Divisions of the Nervous System

Your nervous system is organised into two main parts that work together seamlessly. The Central Nervous System (CNS) consists of your brain and spinal cord, while the Peripheral Nervous System (PNS) connects the CNS to the rest of your body.

The brain serves as your body's command centre, processing sensory information, generating thoughts and emotions, initiating responses, and storing memories. Your spinal cord acts as the information highway, passing signals between your brain and PNS, and controlling reflex responses without needing brain input.

Within the PNS, you have two major divisions. The Autonomic Nervous System controls involuntary processes like heart rate and digestion, and is further divided into the Sympathetic (activating "fight or flight" responses) and Parasympathetic (promoting "rest and digest" functions). The Somatic Nervous System handles voluntary movements and carries sensory information from your sensory organs to the CNS.

Remember this! Think of your nervous system like a sophisticated computer network: the brain is the central processor, the spinal cord is the main cable, and the peripheral nerves are the connections to all your devices (organs and muscles).

Structure and Function of Neurons

Neurons are the communication specialists of your body, sending electrochemical messages throughout your nervous system. Though they all share the same basic structure, neurons come in three main types, each with a specific job.

Sensory neurons have a distinctive unipolar structure (one branch from the cell body) and transmit signals from your sensory receptors toward the CNS. When you touch something hot, these neurons rapidly send that information to your brain and spinal cord.

Relay neurons feature a multipolar structure (multiple branches from the cell body) and act as the middlemen, transferring impulses between sensory and motor neurons. These neurons are found entirely within your CNS.

Motor neurons also have a multipolar structure but serve the opposite function of sensory neurons. They carry commands from your CNS to muscles and glands, enabling both voluntary movements (like writing) and involuntary actions (like digestion).

Did you know? Your nervous system contains approximately 86 billion neurons, and a single neuron can connect with up to 10,000 other neurons, creating a vast communication network more complex than any computer system!

Synaptic Transmission

Synaptic transmission is how neurons talk to each other, and it's essential for everything from basic reflexes to complex thinking. This process happens at special junctions called synapses, where neurons come extremely close but don't actually touch.

When an electrical impulse reaches the end of the presynaptic neuron, it triggers vesicles to fuse with the membrane and release neurotransmitters into the tiny gap called the synaptic cleft. These chemical messengers then diffuse across the gap and bind to specific receptors on the postsynaptic neuron.

This chemical binding initiates electrical changes in the receiving neuron that can either encourage (excitatory) or discourage (inhibitory) the formation of a new action potential. If an action potential is triggered, the communication continues to the next neuron in the chain.

The overall response of a neuron depends on summation – the combined effect of all the excitatory and inhibitory messages it receives. This allows for incredibly complex processing, as a single neuron might be receiving signals from thousands of other neurons simultaneously.

Think about it: Every thought you have, every movement you make, and every sensation you feel depends on millions of these synaptic transmissions happening in precisely coordinated ways throughout your nervous system!

The Endocrine System

The endocrine system works alongside your nervous system to regulate bodily functions, but operates more slowly and with longer-lasting effects. This system consists of a collection of glands that release hormones directly into your bloodstream, affecting everything from growth to mood.

Each gland in the endocrine system has specific functions. The pituitary gland is often called the "master gland" as it controls the release of hormones from other glands using ACTH. The hypothalamus maintains homeostasis using CRH, while the pineal gland regulates your sleep patterns with melatonin.

Other important players include the thyroid (regulating metabolism with thyroxine), adrenal glands (producing adrenaline and cortisol for "fight or flight" responses), and reproductive glands like ovaries and testes (developing secondary sexual characteristics with oestrogen and testosterone).

Fascinating fact: While your nervous system communicates in milliseconds, hormonal messages from the endocrine system can take minutes or even hours to reach their targets – but their effects can last for days!

Structure of the Cerebral Cortex

The cerebral cortex, the wrinkled outer layer of your brain, consists of two hemispheres that look identical but handle different functions. These hemispheres are connected by a thick bundle of nerve fibres called the corpus callosum, which allows them to communicate and coordinate.

Each hemisphere is divided into four lobes with specialised functions. The frontal lobe handles planning and movement, the temporal lobe processes sound and language, the parietal lobe manages sensory information, and the occipital lobe deals with vision.

Specific functions are localised to particular areas within these lobes. The Primary Motor Cortex (frontal lobe) controls voluntary movement, while the Primary Somatosensory Cortex (parietal lobe) processes touch sensations. The Primary Visual Cortex (occipital lobe) interprets visual information, and the Primary Auditory Cortex (temporal lobe) handles sounds.

Language abilities are particularly localised in most people. Broca's area in the left frontal lobe handles speech production, and damage here causes difficulty speaking fluently. Wernicke's area in the left temporal lobe enables language comprehension, with damage leading to difficulty understanding what's heard or read.

Remember: While some brain functions are strictly localised, others are distributed across regions. This is why brain damage can affect people differently depending on exactly which areas are involved!

Brain Plasticity and Functional Recovery

Your brain has an amazing ability to change and adapt throughout your life. Plasticity refers to how the brain modifies its functions and structure in response to environmental changes, development, learning new skills, or recovering from trauma.

When brain damage occurs, functional recovery allows undamaged areas to take over functions previously performed by damaged regions. This happens through several mechanisms. Synaptic pruning strengthens frequently used connections while eliminating unused ones. Axonal sprouting allows existing neurons to grow new connections, while denervation supersensitivity makes remaining neurons more responsive.

Several factors affect recovery potential. Children generally recover better than adults due to greater plasticity. Women typically show better recovery than men. Rehabilitative therapy also significantly improves outcomes by actively engaging and strengthening alternative neural pathways.

Research supports these concepts. Maguire's study of London taxi drivers found their posterior hippocampus (involved in spatial memory) was larger than non-taxi drivers, and the size correlated with time on the job. Case studies like GB, who recovered language abilities after having his left hemisphere removed at age 2, demonstrate the remarkable adaptability of the young brain.

Boost your confidence: Understanding brain plasticity is empowering—it means your brain continues developing throughout your life! Every time you learn something new, your brain physically changes, creating and strengthening neural connections.

Split Brain Research

Split brain research has provided fascinating insights into how our brain hemispheres function. Scientists like Sperry conducted groundbreaking studies on patients who had their corpus callosum (the connection between brain hemispheres) surgically cut to treat severe epilepsy.

In these experiments, researchers carefully controlled what information each hemisphere could access. When shown images only to the right visual field (processed by the left hemisphere), patients could verbally describe what they saw. However, when shown images only to the left visual field (processed by the right hemisphere), patients couldn't verbally identify them but could select or draw them with their left hand.

Gazzaniga's research further revealed that the right hemisphere excels at facial recognition, while the left hemisphere specialises in language processing. These findings suggest our brain hemispheres are capable of independent functioning and have different specialisations.

This research has fundamentally changed our understanding of consciousness and identity. Rather than having a single unified mind, our experience appears to emerge from separate processes working together, with different abilities localised to specific brain regions.

Think deeper: Split brain research raises fascinating questions about the nature of consciousness. If the two hemispheres can operate independently, does this mean we have two separate "minds" within one brain? How does this affect our concept of a unified self?

Ways of Studying the Brain

Scientists use various techniques to study the brain, each with different strengths in spatial resolution (accuracy in identifying where activity occurs) and temporal resolution (accuracy in determining when activity happens).

Post-mortem dissection has been historically significant in understanding brain structure. Broca's area, for instance, was discovered by examining the brain of a patient who could only say "tan" after death. This method offers excellent spatial resolution down to the neuronal level, but can only establish correlational relationships between brain damage and behaviour.

fMRI (functional Magnetic Resonance Imaging) detects blood flow to active brain regions, offering good spatial resolution (1mm) while participants complete tasks. It's non-invasive and safe but has poor temporal resolution as images are taken every few seconds, missing rapid brain processes. The equipment is also expensive, and participants must remain still during scanning.

EEG (Electroencephalography) uses electrodes on the scalp to measure electrical activity of the brain cortex, displayed as brain waves. It offers excellent temporal resolution and is less expensive than fMRI, making it valuable for sleep research and experiments requiring movement. However, it has poor spatial resolution and cannot detect deep brain activity.

For your exams: Remember to evaluate brain imaging techniques using both spatial and temporal resolution. fMRI excels at where but not when, while EEG is great for when but not where. The perfect technique would have both, but currently none do!

Event-Related Potential and Circadian Rhythms

Event-Related Potential (ERP) is an advanced EEG technique that isolates brain responses to specific stimuli. By presenting a stimulus multiple times and statistically averaging the results, researchers can create a waveform showing exactly when cognitive processes occur after stimulus presentation. This method offers excellent temporal resolution (millisecond accuracy) but still suffers from poor spatial resolution.

Our bodies operate on various biological rhythms, with circadian rhythms being the most familiar 24-hour cycles. These rhythms are regulated by two key mechanisms: endogenous pacemakers (internal body clocks) and exogenous zeitgebers (external cues that adjust these clocks).

The sleep-wake cycle is primarily controlled by the Suprachiasmatic Nucleus (SCN) in the hypothalamus, often called the "master clock." When the SCN detects light, it signals the pineal gland to stop producing melatonin, the hormone that promotes sleepiness.

Research by Siffre, who spent six months in a cave without natural light, found that without external cues, the human sleep-wake cycle extends to about 25 hours. This suggests our internal clock naturally runs slightly longer than 24 hours and needs regular entrainment by external cues, primarily light. Additional research by Ralph confirmed the SCN's role by showing that transplanting the SCN between hamsters transferred their circadian rhythm patterns.

Practical application: Understanding circadian rhythms can help you optimise your sleep schedule and performance. Try to maintain consistent sleep times, get morning sunlight to reset your clock, and limit blue light exposure before bedtime to improve your sleep quality!

Infradian and Ultradian Rhythms

Beyond the daily circadian rhythms, your body follows longer infradian rhythms and shorter ultradian rhythms, each regulating different biological processes.

The menstrual cycle is a classic example of an infradian rhythm, typically lasting 28 days. This complex biological process involves releasing an egg during ovulation, thickening the womb lining, and shedding it during menstruation. Hormones like oestrogen and progesterone act as endogenous pacemakers, keeping these processes on schedule. Research into menstrual synchronisation among women living together (McClintock's study) has produced mixed results, with some studies supporting synchronisation and others finding no evidence for it.

Ultradian rhythms occur more frequently than once every 24 hours, with sleep stages being a prime example. Using EEG, scientists can identify distinctive brain wave patterns (delta, theta, alpha, beta, and gamma) that characterise different sleep stages. A complete sleep cycle passes through four stages: N1 (light sleep with theta waves), N2 (deeper sleep with slow theta waves), N3 (deepest sleep with delta waves), and REM (dream sleep with active brain patterns despite body paralysis).

Each sleep cycle lasts approximately 90 minutes and repeats 4-5 times during a typical night's sleep. Research suggests these stages serve different purposes – Shapiro found that ultramarathon runners showed increased N3 sleep, suggesting it aids physical recovery, while Haider observed increased REM sleep in patients recovering from drug overdoses, indicating it may support mental recovery.

Sleep tip: Understanding your sleep cycles can help you wake up feeling refreshed. Try planning your sleep in 90-minute multiples (6, 7.5, or 9 hours) to avoid waking during deep sleep stages, which can leave you feeling groggy!