Lecture day 11 chap 12/ 13 Nervous System & Course Overview

Course Logistics and Assessment Timeline

• 8-week accelerated term ⇒ each class ≈ one regular-semester week.
• Today = Class 11 of 16 (dated 14th, after Lab Test #2).
• Remaining schedule
  • Wed (2 days later): Brain anatomy & nerve identification lab.
  • 28th: Final LAB test – Nervous System (brain structures, spinal & cranial nerves). Instructor warns it is “very difficult”; begin studying now.
  • Final EXAM (lecture):
  • New material: Endocrine System.
  • Comprehensive component: Diseases, illnesses & disorders (content starts next week to give prep time).
• Course/Instructor survey link available on Canvas; please complete within ~2 weeks for constructive feedback.

Sensory–Motor Integration Example (Hand in Hot/Cold Water)

• Stimulus (extreme temperature) detected by cutaneous thermoreceptors in dermis.
• Sensory (afferent) signal – green in diagram – travels via peripheral nerve → dorsal root ganglion → dorsal horn → ascends spinal cord → brain for interpretation.
• Brain evaluates stimulus as “too hot/too cold” and sends motor (efferent) command – red pathway – down corticospinal tract, exiting ventral root to forearm/hand muscles → withdrawal.
• Demonstrates three fundamental functions: sensation, integration, response.

Excitable Cell (Plasma) Membranes

• Phospholipid bilayer = highly regulated barrier (aka plasma membrane).
• Resting ionic asymmetry:
  • Extracellular: high $Na^+$, low $K^+$.
  • Cytosol: high $K^+$, low $Na^+$.
• When specific ion channels open, ions flow down electro-chemical gradients, generating membrane potential changes.
• Voltage definition: $V = V<em>{\text{inside}} - V</em>{\text{outside}}$ (potential difference).

Ion Channel Types & Gating States

• Passive (leak) channels – always partially open; maintain resting potential.
• Voltage-gated – open/close in response to membrane charge.
• Ligand-gated – opened by specific chemical (e.g., acetylcholine on motor end-plate).
• Mechanically-gated – respond to stretch, pressure, or temperature-induced deformation.
• Each channel can cycle through: closed (ready) → open → inactivated.

Neuromuscular Junction (NMJ) Mechanism

• Arrival of neuronal action potential (AP) → $Ca^{2+}$ influx into presynaptic terminal.
• Vesicles exocytose acetylcholine (ACh) into synaptic cleft.
• ACh binds nicotinic receptors (ligand-gated cation channels) on sarcolemma → channel opens → $Na^+$ influx ± $Ca^{2+}$ co-entry, $K^+$ efflux.
• Resulting depolarization triggers AP across skeletal muscle, propagating into T-tubules & sarcoplasmic reticulum → $Ca^{2+}$ release → actin-myosin cross-bridge cycling.
• Acetylcholinesterase (AChE) rapidly degrades ACh to terminate signal (cleanup crew).

Action Potential (AP) Phases in Neuron/Muscle

• Spike amplitude ≈ $\Delta V \sim 160\,\text{mV}$ (from −70 to +90 mV in example).

AP Propagation Modes

• Continuous conduction – along unmyelinated axons; sequential opening of adjacent voltage-gated channels.
• Saltatory conduction – along myelinated axons; AP “jumps” node-to-node (Nodes of Ranvier) ⇒ faster, energy-efficient.

Synapse Classes

• Electrical (gap junctions/connexons)
  • Direct ion flow; virtually instantaneous.
  • Found in brain, eye retina, intercalated discs of heart.
• Chemical
  • Neurotransmitter crosses cleft; converts electrical → chemical → electrical signal.
  • Allows modulation (excitatory vs inhibitory, summation, plasticity).

Neurotransmitters Overview

• Excitatory (EPSP-producing): e.g., Acetylcholine (most), Glutamate, Norepinephrine.
• Inhibitory (IPSP-producing): e.g., GABA, Glycine.
• Major families
  • Biogenic amines: dopamine, serotonin, norepinephrine — mood, reward, attention; imbalances linked to Parkinson’s (dopamine ↓), depression (serotonin ↓), etc.
  • Amino acids: GABA (principal CNS inhibitory).
  • Neuropeptides: endorphins (exercise-induced analgesia; opioid-like potency).
  • Dissolved gases: $NO$ (vasodilation, respiratory drive), $CO$ (toxic in excess).
• First vs Second Messenger Systems
  • Direct (ionotropic): transmitter binds → channel opens immediately (e.g., ACh at NMJ).
  • Indirect (metabotropic): transmitter activates G-protein → produces $cAMP$ or other second messenger → modulates separate channel.

Brain – Gross Anatomy & Functional Areas

Cerebrum

• Two hemispheres separated by longitudinal fissure; interconnected via corpus callosum (white matter bundle).
• Surface features: gyri (ridges) & sulci (grooves).
• Lobes & key cortical zones
  • Frontal: prefrontal cortex (executive functions; matures ~25 yr), primary motor cortex, premotor area, Broca’s (speech production).
  • Parietal: primary somatosensory cortex + association areas (touch, pain, temp, proprioception).
  • Temporal: primary auditory cortex, olfactory cortex, Wernicke’s area (language comprehension).
  • Occipital: primary visual cortex + visual association.

Limbic Components (subcortical)

• Amygdala – emotional processing (fear, pleasure).
• Hippocampus – consolidation of long-term memory.

Diencephalon

• Thalamus – relay & initial processing of all sensory input except smell.
• Hypothalamus – homeostasis, autonomic control, emotion, memory; regulates pituitary hormones.
• Epithalamus (pineal gland) – secretes melatonin (sleep-wake cycles).

Brainstem

• Midbrain, Pons, Medulla Oblongata.
• Vital autonomic centers: cardiac, respiratory, vasomotor reflexes (especially medulla).
• Pons connects cerebellum ↔ cerebrum/brainstem.

Cerebellum

• Two hemispheres posterior/inferior to cerebrum; distinctive arbor vitae (white matter tree).
• Functions: posture, balance, fine-tunes conscious & subconscious motor activity.

Spinal Cord Structure

• Anterior (ventral) median fissure marks front; posterior (dorsal) sulcus at back.
• Gray matter “butterfly” horns
  • Dorsal horns: sensory synapses.
  • Ventral horns: somatic motor neuron cell bodies.
  • Lateral horns (thoracic/lumbar): autonomic (visceral motor).
• White matter columns = ascending (sensory) & descending (motor) myelinated tracts.

Meninges & Cerebrospinal Fluid (CSF)

• Protective connective tissue layers (outer → inner):
  1. Dura mater (tough, fused to skull; forms dural folds along fissures).
  2. Arachnoid mater; subarachnoid space filled with CSF.
  3. Pia mater (delicate, adheres to brain & cord surfaces).
• CSF produced by ependymal cells in choroid plexus; circulates through ventricles, subarachnoid space, central canal.
• Functions: cushions CNS, delivers nutrients, removes waste, maintains pH.
• Clinical: Lumbar puncture/epidural enters subarachnoid space below cord; noticeable “pop” felt when piercing dura.

Peripheral Nervous System (PNS) Organization

• Everything outside brain & spinal cord = PNS.
• Two major subcategories:
  • Sensory (afferent)
  • Motor (efferent) → Somatic & Autonomic divisions.

Nerve Connective-Tissue Sheaths

• $\text{Endoneurium}$ – surrounds individual axon (and myelin).
• $\text{Perineurium}$ – wraps a fascicle (bundle of axons).
• $\text{Epineurium}$ – encloses entire nerve (multiple fascicles + blood vessels).

Cranial Nerves (12 pairs)

Spinal Nerve Plexuses & Major Peripheral Nerves

• Cervical Plexus (C1-C5) – phrenic nerve (diaphragm).
• Brachial Plexus (C5-T1) – axillary, radial, median, ulnar nerves (upper limb control).
• Lumbar Plexus (L1-L4) – femoral, saphenous nerves (anterior thigh, medial leg).
• Sacral Plexus (L4-S4) – sciatic → tibial & common fibular nerves (posterior thigh, entire lower leg/foot).

Key Vocabulary & Concepts to Master

• Resting membrane potential, threshold, depolarization, repolarization, hyperpolarization.
• Saltatory vs continuous conduction.
• EPSP vs IPSP; temporal & spatial summation (mentioned indirectly as “summation”).
• Cholinergic vs adrenergic synapses.
• Gray vs white matter; myelin = oligodendrocytes (CNS) / Schwann cells (PNS).
• Homeostasis role of hypothalamus; endocrine link via pituitary (preview for next unit).

Study & Exam Strategy Tips (from instructor comments)

• Start memorizing cranial nerves, spinal plexuses, and brain regions now; they will appear on both lab & lecture tests.
• Re-draw the AP graph and label ion channel states – commonly tested.
• Utilize extra slide sets provided in LMS for clearer images (e.g., brain region diagrams).
• Condensed course pace means constant review; schedule daily study blocks.