Week 9: Movement & Stability 1

Learning Objectives

Students will be able to describe the structural and functional organization of the central and peripheral nervous systems
Students will be able to explain the mechanism of action potentials including resting membrane potential, depolarization, repolarization, and saltatory conduction
Students will be able to explain the three fundamental functions of the nervous system: sensory input, integration, and motor output
Students will be able to identify and describe the five special senses and their receptor mechanisms
Students will be able to trace the embryological development of the nervous system from neural tube formation through birth
Students will be able to differentiate between the somatic and autonomic nervous systems and their respective functions
Students will be able to compare and contrast the sympathetic and parasympathetic divisions of the autonomic nervous system

Your Nervous System: The Body's Communication Network

Think of your nervous system as the body's communication network, like a complex telephone system that connects everything together. The Central Nervous System (CNS) is like the main switchboard - your brain and spinal cord - where all important decisions are made. The Peripheral Nervous System (PNS) is like the telephone lines that run throughout your body, carrying messages back and forth.

Your special senses are your body's way of gathering information from the world around you. Smell (olfaction) allows you to detect about 10,000 different odors through millions of receptors in your nose. Taste (gustation) works with smell to help you enjoy food and avoid harmful substances. Vision lets you see the world, while hearing and balance (equilibrium) work together using similar structures in your inner ear.

Special senses and cranial nerves: These senses rely on specialised organs and nerve pathways. Olfaction uses receptors in the olfactory epithelium and signals travel through cranial nerve I. Vision depends on light being focused through the eye onto the retina. Hearing and equilibrium use hair cells in the inner ear, and their signals travel through the vestibulocochlear nerve (cranial nerve VIII).

Touch and receptive fields: Touch is not usually grouped with the classic five special senses, but it is still a major sensory system. Different skin receptors detect pressure, vibration, temperature, pain, and stretch. A receptive field is the area monitored by one sensory neuron. Small receptive fields give better detail discrimination, which is why fingertips can detect fine detail better than the back or thigh.

Refraction and common visual changes: Vision depends on refraction - the bending of light so it lands on the retina. If light focuses in front of the retina, the result is myopia (short-sightedness), corrected with concave lenses. If it focuses behind the retina, the result is hyperopia (long-sightedness), corrected with convex lenses. Presbyopia is age-related difficulty focusing on near objects because the lens becomes less flexible, and astigmatism happens when the cornea or lens has uneven curvature.

Adaptation, binocular vision, and smell loss: Sensory systems adapt when a constant stimulus becomes less noticeable over time. Olfaction adapts rapidly, which is why you stop noticing a smell after a short time. Binocular vision means using both eyes together to judge depth. Reduced smell is called hyposmia, while complete loss of smell is anosmia; both become more common with ageing and can affect appetite, safety, and quality of life.

Equilibrium and vestibular organs: Balance depends on structures in the inner ear. The utricle and saccule detect linear acceleration and head position relative to gravity, while the semicircular canals detect rotational movement. These vestibular signals are integrated with vision and muscle/joint information to maintain posture and stability.

When you touch something, sensory receptors in your skin send signals through nerves to your spinal cord and brain, telling you if something is hot, cold, rough, or smooth. The nervous system develops incredibly fast - by just 4 weeks after conception, the neural tube has formed with three distinct brain regions. By birth, your brain has grown significantly and continues developing throughout childhood.

Neural Signaling: How Nerve Cells Communicate

Neurons communicate using electrical signals called action potentials. Think of it like a wave at a sports stadium - when one person stands up, the next person stands up, creating a traveling wave. In neurons, charged particles (ions) move in and out through special gates in the cell membrane.

When a neuron is at rest, it maintains a negative charge inside compared to outside (like a battery waiting to be used) - this is called the resting membrane potential (~-70mV). This is maintained by the sodium-potassium pump, which actively pushes 3 sodium ions out for every 2 potassium ions it brings in.

When stimulated above a threshold (~-55mV), sodium channels open and sodium rushes in, causing depolarization (the inside becomes positive, up to +30mV). This is the action potential - an all-or-nothing electrical signal that travels down the neuron at speeds up to 120 meters per second in myelinated fibers.

Immediately after, potassium channels open and potassium flows out, causing repolarization (returning to negative). There's a brief refractory period where the neuron cannot fire again, ensuring signals travel in one direction. In myelinated axons, the signal jumps between gaps (Nodes of Ranvier) in the myelin sheath - this is called saltatory conduction and is much faster than continuous conduction.

The PNS has two main branches: the somatic nervous system controls voluntary movements like picking up a pen, while the autonomic nervous system manages automatic functions like your heartbeat. The autonomic system has two parts that work like a car's accelerator and brake - the sympathetic system speeds things up when you're stressed (fight or flight), and the parasympathetic system slows things down when you're relaxed (rest and digest). Together, these systems ensure your body responds appropriately to everything happening around you.

Nervous System: Structure, Function, and Organization

The nervous system represents one of the most intricate and compact of the eleven body systems, organized into the Central Nervous System (CNS) comprising the brain and spinal cord, and the Peripheral Nervous System (PNS) encompassing cranial nerves, spinal nerves, ganglia, enteric nervous system plexuses, and sensory receptors. The fundamental operations of the nervous system involve three integrated processes: sensory input (afferent pathways), information processing and integration, and motor output (efferent pathways).

Embryological Development: Nervous system development begins rapidly, with the neural tube forming by week 4 post-fertilization, demonstrating three primary brain regions: forebrain, midbrain, and hindbrain. By weeks 6-7, the cerebrum, diencephalon, cerebellum, pons, and medulla develop alongside ventricular formation and cerebrospinal fluid production. By week 11, visible cerebrum growth exceeds other brain regions, and by term (40 weeks), the cerebrum has expanded to cover most other brain structures, displaying characteristic gyri and sulci.

Special Senses: Olfactory sensation involves approximately 10-100 million receptors located within the olfactory epithelium, capable of detecting an estimated 10,000 distinct odorants. These specialized chemoreceptors transduce chemical signals into neural impulses that travel via the olfactory nerve (cranial nerve I) to the olfactory cortex and limbic system. Gustation involves taste buds containing specialized chemoreceptors that detect sweet, sour, salty, bitter, and umami flavors.

Receptive Fields and Touch: Somatic sensation depends on receptors distributed through skin and deeper tissues. A receptive field is the area of the body surface monitored by a single sensory neuron. Small receptive fields permit finer spatial discrimination, whereas large receptive fields provide less precise localisation. This helps explain why fingertip touch is more detailed than touch on the trunk or limbs.

Vision and Refraction: Clear vision depends on accurate refraction of light by the cornea and lens so that the image focuses on the retina. Myopia occurs when light focuses in front of the retina, usually because of an elongated eyeball or excessive refractive power, and is corrected with concave lenses. Hyperopia occurs when light focuses behind the retina, usually because of a shortened eyeball or insufficient refractive power, and is corrected with convex lenses. Presbyopia reflects age-related loss of accommodation as the lens becomes less elastic, while astigmatism results from irregular curvature of the cornea or lens producing blurred or distorted vision.

Adaptation, Binocular Vision, and Olfactory Loss: Sensory adaptation refers to decreased responsiveness during sustained stimulation. Olfactory receptors adapt rapidly, reducing awareness of a persistent odour. Binocular vision allows both eyes to work together for depth perception and spatial judgement. Clinically, reduced smell is termed hyposmia and complete loss is anosmia; both may occur with ageing, viral illness, head trauma, or neurodegenerative disease.

Hearing and Equilibrium: The cochlea is specialised for hearing, while vestibular organs are specialised for balance. The utricle and saccule are otolithic organs that detect linear acceleration and head position relative to gravity using otoconia. The three semicircular canals detect rotational acceleration. These inputs are integrated with cerebellar, visual, and proprioceptive information to maintain posture, gaze stability, and coordinated movement.

CNS Organization: The brain's structural organization includes the cerebrum (responsible for higher cognitive functions), cerebellum (coordination and balance), brainstem (vital autonomic functions), and diencephalon (thalamus and hypothalamus regulating sensory relay and homeostasis). The spinal cord serves as both a conduit for neural signals and a center for reflex action, with 31 pairs of spinal nerves emerging to innervate the body.

PNS Organization: The somatic nervous system controls voluntary movements of skeletal muscles through motor neurons. The autonomic nervous system regulates involuntary functions of smooth muscle, cardiac muscle, and glands. The autonomic system comprises the sympathetic division (thoracolumbar outflow) activating fight-or-flight responses through the release of norepinephrine, and the parasympathetic division (craniosacral outflow) promoting rest-and-digest activities through acetylcholine release.

Action Potentials and Neural Signaling

Resting Membrane Potential: The resting membrane potential of approximately -70mV is maintained by the sodium-potassium ATPase pump (3 Na+ out, 2 K+ in per ATP) and differential membrane permeability. At rest, the membrane is 25-30 times more permeable to K+ than Na+ due to leak channels. The intracellular environment contains high K+ and impermeant anions (proteins, organic phosphates), while extracellular fluid contains high Na+ and Cl-, creating an electrochemical gradient.

Action Potential Phases: An action potential is a rapid, all-or-none depolarization that propagates along an axon. Depolarization: When threshold (~-55mV) is reached, voltage-gated Na+ channels open rapidly, allowing Na+ influx that drives the membrane potential toward +30mV. Repolarization: Na+ channels close (inactivate) and voltage-gated K+ channels open, allowing K+ efflux that returns the potential toward resting. Hyperpolarization: K+ channels remain open briefly, causing the potential to dip below -70mV before returning to baseline.

Refractory Periods: The absolute refractory period occurs when Na+ channels are inactivated; no new action potential can be generated regardless of stimulus strength. The relative refractory period requires a stronger-than-normal stimulus to generate an action potential because some K+ channels remain open and the membrane is hyperpolarized. These refractory periods ensure unidirectional propagation of action potentials.

Propagation: Action potentials propagate via local current flow - the inward current at the active region depolarizes adjacent membrane to threshold. In unmyelinated axons, this is continuous conduction (~2 m/s). In myelinated axons, voltage-gated channels are concentrated at Nodes of Ranvier, allowing saltatory conduction ("jumping" between nodes) at speeds up to 120 m/s.

Action Potentials Step by Step

Resting membrane potential: A neuron at rest keeps the inside more negative than the outside, usually about -70 mV. This happens because the membrane leaks more K+ than Na+, and the sodium-potassium pump keeps moving 3 Na+ out and 2 K+ in.

Threshold and depolarisation: If a stimulus brings the membrane to about -55 mV, voltage-gated sodium channels open quickly. Sodium rushes in, and the membrane potential rises toward about +30 mV. This is the upstroke of the action potential.

Repolarisation and hyperpolarisation: Sodium channels then inactivate and potassium channels open, so potassium leaves the cell. The membrane returns toward negative values. Because potassium channels close a little slowly, the neuron often dips below resting level for a short time before recovering.

Refractory periods: During the absolute refractory period, another action potential cannot start because sodium channels are inactivated. During the relative refractory period, a stronger-than-usual stimulus is needed because the membrane is still recovering.

Saltatory conduction: In myelinated axons, the signal is regenerated only at the Nodes of Ranvier. This makes conduction much faster than in unmyelinated axons, where each small section of membrane has to depolarise in sequence.

Sensory Pathways and Receptive Fields

Touch and receptive fields: Skin receptors detect pressure, vibration, stretch, temperature, and pain. A receptive field is the patch of body surface monitored by one sensory neuron. Small receptive fields give better detail, which is why fingertips can tell two close points apart better than your back.

Smell pathway: Odour molecules bind receptors in the olfactory epithelium. Signals pass through cranial nerve I to the olfactory bulb and then to the cortex and limbic system, helping explain why smells are closely linked to memory and emotion.

Taste pathway: Taste buds detect sweet, sour, salty, bitter, and umami. Signals travel mainly through cranial nerves VII, IX, and X to the brainstem, then to the thalamus and gustatory cortex.

Vision and hearing pathways: Light is focused onto the retina, where rods and cones turn it into nerve signals. These travel through cranial nerve II, cross partly at the optic chiasm, relay in the thalamus, and reach the occipital cortex. For hearing, vibrations move from the eardrum through the ossicles into the cochlea, where hair cells send impulses through cranial nerve VIII.

Equilibrium and ageing: The utricle and saccule detect linear acceleration, while semicircular canals detect rotation. With ageing, smell often declines, the lens becomes less flexible, hearing commonly loses high-frequency sensitivity (presbycusis), and vestibular decline can increase fall risk.

CNS, PNS, and Autonomic Control

CNS regional functions: The cerebrum handles conscious thought, language, sensation, and voluntary movement. The cerebellum helps smooth movement and maintain balance. The brainstem connects the brain to the spinal cord and contains centres important for breathing, heart rate, swallowing, and other vital reflexes. The spinal cord carries signals and coordinates reflexes.

Meninges: The brain and spinal cord are wrapped by three protective layers: dura mater, arachnoid mater, and pia mater. These layers protect delicate neural tissue and help contain cerebrospinal fluid.

Choroid plexus and CSF: The choroid plexus in the ventricles produces cerebrospinal fluid (CSF). CSF cushions the CNS, helps transport nutrients, and removes waste.

Basal ganglia and limbic system: The basal ganglia help start and smooth voluntary movement, while the limbic system is strongly involved in emotion, motivation, and memory.

PNS organisation: The peripheral nervous system includes cranial nerves, spinal nerves, ganglia, and sensory receptors. Its somatic part controls skeletal muscle through a single motor neuron, while the autonomic part uses a two-neuron chain to control smooth muscle, cardiac muscle, and glands.

Enteric nervous system: The enteric nervous system is the large neural network in the gastrointestinal tract. It can coordinate gut movement and secretion locally, but it is still influenced by sympathetic and parasympathetic input.

Autonomic signalling: All autonomic preganglionic neurons release acetylcholine. Most sympathetic postganglionic neurons release norepinephrine to adrenergic receptors, while parasympathetic postganglionic neurons release acetylcholine to muscarinic receptors.

Autonomic plexuses: Autonomic nerves form named networks called plexuses, such as the cardiac, pulmonary, coeliac, and hypogastric plexuses. These plexuses distribute sympathetic and parasympathetic fibres to organs.

Circumventricular organs: A few regions around the ventricles, called circumventricular organs, can monitor blood chemistry more directly because they have a reduced blood-brain barrier. This helps with homeostatic control such as thirst, vomiting, and hormone regulation.

CNS regional functions: The cerebrum contains primary sensory, motor, and association cortices supporting perception, language, memory, and voluntary action. The cerebellum compares intended with actual movement to improve timing and coordination. The brainstem contains ascending and descending tracts, cranial nerve nuclei, and autonomic centres, while the spinal cord integrates reflex activity and carries long tracts between body and brain.

Meninges: The CNS is enclosed by three connective tissue coverings: the tough outer dura mater, the delicate arachnoid mater, and the vascular pia mater adherent to neural tissue. The subarachnoid space between arachnoid and pia contains CSF and cerebral blood vessels.

Choroid plexus and CSF: The choroid plexus consists of specialised capillaries and ependymal cells within the ventricles that produce most cerebrospinal fluid. CSF circulates through the ventricles and subarachnoid space, cushioning the brain and spinal cord, supporting chemical stability, and assisting waste clearance.

Basal ganglia: Major nuclei including the caudate nucleus, putamen, and globus pallidus help regulate initiation, scaling, and automatic smoothing of movement. Dysfunction contributes to disorders such as Parkinson disease and Huntington disease.

Limbic system: The limbic system includes structures such as the hippocampus, amygdala, and cingulate gyrus. It integrates emotion, motivation, autonomic responses, and memory formation.

PNS and autonomic organisation: The PNS comprises cranial nerves, spinal nerves, sensory ganglia, autonomic ganglia, enteric plexuses, and peripheral receptors. Somatic motor output reaches skeletal muscle through a single lower motor neuron. Autonomic output uses preganglionic and postganglionic neurons, with nicotinic receptors in autonomic ganglia and muscarinic or adrenergic receptors at target tissues.

Enteric and autonomic integration: Enteric plexuses in the gut wall can generate local reflex activity for motility and secretion, but sympathetic input generally suppresses gastrointestinal activity while parasympathetic input generally supports digestion. Autonomic plexuses redistribute these fibres to thoracic, abdominal, and pelvic organs.

Autonomic plexuses and circumventricular organs: Sympathetic and parasympathetic fibres redistribute through autonomic plexuses such as the cardiac, pulmonary, coeliac, mesenteric, and hypogastric plexuses to reach thoracic, abdominal, and pelvic organs. Circumventricular organs such as the area postrema, median eminence, and neurohypophysis have fenestrated capillaries and reduced blood-brain barrier protection, allowing sampling of circulating signals relevant to emesis, endocrine control, and osmoregulation.

🎥 Video Lectures

Overview

Introduction to the nervous system and special senses.

Overview topics:

Special-sense topics:

CNS topics:

PNS topics:

Topic Title

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📄 Lecture Notes

Key Terms

Central Nervous System (CNS)

The brain and spinal cord; the command center that processes information and coordinates body activities

Peripheral Nervous System (PNS)

All neural structures outside the CNS, including cranial nerves, spinal nerves, ganglia, and sensory receptors

Olfaction

The sense of smell, mediated by chemoreceptors in the olfactory epithelium capable of detecting thousands of odors

Gustation

The sense of taste, involving taste buds and chemoreceptors that detect sweet, sour, salty, bitter, and umami flavors

Neural Tube

Embryonic structure that develops into the brain and spinal cord; forms by week 4 of gestation

Cerebrum

The largest part of the brain, responsible for higher cognitive functions, sensory processing, and voluntary movement

Cerebellum

Brain structure posterior to the cerebrum; coordinates movement, balance, and posture

Brainstem

Connection between brain and spinal cord; includes midbrain, pons, and medulla oblongata; regulates vital functions

Cerebrospinal Fluid (CSF)

Clear fluid that cushions the brain and spinal cord, removing waste and delivering nutrients

Somatic Nervous System

Division of PNS controlling voluntary movements of skeletal muscles

Autonomic Nervous System

Division of PNS controlling involuntary functions of smooth muscle, cardiac muscle, and glands

Sympathetic Nervous System

Thoracolumbar division of autonomic system; activates fight-or-flight responses during stress

Parasympathetic Nervous System

Craniosacral division of autonomic system; promotes rest-and-digest activities and energy conservation

Ganglia

Clusters of nerve cell bodies located outside the CNS

Enteric Nervous System

Neural network within the gastrointestinal tract capable of autonomous function; often called the 'second brain'

Receptive Field

The area of the body surface monitored by one sensory neuron; smaller receptive fields allow finer discrimination of touch

Sensory Receptors

Specialized cells that detect stimuli such as touch, temperature, pain, and pressure

Cranial Nerves

Twelve pairs of nerves emerging directly from the brain that control head and neck functions

Spinal Nerves

Thirty-one pairs of nerves emerging from the spinal cord that serve the rest of the body

Action Potential

A rapid, all-or-none electrical depolarization traveling along an axon. Threshold ~-55mV triggers rapid Na+ influx (depolarization to +30mV), then K+ efflux (repolarization). Propagates unidirectionally via local current flow.

Resting Membrane Potential

The stable electrical charge difference across a neuron's membrane at rest (~-70mV), maintained by sodium-potassium pump (3 Na+ out/2 K+ in per ATP) and differential ion permeability.

Threshold

The membrane potential, usually about -55mV, at which voltage-gated sodium channels open rapidly and trigger an action potential

Saltatory Conduction

Rapid action potential propagation in myelinated axons, jumping between Nodes of Ranvier. Achieves speeds up to 120 m/s versus ~2 m/s in unmyelinated fibers.

Refractory Period

Time after an action potential when another AP cannot be generated. Absolute: Na+ channels inactivated, no AP possible. Relative: elevated threshold, only strong stimuli can generate AP.

Sodium-Potassium Pump

ATP-dependent membrane protein transporting 3 Na+ out and 2 K+ into the cell per ATP molecule, maintaining resting membrane potential and ion gradients essential for neural signaling.

Presbycusis

Age-related hearing loss, usually affecting higher frequencies first and often contributing to communication difficulties in older adults

Interactive Activity: Nervous System Matching

Match nervous system terms with their correct definitions in this interactive matching game. Test your knowledge of CNS, PNS, and autonomic system components.

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End of Week Test

Assess your understanding of nervous system organisation, action potentials, special-sense pathways, touch and receptive fields, CNS and PNS functions, autonomic control, and ageing-related sensory change with the full end-of-week assessment.

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Clinical Case Study

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