory impressions (light, sound, touch), and voluntary muscle control.
  • Midbrain: Reduced in humans. Contains the reticular formation (screening relay center connecting hindbrain/forebrain) and auditory relay centers. Controls reflex eye movements.
  • Hindbrain: Includes the Medulla (breathing, heart rate, BP), Pons (sleep/wake cycles, breathing patterns), and Cerebellum (coordinates body movements and position; highly developed in birds).
• Spinal Cord: Neural cylinder in the vertebral column. Cross-section reveals:
  • Inner Portion: Butterfly-shaped grey matter (cell bodies, non-myelinated fibers) with a central canal.
  • Outer Portion: White matter (myelinated nerve fibers).

Peripheral Nervous System (PNS)

• Consists of sensory and motor neurons which may form ganglia (clusters of cell bodies) and nerves (bundles of axons/dendrites).
• Nerve Types (Link-based):
  • Cranial Nerves: $12$ pairs arising from the brain (sensory, motor, or mixed).
  • Spinal Nerves: $31$ pairs arising from the spinal cord (all are mixed).
• Motor Neuron Systems:
  1. Somatic Nervous System: Controls voluntary movements of skeletal muscles.
  2. Autonomic Nervous System: Controls involuntary responses (organs, glands, smooth muscle).
     • Sympathetic System: "Fight or flight" during emergencies; accelerates heart rate, dilates pupils, inhibits digestion. Ganglia arise from middle spinal cord.
     • Parasympathetic System: Associated with relaxed states; contracts pupils, promotes digestion, retards heart rate. Includes the vagus nerve and nerves from the bottom spinal cord.

Nervous Disorders

• Parkinson's Disease (Paralysis Agitans):
  • Symptoms: Tremors, reduced motor power, rigidity, poor balance, speech problems. Mental faculties unaffected.
  • Cause: Death of cells in basal ganglia leading to lack of dopamine. Onset typically in $50s$ or $60s$.
  • Treatment: Levodopa (L-dopa), surgery on globus pallidus or thalamus, and Glial cell-line derived neurotrophic factor (G DNF).
• Epilepsy:
  • Symptoms: Convulsive motor/sensory symptoms and excessive electric discharge in grey matter diagnosed via Electroencephalography (EEG).
  • Cause: Usually starts before age $30$; often triggered by emotional disturbance.
  • Treatment: Anticonvulsant drugs; alcohol must be avoided.
• Alzheimer's Disease:
  • Described by Alois Alzheimer in $1907$.
  • Symptoms: Declined brain function and dementia (memory loss).
  • Causes: Genetic predisposition and potential contribution from high levels of aluminum.

Chemical Coordination and Hormones

• Chemical coordination is performed by the endocrine system via discrete groups of ductless glands.
• Hormones: Organic compounds transported by blood to target tissues. They regulate reactions (stimulate or inhibit) but do not initiate them.
• Chemical Types of Hormones:
  1. Proteins: Insulin, Glucagon.
  2. Amino acids & derivatives: Thyroxin, Epinephrine, Norepinephrine.
  3. Polypeptides: Vasopressin (ADH), Oxytocin.
  4. Steroids: Estrogens, Testosterone, Cortisone.

The Pituitary Gland (Master Gland)

• Connected to the brain by the infundibulum. Weighs $0.5\,g$.
• Anterior Lobe:
  • Somatotrophin (STH): Controls growth and protein synthesis. Excess in youth causes Gigantism; excess later causes Acromegaly. Deficiency causes Dwarfism.
  • Thyroid Stimulating Hormone (TSH): Stimulates thyroid secretary activity.
  • Adrenocorticotrophic Hormone (ACTH): Regulates adrenal cortex; release is controlled by steroids and stress.
  • Gonadotrophic Hormones:
    • FSH: Stimulates follicle development/oestrogen in females; germinal epithelium/sperm in males.
    • LH (ICSH in males): Works with FSH for ovulation and corpus luteum maintenance; stimulates testosterone production in males.
    • Prolactin: Stimulates milk production.
• Median Lobe:
  • Melanophore Stimulating Hormone (MSH): Darkens skin via melanin. High in pregnancy and Addison's disease.
• Posterior Lobe (Stores hormones from the hypothalamus):
  • Antidiuretic Hormone (ADH/Vasopressin): Increases water reabsorption in kidneys. Deficiency causes Diabetes insipidus (large urine volume, thirst).
  • Oxytocin: Stimulates uterine contraction during childbirth and milk ejection from mammary glands.

Thyroid, Parathyroid, and Pancreas

• Thyroid Gland (below larynx): Produces Thyroxin ($T4$), Tri-iodothyronine ($T3$), and Calcitonin.
  • $T3$ and $T4$: Increase basal metabolic rate and work with somatotropin for growth. In amphibians, they control metamorphosis.
  • Grave's Disease (Over-secretion): Exophthalmic goiter, increased metabolism, heart failure risk.
  • Cretinism (Congenital deficiency): Failure to develop normally, mental retardation, sexual immaturity.
  • Myxoedema/Goiter: Caused by iodine deficiency or late-life under-secretion. Swelling of neck, fat accumulation, reduced metabolism.
  • Calcitonin: Lowers blood calcium levels by preventing removal from bone.
• Parathyroid Glands (embedded in thyroid): Produce Parathormone.
  • Function: Raises blood calcium levels. Under-secretion leads to tetany; over-secretion leads to bone demineralization (rickets) and kidney stones.
• Islets of Langerhans (Pancreas):
  • $\beta$-cells: Secrete Insulin to lower blood glucose (glycogen synthesis, glucose utilization).
  • Diabetes Mellitus: Insulin deficiency leads to high blood sugar, sugar in urine, and dehydration.
  • Hypoglycaemia: Excess insulin causes blood sugar to fall too low.
  • $\alpha$-cells: Secrete Glucagon to raise blood sugar (glycogen breakdown in liver).

Adrenal Glands, Gut, and Gonads

• Adrenal Medulla: Produces Adrenaline (dilates vessels, increases heart output) and Noradrenaline (constricts vessels in gut). Secreted during stress.
• Adrenal Cortex: Produces Cortisol (increases glucose from protein), Corticosterone (mineral and glucose regulation), and Aldosterone (conserves $Na^+$ in kidneys).
  • Addison's Disease: Destruction of cortex leading to salt loss and metabolic disturbance.
  • Cushing's Disease: Over-secretion causing protein breakdown and muscular weakness.
• Gut Hormones:
  • Gastrin: Produced by stomach mucosa to stimulate gastric juice.
  • Secretin: Produced by duodenum to release pancreatic juice and bile.
• Gonads:
  • Ovaries: Oestrogens (secondary sexual characters, uterine thickening) and Progesterone (inhibits FSH, maintain pregnancy, suppresses ovulation).
  • Testes: Interstitial cells produce Testosterone and $17\,\beta$-Hydroxytestosterone (initiates sex organ development in fetus, male secondary characters, sex drive).

Feedback Mechanism

• A controlling mechanism controlled by the products of the reactions it regulates.
• Example: Thyroid Function.
  1. Low temperature/stress stimulates the hypothalamus.
  2. Hypothalamus releases Thyrophin Releasing Factor.
  3. Anterior Pituitary releases TSH.
  4. Thyroid releases Thyroxine.
  5. Higher Thyroxine and body temperature then inhibit further release of hormone and TSH.