Unit 1.1 – Introduction to the Nervous System

Overview and Comparison with the Endocrine System

The nervous system (NS) is one of the main communication organ systems in the body, responsible for rapid, precise control of bodily functions. It is contrasted with the endocrine system, which communicates via hormones and is generally slower and longer-lasting in its effects.

• Nervous System: Fast, uses electrical and chemical signals, effects are immediate and short-lived.

• Endocrine System: Slow, uses hormones, effects are prolonged.

Major Subdivisions of the Nervous System

• Central Nervous System (CNS): Brain and spinal cord; integrates information and coordinates activity.

• Peripheral Nervous System (PNS): All nerves outside the CNS; includes sensory and motor divisions.

• Functional Divisions of PNS:

  • Sensory (Afferent) Division: Transmits sensory information to CNS.

  • Motor (Efferent) Division: Transmits commands from CNS to effectors; subdivided into Somatic (voluntary control of skeletal muscles) and Autonomic (involuntary control of smooth muscle, cardiac muscle, glands).

• Enteric Nervous System: Controls gastrointestinal function; will be covered in more detail in advanced courses.

Basic Functions of the CNS

• Contains control centers for spinal reflexes.

• Processes sensory information and sends it to specific brain centers.

• Transmits motor commands from brain centers to PNS motor neurons.

Homeostasis and Negative Feedback

• Homeostasis is maintained via negative feedback mechanisms involving sensors, afferent neurons, control centers, and efferent neurons.

Unit 1.2 – Nervous Tissue & Structure of a Neuron

Cell Types and Functions

Nervous tissue consists of neurons and neuroglial (glial) cells.

• Neurons: Specialized for communication; transmit electrical signals.

• Glial Cells: Support, protect, and nourish neurons.

Neuron Structure

• Dendrites: Receive signals.

• Soma (Cell Body): Contains nucleus and organelles.

• Axon: Conducts impulses away from soma.

• Axon Hillock: Site where action potential begins.

• Axon Terminals: Store and release neurotransmitters.

Signal transmission is unidirectional: dendrites/soma → axon → axon terminal.

Neuron Classification

• Unipolar: Sensory neurons in PNS.

• Bipolar: Special senses (e.g., retina).

• Multipolar: Most common; motor neurons and interneurons.

Chemical Synapses

• Neuro-neuronal synapse: Between two neurons.

• Neuromuscular synapse: Between neuron and muscle cell.

• Neuroglandular synapse: Between neuron and gland cell.

Glial Cells

• CNS: Microglia (immune), Ependymal cells (CSF production), Oligodendrocytes (myelination), Astrocytes (support, blood-brain barrier).

• PNS: Schwann cells (myelination), Satellite cells (support).

Unit 1.3 – Resting Membrane Potential and Action Potential

Electrical Potential in Neurons

Neurons maintain an electrical potential across their plasma membrane, essential for signal transmission.

• Sodium/Potassium Pump: Maintains ion gradients by pumping 3 Na+ out and 2 K+ in per ATP molecule.

• Resting Membrane Potential: Typically -70 mV, due to ion distribution and selective permeability.

Equation for resting membrane potential (Nernst equation):

Molecule Transport Across Membrane

• Solute Permeability: Depends on size, charge, and lipid solubility.

• Primary Active Transport: Uses ATP (e.g., Na/K pump).

• Simple Diffusion: Movement down concentration gradient.

• Facilitated Diffusion: Uses carrier proteins.

• Exocytosis/Endocytosis: Bulk transport mechanisms.

Action Potential (AP)

• Purpose: Rapid transmission of signals.

• Phases:

  • Threshold potential reached

  • Depolarization (Na+ influx)

  • Repolarization (K+ efflux)

  • Undershoot (hyperpolarization)

  • Restoration of resting potential

• Voltage-Gated Channels: Na+ channels set threshold; present only on axonal membrane (axon hillock to terminal).

• Propagation: Depolarization triggers AP in adjacent sections (domino effect).

• Refractory Periods: Ensure unidirectional propagation.

  • Absolute: No new AP possible.

  • Relative: AP possible with stronger stimulus.

• All-or-Nothing Principle: AP occurs fully or not at all.

Unit 1.4 – Synapses and Graded Potentials

Stimulus Coding

• Intensity of stimulus coded by frequency of APs (high frequency = strong stimulus).

Myelination and Conduction

• Non-myelinated Axons: Slow, continuous conduction.

• CNS Myelinated Axons: Oligodendrocytes, saltatory conduction (nodes of Ranvier), fast.

• PNS Myelinated Axons: Schwann cells, saltatory conduction, fast.

• White Matter: Myelinated axons; Grey Matter: Cell bodies, unmyelinated axons.

Axon Classification

Clinical Relevance

• Multiple Sclerosis (MS): Demyelination in CNS; symptoms include muscle weakness, vision problems.

• ALS: Affects motor neurons in PNS; symptoms include muscle atrophy, paralysis.

Synapse Structure and Function

• Neuro-neuronal synapse: Contains neurotransmitter vesicles, ligand-gated channels.

• Presynaptic: Neuron sending signal; Postsynaptic: Neuron receiving signal.

• Sequence of Events: AP arrives → Ca2+ channels open → NT vesicles exocytose → NT diffuses → binds ligand-gated channels.

• NT Removal: Diffusion, uptake, enzymatic degradation.

• Reuptake Inhibitors: Block NT reuptake; e.g., SSRIs for depression.

Graded Potentials

• Depolarizing: Na+ channels (excitatory).

• Hyperpolarizing: K+ or Cl- channels (inhibitory).

• Effect depends on channel type, NT amount, channel density, synapse distance from axon hillock.

Unit 1.5 – Cerebrospinal Fluid (CSF), Meninges, and Brain Ventricles

CSF Composition and Function

• Clear fluid; cushions CNS, removes waste, provides nutrients.

Meninges and Associated Structures

• Dura Mater: Outer and inner layers.

• Arachnoid Mater: Middle layer; contains villi for CSF reabsorption.

• Pia Mater: Inner layer; adheres to CNS surface.

• Spaces: Epidural, subarachnoid (contains CSF).

CSF Circulation Pathway

• Produced in choroid plexuses (ventricles) → lateral ventricles → third ventricle → cerebral aqueduct → fourth ventricle → subarachnoid space → reabsorbed via arachnoid villi into superior sagittal sinus.

Brain Ventricles and Channels

• Lateral, third, fourth ventricles; interventricular foramen; cerebral aqueduct.

CSF Sampling Sites

• Usually lumbar region; avoids injury to spinal cord.

Unit 1.6 – Brain Structure and Function

Major Brain Sections

• Cerebrum: Higher functions.

• Brain Stem: Medulla oblongata, pons, midbrain; vital functions.

• Cerebellum: Coordination, balance.

• Diencephalon: Thalamus, hypothalamus, pineal gland.

Structures and Functions

• Medulla Oblongata: Heart rate, breathing.

• Pons: Relay, breathing.

• Midbrain: Visual/auditory reflexes.

• Cerebellum: Motor coordination.

• Reticular Activating System: Alertness.

• Thalamus: Sensory relay.

• Hypothalamus: Homeostasis, endocrine control.

• Pituitary Gland: Hormone release.

• Pineal Gland: Melatonin, circadian rhythm.

• Cerebrum: Cognition, voluntary movement.

• Corpus Callosum: Connects hemispheres.

Cerebral Lobes and Functional Areas

• Frontal: Prefrontal cortex (cognition), premotor cortex (planning), primary motor cortex (execution), Broca’s area (speech).

• Parietal: Somatosensory cortex (touch), association cortex.

• Occipital: Primary visual cortex, visual association.

• Temporal: Auditory cortex, association, Wernicke’s area (language comprehension).

Limbic System

• Emotion, memory; includes thalamus, hypothalamus, hippocampus, amygdala.

Spinal Cord Structure and Function

Protective Layers

• Vertebrae, meninges, CSF.

White vs. Gray Matter

• White Matter: Myelinated axons; outside.

• Gray Matter: Cell bodies; inside.

Sensory and Motor Neuron Pathways

• Sensory neurons enter via dorsal root; motor neurons exit via ventral root.

Unit 1.7 – Nerve Structure and Nerve Plexus

Nerve Anatomy

• Nerves are part of PNS; essential for effector action.

• Transverse section includes: blood vessels, fascicle, epineurium, perineurium, endoneurium, Schwann cell, axon.

Nerve Types

• Sensory: Carry information to CNS.

• Motor: Carry commands from CNS.

• Mixed: Contain both sensory and motor axons.

Mixed nerves can have APs in both directions due to different axons.

Nerve Roots, Branches, and Plexuses

• Nerve Root: Origin from spinal cord.

• Branch/Ramus: Peripheral extension.

• Nerve Plexus: Network formed by intersection of roots.

Spinal vs. Cranial Nerves

• Spinal Nerves: Emerge from spinal cord; always mixed.

• Cranial Nerves: Emerge from brain; can be sensory, motor, or mixed; part of PNS.

• 12 pairs of cranial nerves; examples: optic (sensory), oculomotor (motor).

Reflexes and Reactions

Spinal Reflex Example

• Patellar reflex arc: stimulus (stretch), receptor (muscle spindle), integration (spinal cord gray matter), effector (quadriceps), response (knee extension).

Spinal vs. Cranial Reflexes

• Spinal: Integrated in spinal cord.

• Cranial: Integrated in brain (e.g., pupillary reflex).

Adaptive Advantage

• Reflexes provide rapid, automatic responses for protection and homeostasis.

Monosynaptic vs. Polysynaptic Reflexes

• Monosynaptic: One synapse; faster.

• Polysynaptic: Multiple synapses; slower, more complex.

Reflex vs. Reaction Pathways

• Reflex: Short, fast.

• Reaction: Longer, involves conscious processing.

Visual Somatic Reaction

• Stimulus → photoreceptor → primary visual cortex → processing → primary motor cortex → spinal motor tracts → skeletal muscle contraction.

Unit 1.8 – Autonomic Nervous System (ANS)

Divisions of ANS

• Sympathetic: Emergency, survival; increases activity.

• Parasympathetic: Vegetative, rest; decreases activity.

Dual Innervation

• Most organs receive both sympathetic and parasympathetic supply; some (e.g., adrenal gland) only sympathetic.

Two-Neuron Chain

• Preganglionic neuron: Cell body in CNS.

• Postganglionic neuron: Cell body in ganglion.

Locations and Nerves

• Sympathetic: Thoracic/lumbar spinal nerves.

• Parasympathetic: Cranial nerves (vagus), sacral spinal nerves.

Neurotransmitters and Receptors

• Preganglionic: Acetylcholine (ACh).

• Postganglionic: Sympathetic: Norepinephrine (NE); Parasympathetic: ACh.

• Receptors: Muscarinic, nicotinic (ACh); alpha, beta (NE).

Sympathetic Effects

• Radial pupillary muscles: dilate pupils.

• Salivary/digestive glands: decrease secretion.

• Digestive smooth muscle: decrease activity.

• Bronchioles: dilate.

• Arterioles: increase blood flow to muscles, decrease to skin/kidneys/digestive.

• Heart: increase rate and force.

• Urinary bladder: relaxes.

Parasympathetic Effects

• Opposite of sympathetic; e.g., constricts pupils, increases digestive activity.

Unit 1.9 – Receptors and Somatosensory Pathways (Pain Emphasis)

Stimulus and Receptors

• Stimulus: Any change detected by receptors (e.g., light, heat, pain).

• Receptors: Specialized cells (photoreceptors) or dendrites (nociceptors).

Receptor Classification

Somatosensory Stimuli

• Touch, temperature, pain, proprioception.

Detection vs. Perception

• Detection: Receptors and spinal tracts.

• Perception: Primary sensory cortices (e.g., somatosensory cortex).

Spinal Tract Example: Spinothalamic Pathway

• Sensory neuron enters via dorsal root.

• Dorsal root ganglion houses cell body.

• First synapse: dorsal horn of gray matter.

• Second synapse: thalamus; third-order neuron to cortex.

• Decussation (crossing) occurs at spinal nerve level.

• Thalamus acts as relay station.

Pain Pathway and Modulation

• Presynaptic Inhibition: Reduces pain transmission.

• Aδ fibers: Fast, sharp pain.

• C fibers: Slow, dull pain.

Unit 1.10 – Additional Concepts

Anaphylactic Shock and Beta Agonists

• Beta agonists (epinephrine, salbutamol, salmeterol) stimulate bronchodilation via sympathetic NS.

Parasympathetic Reflex Actions

• Control of digestive and urinary systems; internal sphincter (smooth muscle, involuntary), external sphincter (skeletal muscle, voluntary).

• Spinal cord injury above lumbar nerves results in loss of voluntary control.

Pain Modulation: Gate-Control Theory

• Non-painful input closes "gates" to painful input, reducing pain perception.

Somatic vs. Visceral Pain and Referred Pain

• Somatic Pain: From skin, muscles.

• Visceral Pain: From internal organs.

• Referred Pain: Perceived at a location other than origin (e.g., heart attack pain in arm).

Opioids

• Endogenous: Endorphin, enkephalin.

• Exogenous: Morphine, heroin, fentanyl.

Practice Questions: Available in reviews, chapter ends, and Pearson website.