NSB 617 Chapter 1 Notes: Applying Research to Everyday Exercise and Sport — Neurobiology
Somatic vs Autonomic Nervous System
Somatic = nerves that innervate the skin, the joints, and the muscles under voluntary control.
Autonomic (aka Visceral PNS) = neurons that innervate internal organs, blood vessels, and glands.
Ganglion = a cluster of neural cell bodies outside the central nervous system.
Nerve = bundle of fibers/axons that allows communication between CNS and peripheral targets.
Major divisions and brain terminology (overview gleaned from Chapter 1 slides)
CNS components include the brain and spinal cord; PNS comprises all nerves outside the CNS (cranial, spinal, autonomic components).
The human brain can be broadly segmented into major divisions visible in midline cuts and imaging:
Cerebral hemispheres
Corpus callosum (major white-matter tract connecting left and right hemispheres)
Diencephalon
Mesencephalon (midbrain)
Pons
Cerebellum
Medulla oblongata
Spinal cord
The CNS divisions are useful for quickly locating structures and understanding functional relationships (e.g., corpus callosum as the major commissural connector).
Basic Anatomy and Orientation
Directional and sectional references are essential for locating brain regions:
Anterior vs Posterior (rostral vs caudal in many contexts)
Dorsal vs Ventral (superior vs inferior in some orientations)
Medial vs Lateral (toward vs away from midline)
Coronal (frontal, cross-section separating anterior and posterior), Sagittal (midsagittal separates left and right), Horizontal (axial, superior and inferior parts)
In humans, coronal and horizontal views provide complementary perspectives on internal structures.
Basic Anatomical References (Key Structures and Landmarks)
Forebrain structures (overview): cerebral cortex, basal ganglia, thalamus, hypothalamus, amygdala, hippocampus.
Deep structures include the tectum (superior/inferior colliculi in midbrain), brainstem components (pons and medulla), and the cerebellum.
Brainstem and spinal cord organization (from rostral to caudal): midbrain, pons, medulla, spinal cord with cervical, thoracic, lumbar, and sacral segments.
Lateral, anterior (rostral), medial, dorsal, ventral, and transverse (coronal) views help describe relative positions.
Glia and Neurons: Core Players of the Nervous System
Glia: insulate, support, nourish neurons; essential for homeostasis and metabolic support.
Neurons: process information, sense environmental changes, communicate to other neurons, and command body responses.
Analogy: Neurons = chocolate chips; Glia = dough (glia provide the supportive matrix for neuronal function).
The nervous system consists of neurons and glia; glia also contribute to immune functions and structural integrity.
Neurons of the CNS: Structure and Common Staining Approaches
Nissl stain: highlights rough endoplasmic reticulum (Nissl bodies) in neuron cell bodies; useful for counting neurons and assessing cytoarchitecture.
Golgi stain: silver chromate solution randomly labels a small subset of neurons in their entirety, enabling detailed morphology.
Nucleic acid staining (e.g., Nissl) labels RNA and DNA-rich regions; Golgi stain reveals full morphology.
Key neuron features observed in histology include dendrites, cell body (soma), axon, and axon terminals.
Dendrites receive information from other neurons; dendritic spines are common postsynaptic sites.
The axon propagates action potentials for long distances; axon terminals release neurotransmitters to synapse on target cells.
The Neuron Doctrine and Historical Foundations
Santiago Ramón y Cajal (1852–1934) is considered the Father of Neurobiology; his work established the neuron doctrine.
Two major insights:
1) Every neuron is a discrete cell (not a continuous network).
2) Neurons communicate across synapses, not via a continuous cytoplasmic connection.
Non-Neuronal Cells (Glia) in the CNS and PNS
Central Nervous System (CNS) glia:
Ependymal cells
Oligodendrocytes
Astrocytes
Microglia
Peripheral Nervous System (PNS) glia:
Schwann cells
Roles:
Ependymal cells produce cerebrospinal fluid (CSF) and line ventricles; influence cell migration during brain development.
Oligodendrocytes (CNS) form myelin sheaths around axons and contribute to nodes of Ranvier.
Schwann cells (PNS) form myelin in the peripheral nerves and contribute to nodes of Ranvier.
Astrocytes support neurites, regulate extracellular chemical content, provide metabolic support, and interact with blood vessels.
Microglia act as immune cells in the CNS, performing phagocytosis and contributing to vascular integrity and homeostasis.
Glial Functions in Myelination and Signaling
Oligodendroglial cells (CNS) create myelin around axons and contribute to nodes of Ranvier, speeding action potential propagation.
Schwann cells (PNS) perform a similar myelinating role in the peripheral nervous system.
Myelination increases conduction velocity; unmyelinated segments conduct more slowly.
Neuronal Morphology: Diversity and Common Features
Neurons show morphological diversity (examples from various species):
Cortical pyramidal cell (rabbit)
Cerebellar basket cell (mouse)
Spinal motor neuron (cat)
Sensory neuron (mammal)
Spinal motor neuron (fruit fly) – simplified representation
Common structural components:
Dendrites: receive signals from other neurons.
Cell body (soma): metabolic center; contains nucleus and DNA.
Axon: transmits signals long distances; typically from ~ to > in length.
Axon terminals: form synapses with other neurons to transfer signals.
Signal strength and threshold: not every incoming signal triggers an action potential; the axon conducts only if the signal reaches threshold.
Dendrites, Spines, and Postsynaptic Specializations
Dendrites are the main receiving apparatus; they form synapses with axon terminals.
Dendritic spines are small protrusions that host many excitatory synapses and can change with learning.
Postsynaptic site receives signals from the presynaptic axon terminal; integration occurs at the soma and dendrites.
Axons: Structure and Specializations
Axon hillock is the origin of the axon where action potentials are initiated.
The axon proper conducts the action potential; the axon terminal releases neurotransmitters.
The axon does not contain rough endoplasmic reticulum (ER) like the soma; protein synthesis is specialized and distributed differently.
Axonal structure includes a high density of microtubules and motor proteins for transport along the axon.
Axon Terminals and Synaptic Machinery
Axon terminals contain synaptic vesicles filled with neurotransmitters.
They have a high density of membrane proteins and mitochondria to fuel synaptic transmission and vesicle cycling.
The terminal region is a highly energy-demanding site due to rapid recycling and neurotransmitter release.
The Chemical Synapse: Transmission is Electrical-to-Chemical-to-Electrical
Chemical synapse steps (overview):
An action potential arrives at the presynaptic terminal and opens voltage-gated calcium channels.
Calcium entry triggers vesicle fusion and neurotransmitter release into the synaptic cleft.
Neurotransmitters diffuse across the cleft and bind to receptors on the postsynaptic membrane.
Receptor activation leads to postsynaptic potentials and neural signaling; termination occurs via reuptake, enzymatic degradation, or diffusion.
The process is a chemical relay that converts an electrical signal into a chemical message and back into an electrical signal in the postsynaptic neuron.
Dysfunction in synaptic transmission is associated with various mental disorders, highlighting its clinical relevance.
Electrical Synapses: Gap Junctions and Direct Ionic Coupling
Electrical synapses provide direct cytoplasmic continuity between cells via gap junctions.
Gap junctions consist of connexons, each formed by six connexin proteins.
Electrical coupling allows ions to pass directly between cells, enabling rapid, bidirectional signaling with minimal delay.
Examples include coordination in cardiac tissue, certain brain circuits, and other fast signaling systems.
The Fundamentals of Neuronal Communication (Chemical Synapses) – Stepwise View
Key sequence (summary):
presynaptic action potential reaches terminal
voltage-gated Ca^{2+} channels open; Ca^{2+} influx triggers vesicle fusion
neurotransmitter release into synaptic cleft
neurotransmitter binds postsynaptic receptors
postsynaptic ion channels open, generating postsynaptic potentials
signal termination mechanisms restore basal conditions
The strength and probability of signal transmission depend on multiple factors, including neurotransmitter type, receptor density, receptor kinetics, and the state of the postsynaptic cell.
Neural Coding and Synaptic Weights
Neurons communicate via the number and timing of spikes (rate coding and temporal coding).
Information is encoded in both spike counts (rate) and precise spike timing; synaptic strength (weight) modulates the postsynaptic response.
Synaptic plasticity mechanisms adjust weights based on activity, contributing to learning and memory.
Neural Circuits and Reflexes
Neurons function within specialized neural circuits.
Example: involuntary knee-jerk reflex involves sensory input entering via dorsal roots, integration in the spinal cord, and motor output to muscles.
In a reflex arc, dorsal (sensory) input can drive or inhibit motor neurons to produce a reflexive action, with potential inhibitory interneurons shaping the response.
Common Circuit Motifs in Neural Networks
Convergent excitation: multiple inputs excite a single neuron.
Divergent excitation: a single input diverges to excite multiple neurons.
Feedforward excitation: a pathway where signals propagate in one direction from input to output.
Feedback excitation: excited output feeds back to reinforce the input pathway.
Recurrent (lateral) excitation: neurons excite neighboring neurons within a circuit, supporting persistent activity.
Feedforward inhibition: an intermediate interneuron suppresses downstream targets, shaping the flow of activity.
Feedback inhibition: output inhibits earlier stages to regulate circuit activity.
Recurrent inhibition and cross/inhibition: inhibition that stabilizes networks and implements competition.
Lateral inhibition: enhancing contrast by inhibiting neighboring neurons.
Disinhibition: inhibiting an inhibitor to release excitation in a downstream target.
Sensory and Motor Pathways: Major Tracts and Hierarchies
Dorsal column–medial lemniscal pathway (large-diameter, myelinated fibers):
carries fine touch, vibration, proprioception from the body to the brain.
Route: dorsal root entry → dorsal column → nucleus gracilis/cuneatus in the medulla → medial lemniscus → thalamus → primary somatosensory cortex.
Spinothalamic pathway (lateral and anterior):
carries pain and temperature information; crosses early in the spinal cord and ascends via the anterolateral system to the thalamus and cortex.
Thalamus acts as a relay and processing hub before cortex; brainstem and reticular nuclei participate in arousal and attention.
Cortico-spinal (pyramidal) tract connects motor cortex to the spinal cord to drive voluntary movement.
Other brainstem and midbrain structures involved in motor control include the red nucleus, superior colliculus, vestibular nuclei, reticular nuclei, and the cerebellum.
Ventromedial pathways contribute to postural control and coordination of movement via brainstem circuits.
Sensory and Motor Cortical Maps: Homunculi
Motor cortex: motor homunculus represents body parts along the primary motor cortex; body regions vary in size to reflect cortical representation (e.g., hands and face have large representations).
Sensory cortex: sensory homunculus similarly maps body surface to cortical areas; a detailed map shows preferential representation for touch and proprioception in different body regions.
Representations commonly include sections for shoulder, elbow, wrist, hand, fingers, face, lips, tongue, jaw, etc. (These maps illustrate how the brain allocates cortical real estate to different body parts for motor control and sensory perception.)