Chapter 1: Applying Research to Everyday Exercise and Sport — CNS, Neurons, and Glia (NSB 617)
Central Nervous System (CNS) Organization and Basic Anatomy
The nervous system is divided into two major divisions:
Peripheral Nervous System (PNS): includes somatic and autonomic components.
Central Nervous System (CNS): consists of the brain and spinal cord.
Somatic PNS (aka. somatic nervous system): nerves that innervate the skin, joints, and muscles under voluntary control.
Autonomic PNS (aka Visceral PNS): neurons that innervate internal organs, blood vessels, and glands.
Ganglion: a cluster of neural cell bodies located outside the CNS.
Nerve: a bundle of fibers/axons.
CNS anatomy in the brain shows seven main divisions (Figure references from slides): Forebrain, Midbrain, Hindbrain; Cerebral hemispheres; Corpus callosum; Diencephalon; Brainstem components (Pons, Medulla oblongata); Cerebellum; Spinal cord. Major labeled parts include:
Forebrain: Cerebrum (cerebral cortex and subcortical structures) and Diencephalon (thalamus and hypothalamus).
Midbrain: tectum and tegmentum (part of brainstem anatomy).
Hindbrain: Pons, Medulla oblongata, Cerebellum.
The four lobes of the cerebral cortex: Frontal, Parietal, Temporal, Occipital.
The major brain divisions are visible in midline sections and MRI (as per Figures 1-2 and 1-3):
Corpus callosum connects left and right hemispheres.
Diencephalon lies between the cerebral hemispheres.
Brainstem includes Pons and Medulla oblongata; Cortex surrounds deep structures.
Basic Anatomical References and Orientation
Planes and directional terms used in neuroanatomy:
Anterior or rostral vs Posterior or caudal.
Dorsal (superior) vs Ventral (inferior).
Medial vs Lateral; Midline.
Midsagittal plane, Horizontal plane, Coronal plane.
Rostral vs Caudal in the brain; Dorsal vs Ventral in the spinal cord context noted.
These terms help describe relationships in brain and spinal cord across sections (Figures showing anatomical references).
For cross-sections, it helps to visualize: midline (line that divides left/right), lateral structures, and dorsal/ventral axes in both brain and spinal cord.
Forebrain, Midbrain, Hindbrain – Substructures
Forebrain (prosencephalon) includes:
Cerebral cortex (cerebral hemispheres) with lobes: Frontal, Parietal, Temporal, Occipital.
Basal ganglia (caudate, putamen, globus pallidus, etc.).
Thalamus (sensory relay station) and Hypothalamus (autonomic/behavioral regulation).
Hippocampus (memory formation).
Midbrain (mesencephalon) includes:
Tectum and Tegmentum; part of brainstem.
Hindbrain (rhombencephalon) includes:
Cerebellum; Pons; Medulla oblongata.
Spinal cord segments: Cervical, Thoracic, Lumbar, Sacral.
Note on terminology: “forebrain, midbrain, hindbrain” correspond to early developmental divisions and relate to adult anatomy in the order listed above.
Gross Brain Landmarks and Cross-Section Visualization
The corpus callosum is a large bundle of nerve fibers that connects the left and right cerebral hemispheres.
The main divisions visible on midline cuts are Forebrain, Midbrain, Hindbrain; the brainstem components (pons and medulla) lie medially to the cerebellum.
MRI illustrations reflect these internal landmarks in living brains.
Basic Anatomical References: Orientation, Planes, and Axes
Anterior/rostral vs posterior/caudal orientation is applied to both brain and spinal cord.
Dorsal (superior) vs Ventral (inferior) orientation is used for the brain; for the spinal cord, dorsal/ventral refers to the back/front surfaces.
Medial-lateral axis describes the relative position to the midline.
Planes: midsagittal (vertical plane cutting the body into left/right halves), horizontal (axial), coronal (frontal).
Neural Tissues: Neurons and Glia
Neurons and glia constitute the nervous system.
Glia insulate, support, and nourish neurons; neurons process information, sense environment, communicate, and command body responses.
Non-neuronal cells in CNS include: Ependymal cells, Oligodendrocytes, Astrocytes, Microglia.
In the peripheral nervous system (PNS): Schwann cells.
Neurons in the CNS – Morphology and Types
Neuronal morphologies include:
Unipolar neuron: single axon; typical of some sensory systems.
Bipolar neuron: two processes (one axon, one dendrite).
Pseudo-unipolar neuron: a single process that divides into two branches (peripheral axon to skin/muscle; central axon toward CNS).
Multipolar neurons: multiple dendrites and one axon (e.g., pyramidal cells, Purkinje cells, motor neurons).
Example structures:
Pseudo-unipolar: dorsal root ganglion neuron.
Multipolar: pyramidal neuron of hippocampus, Purkinje cell of cerebellum, motor neuron of spinal cord.
Neuron visualizations (from slides): Unipolar, Bipolar, Pseudo-unipolar, Multipolar cell schemes.
Nissl stain (nucleic acid staining) and Golgi stain (silver chromate labeling):
Golgi stain labels a small subset of cells in their entirety; useful for visualizing full neuron morphology in hippocampus and cerebellum.
Nissl staining highlights RNA-rich structures (RER—rough endoplasmic reticulum) and is used to reveal cell bodies; Toluidine blue or Cresyl violet are common Nissl components.
Nissl stain is also historically connected to ER labeling terminology.
Cajal and the neuron doctrine (Ramon y Cajal, 1852–1934):
First to clearly observe neurons as discrete units.
Two key insights: (1) every neuron in the brain is separate; (2) neurons communicate across synapses.
Major non-neuronal cells (glia) and their general roles:
Ependymal cells: line ventricles and produce cerebrospinal fluid (CSF); contribute to directing neural development.
Oligodendrocytes: myelinate CNS axons; nodes of Ranvier exist at gaps in myelin.
Astrocytes: most abundant glia in brain; influence neurite growth; regulate extracellular chemical content; nutritive function by contacting neurons and capillaries.
Microglia: CNS phagocytes; participate in immune defense and vasculature support.
Schwann cells: PNS myelinating glia; also involved in nodes of Ranvier in the PNS.
Node of Ranvier:
Specific region along a myelinated axon where the axonal membrane is exposed, enabling saltatory conduction.
Neuronal Structure and Functional Compartments
Neuron anatomy overview:
Dendrites: receiving inputs; dendritic spines are postsynaptic structures.
Cell body (soma): metabolic center of the neuron.
Axon: carries signals over long distances; typical lengths range from to >1\,\text{m} in some circuits; axon terminals at the end of the axon transfer signals to other neurons.
Axon terminals: sites of neurotransmitter release; abundant membrane proteins and mitochondria; lack of microtubules in terminals.
Dendrites as the primary receptive structures; axon hillock as the initiation zone for action potentials.
The axon proper vs soma: ER is not present in the axon; axons have a unique protein composition.
Synapses: chemical vs electrical
Chemical synapses: electrical-to-chemical-to-electrical signaling; neurotransmitter release at the presynaptic terminal, diffusion across the synaptic cleft, and binding to postsynaptic receptors to generate postsynaptic potentials.
Electrical synapses (gap junctions): direct cytoplasmic connection via connexons forming a gap junction channel; allow ions to flow directly between cells.
Synaptic cleft and postsynaptic specialization are visible in electron micrographs; vesicles in presynaptic terminal indicate neurotransmitter release.
Gap junctions and connexins:
Connexon: channel formed by six connexin subunits; forms the basis of electrical coupling between neurons.
Electrical vs Chemical Synapses – Functional Implications
The chemical synapse sequence (basic steps):
An action potential travels along a presynaptic axon to the presynaptic terminal.
Voltage-gated Ca2+ channels open, Ca2+ enters, triggering transmitter-containing vesicles to fuse with the membrane.
Neurotransmitter is released into the synaptic cleft and binds to postsynaptic receptors.
Postsynaptic receptor activation causes membrane potential changes (graded potentials).
The graded potentials are integrated at the postsynaptic soma/axon hillock and may trigger an action potential if threshold is reached.
Electrical synapses provide faster, bidirectional signaling and synchronization in some circuits, albeit with less plasticity than chemical synapses.
How Neurons Operate Within Circuits
Neurons function within specialized neural circuits to produce complex behaviors.
Classic example: involuntary knee-jerk reflex demonstrates a simple circuit structure.
extensor muscle → sensory neuron →
send information to the dorsal part of the spinal cord→ interneurons → motor neuron → activated extensor muscle, resulting in the leg kicking forward.
or inhibitory interneuron → flxor motor neuron → flexor muscle
Common circuit motifs (illustrative repertoire):
Convergent excitation: a postsynaptic neuron that receives excitatory input from multiple presynaptic neurons
Divergent excitation: a single neuron that uses branched axons to send excitatory signals to multiple postsynaptic neurons
Feedforward excitation: one neuron directly activates a subsequent neuron in a pathway
Feedback excitation: excitatory neurons send signals back to each other
Recurrent (lateral) excitation: excitatory neurons that receive input from and send output back to other neurons within the same network
Feedforward inhibition: an excitatory neuron excite onto both an excitatory neuron and an inhibitory neuron, and the inhibitory neuron inhibit the excitatory postsynaptic neuron
Feedback inhibition: postsynaptic excitatory neuron excite an inhibitory neuron to inhibit back to postsynaptic excitatory neuron
Recurrent (cross) inhibition: two parallel excitatory pathways cross inhibit each other via inhibitory neuron intermediates; Ex. knee-jerk reflex.
Lateral inhibition: one or several parallel excitatory neurons excite an inhibitory neuron→ inhibit the postsynaptic excitatory neurons
Disinhibition: inhibitory neuron(presynaptic) inhibit a inhibitory neuron to inhibit
Sensory and Motor Pathways – Core Routes
Dorsal column-medial lemniscal pathway (touch, vibration, two-point discrimination, proprioception):
Primary sensory neurons: dorsal root axons (Aα, Aβ, Aδ types).
Ascends in the dorsal column of the spinal cord to the dorsal column nuclei in the medulla.
Crosses at the level of brainstem, then ascends via the medial lemniscus to the thalamus.
Thalamus relays to cerebral cortex: primary somatosensory cortex.
In the cerebral cortex, touch and proprioceptive information is processed.
Spinothalamic pathway (pain, temperature, some touch):
Lateral spinothalamic tract carries pain and temperature information.
Dorsal root axons convey initial sensory input and synapse onto second-order neurons in the spinal cord.
Additional pathway components and features include:
Medulla, brainstem, thalamic relays, and cortical targets (motor and sensory areas).
Dorsal column nuclei (nucleus cuneatus and nucleus gracilis) in the medulla serve as relay stations before the medial lemniscus.
Motor control pathways involve the cerebral cortex and brainstem circuitry:
Primary motor cortex communicates with spinal motor neurons via corticospinal tracts.
Thalamus and brainstem structures (reticular nuclei, superior colliculus, vestibular nuclei) contribute to motor control and reflexive actions.
Ventromedial pathways and other controls integrate posture and movement planning.
Key brain regions involved in sensory and motor processing:
Primary motor cortex (execution of voluntary movement).
Primary somatosensory cortex (somatic sensation).
Thalamus (relay and integration of sensory information).
Brainstem nuclei (reticular formation, colliculi) and spinal cord circuits contribute to reflexive and autonomic components.
Cortical Representations – Homunculi
Sensory and motor homunculi illustrate somatotopic organization in cortex:
Motor homunculus (in primary motor cortex): body map with face, lips, tongue, mandible, neck, trunk, limbs, digits, and proximal-to-distal arrangement; reflects motor control priorities.
Sensory homunculus (in primary somatosensory cortex): body surface map, showing sensitivity distribution (e.g., fingers, lips, face occupy disproportionately large areas due to dense innervation).
The slide examples show the following body parts represented in each homunculus:
Motor: wrist, elbow, shoulder, trunk, knee, toes, face features; tongue, jaw, lips; mastication and swallowing muscles involved.
Sensory: wrist, hand, fingers; nose; face; lips; teeth and gums; tongue; pharynx; trunk; limbs; genitalia.
Morphological Diversity of Neurons (Representative Examples)
Neurons exhibit diverse shapes and sizes across brain regions:
Cortical pyramidal cell (rabbit)
Cerebellar basket cell (mouse)
Spinal motor neuron (cat)
Sensory neuron (mammal)
Spinal motor neuron (fruit fly) [illustrative cross-species example]
Historical and Imaging Contexts
Golgi stain (Camillo Golgi, 1873):
Silver chromate solution randomly darkly labels a small number of cells, enabling visualization of individual neurons in tissue (e.g., rat hippocampus).
Nissl stain: nucleic acid staining method highlighting RNA and ribosomes in cell bodies; commonly used with cresyl violet or toluidine blue.
The Golgi and Nissl methods provide complementary views of neurons and glia to study cellular morphology and organization.
The neuron doctrine (Cajal):
Neurons are discrete units, not a continuous network.
Neurons communicate via synapses; information is transmitted across gaps between cells.
Summary of Key Concepts and Implications
CNS and PNS organization and the basic cellular constituents (neurons and glia) underpin all neural function.
Glial cells provide support, insulation (myelination), metabolic support, and immune functions; glial diversity is essential for CNS health and development.
Neurons have specialized compartments (dendrites, soma, axon, terminals) with structural specializations (dendritic spines, axon hillock, synaptic vesicles) that support wired communication and plasticity.
Synapses come in two main flavors: chemical (neurotransmitter-mediated) and electrical (gap junctions). Each supports different speeds and plasticity in signaling.
Neural circuits implement information processing through recurrent and feedforward motifs, with excitatory and inhibitory components shaping responses.
Sensory and motor pathways organize information flow from peripheral receptors to cortical centers and from cortical commands to muscles, with defined relay stations (dorsal columns, thalamus, brainstem nuclei).
Cortical homunculi illustrate the topographic mapping of body parts in motor and sensory cortices, reflecting functional anatomy and innervation density.
Historical staining methods (Golgi and Nissl) and the neuron doctrine shaped our understanding of neural organization and communication.
Frequently used quantitative references and terms:
Axon lengths span from roughly to greater than in some circuits.
Node of Ranvier marks the gaps in myelin where ion channels are concentrated to enable saltatory conduction.
Neuron types include unipolar, bipolar, pseudo-unipolar, and multipolar classes with distinct functional roles.
Ethical, philosophical, or practical implications:
Understanding neuron-glia interactions informs clinical approaches to mental disorders, neurodegenerative diseases, and brain injury.
The neuron doctrine underscored the concept of discrete cellular units; modern neuroscience continues to explore how networks of neurons give rise to complex behaviors while acknowledging glial contributions.
Connections to foundational principles:
Structure determines function: neuronal morphology and synaptic organization underlie signaling properties and circuit dynamics.
Electrical and chemical signaling provide complementary mechanisms for rapid and plastic communication in the nervous system.
Real-world relevance:
Knowledge of sensory and motor pathways informs neurorehabilitation strategies following CNS injury.
Understanding common circuit motifs helps in analyzing how diseases disrupt normal information processing and how interventions might restore function.
Note: Figures referenced (e.g., Figures 1-2, 1-3) illustrate the anatomical divisions and landmarks described above; the content aligns with standard CNS organization and neuroanatomical terminology as presented in the slides.