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Nervous System — Comprehensive Study Notes (no overarching title in content)

LO 1 — Describe the organisation of the nervous system

  • Central Nervous System (CNS)

    • Brain and spinal cord (command centre)

  • Peripheral Nervous System (PNS)

    • Nerves arising from brain and spinal cord (employees)

  • Functions overview

    • PNS sensory function (ingoing to CNS information)
    • Receptors in the PNS detect changes in the internal and external environment (sensory input)
    • Pass the information on to the CNS
    • CNS integrative function
    • Receives, identifies, analyses for decision
    • Outgoing to the PNS
    • PNS motor function
    • Respond to stimuli (sent from CNS)
    • Specific nerve impulses, called motor output, travel to effectors (muscles or glands) to respond to environment

  • Summary: Functions of the nervous system

    • Sensory input, integration/processing, motor output

  • Organisation sketch (CNS vs PNS in more detail)

    • CNS: brain and spinal cord
    • PNS: all nervous tissue outside the CNS

LO 1 — Detailed organisation of the nervous system

  • CNS (Central Nervous System)
    • Brain and spinal cord
    • Somatic and special sensory receptors and somatic sensory neurons
    • Somatic motor neurons (voluntary)
    • Autonomic sensory receptors and autonomic sensory neurons
    • Autonomic motor neurons (involuntary): sympathetic and parasympathetic divisions
    • Enteric motor neurons (involuntary) in enteric plexuses
    • Enteric sensory receptors and enteric sensory neurons in GI tract and enteric plexuses
  • PNS (Peripheral Nervous System)
    • All nervous tissue outside the CNS

LO 2 — Describe the structure and function of neurons and neuroglia

  • Neuron
    • Type of cell that receives and sends messages from the body to the brain and back
  • Neuroglia (glia)
    • Cells that hold nerve cells in place and support neurons to work effectively

NEURONAL FEATURES

  • Neurons characteristics
    • Electrically excitable cells that initiate, transmit, and receive nerve impulses
    • High metabolic rate: survival depends on continuous and abundant supplies of glucose and oxygen
    • Extreme longevity: most form during fetal development and survive our whole lives
    • Non-mitotic: most are unable to divide and produce new neurons
  • Neuroglia characteristics
    • Nonconductive
    • Provide support and protection to neurons
    • Make up about half the volume of the nervous system
    • Can multiply and divide
    • Found in both CNS and PNS: oligodendrocytes (CNS) and Schwann cells (PNS)
    • Smaller than neurons
    • More numerous than neurons

Neuron structure

  • Parts of a neuron (key components)
    • Axon
    • Dendrites
    • Neurolemma
    • Axon hillock
    • Nissl bodies
    • Myelin
    • Axon terminal
    • Synaptic end bulb
  • Dendrites
    • Short processes that branch from the cell body
    • Receive nerve impulses and carry them to the cell body
  • Axon
    • Transmit nerve impulses (electrical) away from the cell body to other cells
  • Myelin sheath
    • Insulates and speeds up electrical transmission
  • Axon terminals and synaptic knobs (chemical signaling)
  • Neuroglia nuclei (in the surrounding tissue)

Neuron structure — visual cues

  • Dendrites, Axon hillock, Axon, Nuclei of neuroglia

Structural classification of neurons

  • (a) Multipolar
    • Many dendrites and a single axon
    • Most common type; e.g., motor neurons
  • (b) Bipolar
    • Two processes, one dendrite and one axon
    • Uncommon; associated with special senses
  • (c) Unipolar (pseudounipolar)
    • Single, short process that branches like a ‘T’
    • Dendrite and axon fused as they leave the cell body
    • Most sensory neurons in the PNS

Functional classification of neurons

  • Sensory neuron
  • Motor neuron
  • Interneuron

Neuroglia of the CNS

  • Key CNS glial cells
    • Oligodendrocytes
    • Microglial cells
    • Astrocytes (protoplasmic and fibrous)
    • Ependymal cells
    • (Pia mater and meninges are protective coverings rather than glial cells)
  • Myelination in CNS
    • Oligodendrocytes
    • Myelin wraps around axons; one oligodendrocyte can myelinate many axons

Neuroglia of the PNS

  • Schwann cells (neurolemmocytes)
    • Associated with PNS axons
    • Wrap around axons; one Schwann cell myelinates part of one axon
    • Produce myelin which insulates electrical activity
    • Equivalent to oligodendrocytes in the CNS
  • Satellite cells
    • Surround neuron cell bodies in ganglia

LO 2 — Quick questions (practice concepts)

  • What are the functions of the dendrites, cell body, and axon of a neuron?
  • Which type of structural class of neuron is the most abundant?
  • What are the three functional classes of neurons?
  • Compare and contrast Schwann cells and oligodendrocytes.

LO 3 — Regeneration and repair of nervous tissue

  • Nervous tissue has plasticity but limited regenerative ability

  • Plasticity: ability to change based on experience

  • Regenerate: capability to replicate or repair

  • Neurogenesis in the CNS

    • Limited repair due to:
    • Limited stem cells
    • Non-supportive microenvironment for neuron generation
    • Absence of growth-stimulating cues present during development
    • Rapid scar tissue formation by astrocytes
    • Energy demands to create new cells
    • Neurogenesis = creation of new neurons

  • Damage and repair in the PNS

    • Repair is possible if:
    • The cell body is intact
    • Schwann cells are functional
    • Scar tissue formation does not occur too rapidly
    • Note: do not learn the detailed steps involved

LO 3 — Quick questions (PNS/CNS regeneration)

  • What prevents most CNS neurons from repairing?
  • What structures allow PNS nerves to repair?

LO 4 — Grey matter vs White matter in the brain

  • Grey matter
    • Houses motor neuron and interneuron cell bodies, dendrites, unmyelinated axons, axon terminals, neuroglia
    • Forms the cortex (surface) and discrete internal clusters called cerebral nuclei
  • White matter
    • Made up of myelinated axons
    • Internal in the brain; external layer in the spinal cord

LO 4 — Grey vs White matter (histology cues)

  • Histology references: grey matter is laminated (six layers) in the cerebrum; white matter contains axons and tracts

LO 5 — Identify brain regions and general functions

  • Major parts of the adult brain (sagittal view)

    • Cerebrum
    • Diencephalon
    • Brainstem
    • Cerebellum

  • Brainstem functions

    • Regulates: blood pressure, heart rate, breathing, facial sensations, hearing, swallowing
    • Brainstem injury can cause dizziness, motor function loss, paralysis, coma, or death

  • Brainstem anatomy and connections

    • Bidirectional pathway for all tracts between cerebrum and spinal cord
    • Houses nuclei of many cranial nerves

  • Diencephalon components and roles

    • Epithalamus: links limbic system to other brain parts
    • Thalamus: sensory information (except smell) must be processed here before cortex interpretation; also involved in sleep, wakefulness, consciousness, learning, memory; contains subthalamus
    • Hypothalamus: hormone production; regulates body temperature, hunger, thirst, mood, sex drive, blood pressure

  • Cerebrum features

    • Coronal section landmarks: gyrus (ridge), sulcus (groove), fissure (deep groove; e.g., longitudinal fissure)
    • Outer gray matter: billions of neurons; internal white matter: tracts connecting cortex to brain/spinal cord; basal nuclei: deep gray matter influencing motor function and some cognitive processes; corpus callosum connects left and right hemispheres

  • Lobes of the Cerebrum

    • Frontal lobe: voluntary motor function, verbal communication, higher intellect
    • Parietal lobe: conscious awareness of sensations
    • Occipital lobe: vision
    • Temporal lobe: smell, hearing
    • Insula lobe: memory, taste
    • Note: lobes can have overlapping functions; primary motor cortex and primary somatosensory cortex are located near the central sulcus

  • Somatosensory and motor homunculi

    • Sensory Homunculus: depicted on the somatosensory cortex
    • Motor Homunculus: depicted on the primary motor cortex

  • Cerebellum

    • Coordinates and fine-tunes skeletal muscle movements initiated by the cerebral cortex to produce smooth, coordinated movements
    • Stores memories of learned movements
    • Receives proprioceptive information from muscles and joints to regulate body position
    • Helps maintain balance and posture
    • Alcohol inhibits cerebellar activity

LO 6 — Brain protection mechanisms

  • Protection mechanisms include:
    • Cranial bones
    • Cranial meninges
    • Blood-brain barrier (BBB)
    • Cerebrospinal fluid (CSF) and ventricles

Cranial meninges (layers — superficial to deep)

  • Dura mater: tough, bilayered (periosteal and meningeal layers)
  • Subdural space
  • Arachnoid mater: middle layer, thin, avascular; spider’s web of collagen and elastin
  • Subarachnoid space
  • Pia mater: deepest, thinnest layer; adheres to brain surface; unicellular, highly vascularized
  • Dural venous sinus: between dural layers; drains brain blood

Blood–Brain Barrier (BBB)

  • Tightly packed endothelial cells line brain vessels
  • Purpose: protect CNS by keeping dangerous substances out of brain tissue
  • Blood supply path: vertebral and carotid arteries bring blood to the brain; jugular veins return blood to heart
  • BBB is lipid-based; hydrophobic, and restricts large molecule entry
  • Examples: many antivirals and antibiotics cannot cross BBB; some antidepressants and ibuprofen can cross

Ventricles and CSF

  • Ventricles: four interconnected cavities in the brain continuous with the central canal of the spinal cord
  • CSF (cerebrospinal fluid): clear, colorless; circulates in ventricles and subarachnoid space; contains small amounts of water, oxygen, and glucose
  • Functions of CSF
    • Mechanical protection: brain floats in CSF; shock absorption and protection from sudden movements
    • Circulation: transports nutrients and removes waste from the brain
  • Key structures associated with CSF production and circulation
    • Choroid plexus of lateral, third, and fourth ventricles
    • Central canal of spinal cord; Subarachnoid space

LO 6 — Quick questions (brain protection)

  • What are the three layers of the cranial meninges, from superficial to deep?
  • Which cranial meningeal layer is the toughest?
  • What is the purpose of the blood–brain barrier?
  • Where are CSF-producing structures located?

LO 7 — Describe the structure and function of the spinal cord

  • External structure
    • Part of CNS; continuous with brainstem
    • Conus medullaris: end of spinal cord (around L1/L2)
    • Cauda equina: bundle of nerve roots inferior to the conus medullaris
  • Spinal meninges
    • Pia mater
    • Arachnoid mater
    • Dura mater
  • Spinal nerve structure
    • Nerves are bundles of axons surrounded by three connective tissue layers: epineurium (around the entire nerve), perineurium (around each fascicle), endoneurium (around each axon)
    • Cross-sectional diagram shows epineurium, perineurium, endoneurium, fascicles, axons, blood vessels
  • Internal spinal cord structure
    • Gray matter: neuron's dendrites and cell bodies, unmyelinated axons, glial cells
    • White matter: myelinated axons
  • Spinal cord function
    • Sensory and motor information processing
    • Contains interneurons (cell bodies and axons)
    • Pathways: sensory neuron axon, sensory neuron cell body, motor neuron axon (to skeletal muscles), motor neuron cell body, autonomic neuron cell body, autonomic neuron axon (to cardiac and smooth muscles and glands)

LO 8 — Describe the structure of a spinal nerve

  • Spinal nerves are cable-like bundles of axons
  • Surrounded by protective connective tissue layers in this order (from outside to inside): epineurium, perineurium, endoneurium
  • Each nerve contains blood vessels and fascicles
  • Spinal nerves are part of the PNS and are mixed nerves (contain both sensory and motor fibers)
  • Segmentation and plexuses
    • 31 pairs total: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal
  • Brachial plexus (C5–C8, T1) example nerves: musculocutaneous, axillary, radial, median, ulnar (note: a learning cue says “Don’t learn this!” for some slide content)

LO 8 — Quick questions (spinal nerves)

  • Why are all spinal nerves classified as ‘mixed’ nerves?
  • How many spinal nerves are in each vertebral segment – cervical, thoracic, lumbar?
  • What is a nerve plexus?
  • Gray matter is the site of neuron cell bodies and dendrites; white matter contains axonal tracts that connect brain regions and spinal cord
  • The cortex contains primary motor and primary somatosensory areas near the central sulcus; the homunculi illustrate the body’s mapping to these cortical regions
  • The cerebellum’s role in balance and coordination relies on proprioceptive input; alcohol can impair cerebellar function
  • The BBB’s lipid-based exclusion explains why certain medications must be specially formulated to reach the CNS
  • The Enteric Nervous System (ENS) operates semi-independently within the GI tract but is connected to the CNS via autonomic pathways