The mammalian nervous system consists of two main divisions:

• Central Nervous System (CNS) — brain and spinal cord; processes and integrates information
• Peripheral Nervous System (PNS) — nerve fibers branching from CNS to all body parts

Neurones (nerve cells) are the basic functional units that transmit nerve impulses. Each neurone has a cell body (containing nucleus), dendrites (receive signals), and a long axon (transmits signals). Supporting cells called neuroglia (glial cells) provide protection, nourishment, and insulation.

Types of Neurones

1. Sensory (Afferent) neurones — transmit impulses from sensory receptors to CNS
2. Interneurones — relay impulses within CNS between sensory and motor neurones
3. Motor (Efferent) neurones — transmit impulses from CNS to effectors (muscles or glands)

Resting Potential

When a neurone is not conducting an impulse, it maintains a resting potential of about -70 mV (inside negative relative to outside). This occurs because:

• Sodium ions (Na⁺) are actively pumped out of the axon
• Potassium ions (K⁺) accumulate inside the axon
• The membrane is polarized with positive charges outside and negative charges inside
• The Na⁺/K⁺ pump (using ATP) maintains this ion distribution

Action Potential

An action potential is a rapid change in membrane potential when a stimulus reaches threshold (about -55 mV). It involves three phases:

1. Depolarisation — Sodium channels open, Na⁺ rushes into the axon, changing membrane potential from -70 mV to +40 mV

2. Repolarisation — Sodium channels close, potassium channels open, K⁺ flows out, restoring negative interior

3. Hyperpolarisation — Membrane potential briefly becomes more negative than resting potential (-70 mV) before returning to normal

Conduction Along the Axon

Impulse transmission follows these steps:

1. Polarisation — At rest, Na⁺ is high outside, K⁺ is high inside; membrane is polarized
2. Stimulus arrival — Causes local depolarisation opening voltage-gated Na⁺ channels
3. Action potential — Na⁺ enters, creating depolarisation that triggers adjacent regions
4. Repolarisation — K⁺ exits behind the impulse, restoring the membrane
5. Saltatory conduction — In myelinated axons, impulses "jump" between nodes of Ranvier, increasing speed

Characteristics of Nerve Impulses

• All-or-nothing law — If stimulus exceeds threshold, impulse is generated with constant magnitude
• Unidirectional flow — Impulses flow from dendrite → cell body → axon → synapse
• Refractory period — Brief period after an impulse when the axon cannot fire again, ensuring one-way flow
• Speed — Determined by axon diameter (larger = faster) and presence of myelin sheath (myelinated = faster)

Synaptic Transmission

The synapse is the junction between two neurones. A chemical synapse operates as follows:

1. Action potential arrives at the synaptic knob
2. Voltage-gated calcium channels open, Ca²⁺ enters
3. Synaptic vesicles fuse with presynaptic membrane (exocytosis)
4. Neurotransmitter (e.g., acetylcholine) released into synaptic cleft
5. Neurotransmitter binds to receptors on postsynaptic membrane
6. Ion channels open, creating excitatory postsynaptic potential (EPSP)
7. If sufficient EPSPs accumulate (temporal or spatial summation), action potential is triggered
8. Neurotransmitter is broken down by enzymes (e.g., acetylcholinesterase) to prevent continuous stimulation

Structure of the Eye

The eye consists of:

• Cornea — Refracts light entering the eye
• Iris — Controls pupil size (like camera aperture)
• Lens — Focuses light onto retina; changes shape for accommodation
• Retina — Contains photoreceptor cells (rods and cones)
• Optic nerve — Transmits impulses to brain

Accommodation

Accommodation is the process of focusing light on the retina for objects at different distances:

Retina: Rods and Cones

The retina has three cell layers:

• Outer layer — Photoreceptors (rods and cones)
• Middle layer — Bipolar neurones
• Inner layer — Ganglion cells (axons form optic nerve)

Physiology of Seeing

1. Light enters through cornea and pupil
2. Lens adjusts curvature (accommodation) to focus light on retina
3. Light strikes rods/cones in retina
4. Pigments (rhodopsin in rods, iodopsins in cones) undergo chemical changes (bleaching)
5. Photoreceptors generate nerve impulses
6. Bipolar neurones transmit to ganglion cells
7. Optic nerve carries impulses to visual cortex in brain
8. Brain interprets impulses as images

Structure of the Ear

The ear has three main parts:

Outer ear — Pinna collects sound waves; auditory canal leads to tympanic membrane (eardrum)

Middle ear — Contains three ossicles (malleus, incus, stapes) that amplify and transmit vibrations; eustachian tube equalizes pressure

Inner ear — Contains cochlea (hearing) and vestibular apparatus (balance)

Mechanism of Hearing

1. Sound waves enter through pinna
2. Tympanic membrane vibrates
3. Ossicles (malleus → incus → stapes) amplify and transmit vibration
4. Stapes pushes against oval window, creating fluid waves in cochlea
5. Waves travel through perilymph, causing basilar membrane to vibrate
6. Hair cells in organ of Corti are stimulated
7. Nerve impulses generated in auditory neurones
8. Impulses travel via auditory nerve to cerebral cortex
9. Brain interprets as sound

Loudness is determined by the number of hair cells stimulated; pitch is determined by which region of the basilar membrane vibrates (high频率 = base, low频率 = apex).

Body Balance

The vestibular apparatus in the inner ear maintains balance through:

Semicircular canals — Three curved tubes at right angles to each other, detecting rotational movement:

• Each canal has an ampulla with cupula (gelatinous plate)
• Head movement causes endolymph to flow
• Cupula bends, stimulating hair cells
• Impulses sent via vestibular nerve to brain
• Brain sends corrective signals to muscles

Utricle and saccule — Detect linear acceleration and head position relative to gravity:

• Contain otoliths (calcium carbonate crystals) embedded in gelatinous matrix
• Otoliths move with gravity, stimulating hair cells
• Detect vertical (utricle) and horizontal (saccule) movements
• Send information to brain for posture adjustment

A typical coordination response follows this sequence:

1. Stimulus — Detected by sensory receptor
2. Transduction — Stimulus converted to nerve impulse in receptor
3. Transmission — Impulse travels via sensory neurone to CNS
4. Integration — CNS processes information
5. Response — Motor neurone carries impulse to effector
6. Action — Muscle contracts or gland secretes

In Tanzania, understanding the physiology of coordination helps explain everyday activities. For example, when a student reads from a chalkboard in class, the eyes must accommodate to focus distant letters — this involves coordinated changes in lens shape controlled by ciliary muscles. Similarly, farmers handling livestock need to understand that animals respond to sounds and touches through their nervous systems; loud noises or improper handling can trigger stress responses because the animals' sensory systems detect and transmit these stimuli rapidly through neural pathways, affecting their welfare and productivity.