Foundations of the Nervous System – Core Vocabulary

Basic Anatomical Terminology

The anatomical position is the universally accepted starting point for describing body structures. The subject stands erect, faces the observer, eyes forward, lower limbs parallel with feet flat and directed forward, and upper limbs at the sides with palms facing anteriorly.

Directional terms are paired opposites that locate structures:
• Superior vs. Inferior
• Anterior (ventral) vs. Posterior (dorsal)
• Medial vs. Lateral
• Proximal vs. Distal
• Superficial vs. Deep
Using this common language avoids ambiguity when communicating in healthcare settings.

Anatomical Planes

• Sagittal plane – divides left/right. A midsagittal (median) plane passes the midline; parasagittal planes are offset. Used to visualise structures between cerebral hemispheres (e.g., size of the corpus callosum).
• Frontal (coronal) plane – divides anterior/posterior. Helpful in assessing ventricles of the brain.
• Transverse (horizontal) plane – divides superior/inferior. Favoured for imaging the brainstem.
• Oblique plane – any non-90^{\circ} cut; e.g., viewing hippocampal relations.

Organisation of the Nervous System

The nervous system consists of billions of neurons plus even more neuroglia and is split into two anatomical divisions:

1. Central Nervous System (CNS) – brain (within skull) and spinal cord (within vertebral canal and continuous with the brain at the foramen magnum). Functions: process sensory input; coordinate motor output.

2. Peripheral Nervous System (PNS) – all nervous tissue outside the CNS:
   • 12 pairs of cranial nerves
   • 31 pairs of spinal nerves
   • Ganglia (collections of neuron cell bodies)
   • Sensory receptors

Information flow:
• Afferent (sensory) fibres – toward CNS.
• Efferent (motor) fibres – away from CNS to muscles & glands.
Example: Bilateral laryngeal motor control adducts vocal folds for phonation; unilateral paralysis may reduce speech to a whisper.

Functional Divisions of the PNS

1. Somatic Nervous System (SNS) – voluntary control.
   • Somatic sensory neurons: receptors in skin, body wall, limbs, special senses.
   • Somatic motor neurons: skeletal muscles (e.g., hypoglossal nerve to tongue).

2. Autonomic Nervous System (ANS) – involuntary visceral regulation.
   • Autonomic sensory neurons: internal organs.
   • Autonomic motor neurons: smooth & cardiac muscle, glands.
   • Two branches:
   ◦ Sympathetic ("fight-or-flight"). Preganglionic neurons emerge from T1\text{–}L2; postganglionic fibres distribute via every spinal nerve (31 pairs). Effects: ↑ heart rate, bronchiole dilation, muscle blood-flow redistribution, etc.
   ◦ Parasympathetic ("rest-and-digest"). Preganglionic neurons arise from brainstem nuclei & sacral cord S2\text{–}S4; travel in cranial nerves (e.g., facial nerve – lacrimal & salivary glands).
   Many organs receive dual, often antagonistic innervation.

3. Enteric Nervous System (ENS) – the "brain of the gut." Networks within the gastrointestinal wall span most of its length.
   • Sensory neurons monitor luminal chemistry.
   • Motor neurons control smooth-muscle motility & gland secretion.
   • Operates semi-independently but is modulated by ANS.

Histology of Nervous Tissue – Neurons

A neuron contains:
• Cell body (soma) – nucleus + organelles (ribosomes, mitochondria, lysosomes, Golgi) for metabolism & protein synthesis.
• Dendrites – branching receptive extensions with neurotransmitter receptors.
• Axon – single, long efferent process conveying action potentials; usually myelinated.
• Axon terminals (terminal boutons) – form synapses; contain vesicles of neurotransmitter.

Each neuron forms from a few up to hundreds-of-thousands of synapses. Synaptic plasticity (activity-dependent modification of synaptic strength) underlies development, learning, and recovery after injury.

Myelin & Nodes of Ranvier

Myelin is concentric lipid wrapping around axons, produced by:
• Oligodendrocytes (CNS) – myelinate multiple axons.
• Schwann cells (PNS) – myelinate one axon segment each.
Nodes of Ranvier are interruptions permitting saltatory conduction, greatly accelerating impulse propagation.

Classification of Neurons

Structural
• Multipolar – many dendrites, one axon; dominant in CNS (e.g., pyramidal cells influencing cognition & neuroplasticity).
• Bipolar – one dendrite + one axon; specialised senses (retina, olfactory epithelium).
• Unipolar (pseudounipolar) – single process splitting into peripheral & central branches; sensory ganglia of PNS.

Functional
• Sensory (afferent) – detect internal/external stimuli; relay to CNS.
• Motor (efferent) – transmit commands from CNS to effectors (muscle/gland).
• Interneurons – confined to CNS; integrate information; basis of learning, memory, planning.

Location
• Central neurons – reside within brain/spinal cord (e.g., thalamic relay to cortex).
• Peripheral neurons – within cranial/spinal nerves, sensory receptors, ganglia.

Neuroglial Cells

CNS glia:

1. Astrocytes – most abundant; structural support; regulate extracellular ion & neurotransmitter levels; blood-brain barrier contribution; guide synaptogenesis.

2. Oligodendrocytes – form/maintain CNS myelin.

3. Microglia – immune surveillance; phagocytosis; synaptic pruning; neuroinflammation. Brain tumours (gliomas) often arise from glia and may impair speech/language.

4. Ependymal cells – line ventricles & spinal cord canal; produce/circulate cerebrospinal fluid (CSF).

PNS glia:
• Schwann cells – produce myelin; aid axon regeneration.
• Satellite cells – surround neuron cell bodies in ganglia; regulate chemical milieu & metabolic support.

Collections of Nervous Tissue

• Grey matter – neuron cell bodies, dendrites, unmyelinated axons, glia.
• White matter – myelinated axons.

Terminology
• Nucleus – cluster of neuronal somata inside CNS.
• Ganglion – cluster of somata in PNS.
• Tract – bundle of axons in CNS.
• Nerve – bundle of axons in PNS.

Nerve connective tissue layers:
• Epineurium – surrounds entire nerve.
• Perineurium – encloses individual fascicles.
• Endoneurium – wraps each axon.

Electrical Properties of Neurons

Resting membrane potential: -70\,\text{mV} (inside negative relative to outside).

Graded Potentials
• Localised, stimulus-dependent changes in dendritic/somatic membrane.
• Depolarising: Na^+ or Ca^{2+} influx.
• Hyperpolarising: K^+ efflux or Cl^- influx.

Action Potential (All-or-Nothing Electrical Impulse)

Key phases (completed in \approx 2\,\text{ms}):

1. Depolarisation – stimulus triggers opening of voltage-gated Na^+ channels; membrane potential rises toward +30\,\text{mV}. Threshold: \approx -55\text{ to }-50\,\text{mV}.

2. Repolarisation – Na^+ channels inactivate; voltage-gated K^+ channels open; potential returns negative.

3. Hyperpolarisation (undershoot) – continued K^+ efflux drives membrane more negative than resting.

Refractory Periods
• Absolute – another AP impossible while Na^+ channels reset.
• Relative – super-threshold stimulus required during late hyperpolarisation.

Propagation Mechanisms

• Continuous conduction – sequential opening of channels along unmyelinated axon.
• Saltatory conduction – in myelinated axons depolarisation "jumps" node to node; faster because myelin minimises ion leakage and decreases membrane capacitance. Node spacing is tuned so depolarisation stays above threshold at next node.

Synaptic Transmission

Chemical synapse steps:

1. AP arrives at presynaptic terminal → voltage-gated Ca^{2+} channels open.

2. Ca^{2+} influx binds vesicle proteins → vesicles fuse with membrane (exocytosis).

3. Neurotransmitter diffuses across \approx 20\,\text{nm} synaptic cleft.

4. Binding to postsynaptic receptors (lock-and-key specificity) opens ion channels or activates second-messenger cascades, generating a postsynaptic potential.

Electrical synapses (gap junctions) also exist but are less common in the human CNS.

Clinical & Functional Connections

• Speech motor control requires intact bilateral laryngeal pathways; lesions produce dysphonia.
• Stress, emotion, and cognitive load evoke sympathetic activity (↑HR, bronchodilation).
• Glial tumours (gliomas) can impair speech/language by disrupting regional networks.
• Saltatory conduction underlies the rapid transmission in CNS white matter tracts; demyelinating diseases (e.g., multiple sclerosis) slow or block conduction leading to diverse neurological deficits.

Summary

Understanding anatomical terminology, nervous-system organisation, cellular constituents, and electrophysiology is foundational for analysing higher-level neuro-behavioural functions such as communication and swallowing. These principles recur in later modules, especially when exploring cranial-nerve control and neuroplastic adaptations after injury.