The nervous system

Central vs Peripheral Nervous System

The CNS comprises the brain and spinal cord (enclosed in bone for protection). The PNS comprises all nerves outside the CNS, including sensory neurons carrying information in (afferent) and motor neurons carrying commands out (efferent). The spinal cord is segmented: each vertebral segment receives sensory input from and sends motor output to a specific body region.

Glial Cells

Non-neuronal cells making up approximately half of all brain cells. Oligodendrocytes (CNS) and Schwann cells (PNS) produce myelin. Astrocytes maintain the blood-brain barrier, regulate the chemical environment, mop up excess glutamate (preventing excitotoxicity), and release neurotrophic factors. Microglia are the brain’s immune cells – they survey the environment, clear debris by phagocytosis, and respond to damage, but can also cause neuroinflammation. Ependymal cells line the brain ventricles and regulate cerebrospinal fluid.

Evolution of Nervous Systems

Hydra/cnidarians: nerve net (diffuse, no superhighways). Starfish: circular nerve ring with radial branches. Planaria: cerebral ganglia (‘brain’) + paired ventral nerve cords with lateral connections (~20,000 neurons). Arthropods/insects: centralised brain + segmented ventral nerve cord. Vertebrates: large brain + spinal cord with segmental nerve roots. Complexity increases with cephalisation and larger cerebrum.

Autonomic Nervous System

Controls involuntary body functions. Sympathetic division: fight-or-flight response – increases heart rate, respiration, blood flow to muscles; decreases digestion. Parasympathetic division: rest-and-digest – slows heart rate, promotes digestion and energy conservation. These two divisions generally have opposing effects on the same organs.

Vertebrate Brain Regions

Hindbrain (brainstem/medulla oblongata): controls automatic survival functions (heart rate, breathing, blood pressure). Midbrain: processing relay station routing sensory information to appropriate brain areas. Cerebellum: coordinates and fine-tunes motor movements, timing, spatial awareness; does not initiate movement. Cerebrum/forebrain: higher-order processing, learning, memory, language, decision-making; proportionally larger in more complex vertebrates.

Cerebral Cortex Lobes

Frontal lobe: decision-making, planning, personality, risk assessment; contains the motor cortex for voluntary movement initiation; last brain region to mature (mid-to-late 20s). Parietal lobe: somatosensory cortex (processing touch, pain, temperature); spatial perception. Temporal lobe: auditory processing; language comprehension (Wernicke’s area). Occipital lobe: visual processing (visual cortex at the back of the brain).

Lateralisation and Contralateral Control

The left hemisphere controls the right side of the body and vice versa. The corpus callosum is the main white-matter highway connecting the two hemispheres. In most right-handed people, the left hemisphere is dominant for language.

Afferent vs Efferent Neurons

Afferent (sensory) neurons carry information from the periphery into the CNS. Efferent (motor) neurons carry commands from the CNS to muscles and organs. Interneurons connect afferent and efferent neurons within the CNS and are involved in processing and integration.

Central nervous system (CNS)

The brain and spinal cord; the main processing and integration centre of the nervous system.

Peripheral nervous system (PNS)

All nerves and ganglia outside the brain and spinal cord; connects the CNS to the body.

Afferent neuron

A sensory neuron that carries information from the periphery into the CNS.

Efferent neuron

A motor neuron that carries commands from the CNS to muscles or glands.

Interneuron

A neuron within the CNS that connects sensory and motor neurons; involved in information processing and reflex arcs.

Glial cell

A non-neuronal cell in the nervous system that supports, protects, and maintains neurons; includes astrocytes, oligodendrocytes, microglia, and Schwann cells.

Astrocyte

A star-shaped glial cell that maintains the blood-brain barrier, regulates the chemical environment, and provides neurotrophic support.

Microglia

The resident immune cells of the brain; perform surveillance, phagocytosis of debris, and can mediate neuroinflammation.

Blood-brain barrier

A selective barrier formed by tight junctions in brain blood vessels and astrocyte end-feet; prevents most pathogens and large molecules from entering the brain.

Ependymal cell

A glial cell lining the ventricles of the brain; involved in producing and circulating cerebrospinal fluid.

Cerebrospinal fluid (CSF)

A clear fluid filling the ventricles and surrounding the brain and spinal cord; provides cushioning, nutrients, and waste removal.

Autonomic nervous system

The division of the PNS that controls involuntary body functions (heart rate, digestion, breathing).

Sympathetic nervous system

The ‘fight-or-flight’ division; increases heart rate, respiration, and blood flow to muscles during stress.

Parasympathetic nervous system

The ‘rest-and-digest’ division; decreases heart rate, promotes digestion, and conserves energy.

Cerebrum

The largest part of the brain; responsible for higher-order processing, learning, memory, language, and voluntary movement.

Cerebral cortex

The outer layer of the cerebrum; composed of grey matter (cell bodies and synapses); highly folded to increase surface area.

Grey matter

Brain tissue composed primarily of neuronal cell bodies, dendrites, and synapses; found in the cortex and deep nuclei.

White matter

Brain tissue composed primarily of myelinated axons; appears white due to the fatty myelin; connects brain regions.

Cerebellum

A separate brain lobe that coordinates motor movements, balance, timing, and spatial awareness; does not initiate movement.

Brainstem / Medulla oblongata

The hindbrain region controlling automatic survival functions: heart rate, respiration, blood pressure.

Thalamus

A midbrain structure serving as the gateway to the cortex; relays sensory information to appropriate cortical areas.

Corpus callosum

A large bundle of white matter connecting the left and right cerebral hemispheres.

Motor cortex

A strip of cortex anterior to the central sulcus that initiates voluntary skeletal muscle movements.

Somatosensory cortex

A strip of cortex posterior to the central sulcus that processes touch, pain, temperature, and proprioception.

Broca’s area

A cortical region (frontal lobe) involved in speech production and language processing.

Nerve net

The simplest nervous system: a diffuse, non-centralised network of neurons found in cnidarians (e.g., Hydra).

Ganglion (pl. ganglia)

A cluster of neuronal cell bodies outside the CNS; or in invertebrates, a simple ‘brain’ structure.

Neuronal Plasticity

The brain is not fixed – it continuously remodels its connections in response to activity. Connections that are used frequently become stronger (more synapses, larger synapses); connections that are unused weaken and may be eliminated. This ‘use it or lose it’ principle applies to learning, motor skills, and sensory processing.

Long-Term Potentiation (LTP)

The molecular mechanism underlying learning and memory. At glutamatergic synapses: (1) First stimulus – glutamate binds NMDA receptors, but a Mg²⁺ block prevents ion flow. (2) Repeated stimulation – depolarisation removes the Mg²⁺ block; Ca²⁺ and Na⁺ enter through NMDA receptors. (3) Ca²⁺ triggers insertion of additional AMPA receptors into the postsynaptic membrane. (4) Subsequent stimulation – glutamate activates both NMDA and AMPA receptors, producing a much stronger postsynaptic response. This ‘supercharging’ of the synapse is why repeated study strengthens memory.

Memory Systems

Short-term memory is processed in the hippocampus (seconds to minutes). Consolidation into long-term memory requires repeated reinforcement (LTP) and transfer to distributed cortical areas. Memories are not stored in one place – different aspects (emotional, visual, auditory) are stored across multiple brain regions. Emotional significance enhances memory consolidation (limbic system involvement).

The Limbic System

A group of brain structures involved in emotion, memory, and motivation. Key components: hippocampus (short-term memory processing and consolidation), amygdala (fear and emotional responses), thalamus (sensory relay gateway), hypothalamus (hormonal regulation, homeostasis). Emotional states strongly influence which memories are formed and retained.

Synaptic Pruning

During early development, the brain produces an excess of synaptic connections. Between ages 2–4, unused connections are eliminated (pruned) while important ones are strengthened. This process is essential for efficient brain function. Abnormal pruning is implicated in neurological conditions: too few synapses pruned (autism spectrum – sensory overload due to excess connections) or too many pruned (schizophrenia – deficit of prefrontal connections).

Reflex Arcs

Reflexes bypass higher brain processing for speed. Sensory neurons carry information into the dorsal horn of the spinal cord; motor neurons exit from the ventral horn. The simplest reflex (monosynaptic, e.g., knee-jerk) involves only one synapse between sensory and motor neurons. Protective reflexes (muscle spindles, Golgi tendon organs) prevent muscle/tendon damage. Motor neurons conduct at up to 120 m/s – the fastest signalling in the body.

Neurotransmitter-Related Disorders

Parkinson’s disease: loss of dopaminergic neurons in the substantia nigra; causes tremors, poor balance, impaired motor coordination; treated with L-DOPA (dopamine precursor). Depression: decreased serotonin signalling; treated with SSRIs. Addiction: all drugs of abuse ultimately increase dopamine in the reward pathway but via different mechanisms – opioids inhibit inhibitory neurons (disinhibition); nicotine directly stimulates dopaminergic neurons; cocaine/amphetamines block dopamine reuptake. Chronic use depletes dopamine.

Brain Imaging Techniques

PET scans (positron emission tomography): use radio-labelled tracers to visualise which brain areas are active during specific tasks. fMRI (functional magnetic resonance imaging): measures oxygen consumption in real-time, revealing brain areas that are more active during specific functions. Both have been essential for mapping brain regions to functions.

Neuroplasticity

The capacity of the nervous system to remodel its connections in response to activity, experience, or injury.

Long-term potentiation (LTP)

A persistent strengthening of synaptic connections through repeated activation; the molecular basis of learning and memory.

NMDA receptor

A glutamate receptor/ion channel blocked by Mg²⁺ at rest; unblocked by repeated depolarisation; allows Ca²⁺ and Na⁺ entry; critical for LTP.

AMPA receptor

A glutamate receptor/ion channel that allows Na⁺ entry; additional AMPA receptors are inserted into the membrane during LTP, strengthening the synapse.

Hippocampus

A limbic system structure essential for short-term memory processing and consolidation into long-term memory.

Amygdala

A limbic structure involved in processing fear, rage, and emotional memories.

Limbic system

A group of brain structures (hippocampus, amygdala, thalamus, hypothalamus) collectively involved in emotion, memory, and motivation.

Synaptic pruning

The developmental process of eliminating unused synaptic connections to streamline brain function; occurs primarily in early childhood.

Reflex arc

A neural pathway that mediates a reflex; bypasses the brain for speed, processing at the spinal cord level.

Dorsal horn

The posterior portion of the spinal cord grey matter; receives sensory (afferent) information from the periphery.

Ventral horn

The anterior portion of the spinal cord grey matter; contains motor neurons that send efferent signals to muscles.

Muscle spindle

A sensory receptor embedded in muscle tissue that detects stretching; triggers reflexive contraction to prevent muscle damage.

Golgi tendon organ

A sensory receptor in tendons that detects tension; triggers reflexive relaxation to prevent tendon damage.

Substantia nigra

A midbrain structure containing pigmented dopaminergic neurons; their loss causes Parkinson’s disease.

Parkinson’s disease

A neurodegenerative disorder caused by loss of dopaminergic neurons in the substantia nigra; characterised by tremors and impaired motor control.

L-DOPA

A dopamine precursor used to treat Parkinson’s disease; crosses the blood-brain barrier and is converted to dopamine in the brain.

Disinhibition

The process by which inhibiting an inhibitory neuron results in increased activity of the downstream neuron; the mechanism by which opioids increase dopamine release.

PET scan

Positron Emission Tomography; a brain imaging technique using radiolabelled tracers to visualise metabolic activity.

fMRI

Functional Magnetic Resonance Imaging; a brain imaging technique measuring oxygen use to map active brain regions in real-time.

Excitotoxicity

Neuronal damage or death caused by excessive stimulation by glutamate; astrocytes help prevent this by clearing excess glutamate.

Motor cortex homunculus

A mapping of the motor cortex showing the disproportionate representation of body parts based on the precision of their motor control (e.g., hands and lips are overrepresented).