Source: The Biological Mind, pp. 111–115 (4-3a), p. 130 (4-4b), pp. 136–143 (4-5b, 4-5c)
Tags: spinal cord, brainstem, cerebellum, autonomic nervous system, sympathetic, parasympathetic, neural signaling, action potential, neurotransmitters, synapse, dopamine, serotonin, GABA, acetylcholine, reuptake, Psychology 1100, Ohio State
Difficulty: Intermediate Prerequisites: A basic understanding that the nervous system is divided into central and peripheral components. If you are unsure what "CNS" and "PNS" mean, review the nervous system overview before reading these notes.
This chapter covers the biological machinery that makes behaviour possible. Four frequently missed topics live here: the structures of the spinal cord, brainstem, and cerebellum; the autonomic nervous system and its two branches; how neurons communicate through electrical and chemical signals; and the major neurotransmitters and what they do. These topics are densely factual, which is likely why so many students missed them. If you can trace a signal from a sensory receptor through the spinal cord to the brain and back out to a muscle, and explain what chemicals are involved at each synapse, you have the core of this chapter.
The spinal cord, brainstem, and cerebellum handle reflexes, basic life functions, and movement coordination. The autonomic nervous system controls involuntary processes through two opposing branches (sympathetic for arousal, parasympathetic for calm). Neurons communicate via electrical impulses (action potentials) that trigger chemical messengers (neurotransmitters) at synapses. Different neurotransmitters serve different functions, from movement to mood.
Spinal cord
A long, thin bundle of neural tissue running from the brainstem down through the vertebral column. It carries sensory information up to the brain and motor commands down to the body, and it manages simple reflexes on its own.
Brainstem
The stalk-like structure connecting the spinal cord to the rest of the brain, comprising the medulla, pons, and midbrain. Think of it as the life-support system: it handles breathing, heart rate, and other functions you do not consciously control.
Medulla (medulla oblongata)
The lowest part of the brainstem, directly above the spinal cord. It regulates heart rate, blood pressure, breathing, and reflexes like swallowing and vomiting. Damage here is typically fatal.
Pons
The brainstem structure above the medulla, involved in sleep, arousal, and relaying information between the cerebellum and the cortex. In simple terms, it is a bridge (pons means "bridge" in Latin) that connects lower and higher brain regions.
Midbrain
The uppermost part of the brainstem, involved in eye movement, auditory and visual processing, and motor control. It contains the substantia nigra, a structure relevant to Parkinson's disease.
Cerebellum
The "little brain" at the back of the skull, responsible for coordination, balance, and fine motor control. Think of it as the quality-control centre for movement: it does not initiate actions, but it makes them smooth and precise.
Reflex arc
The neural pathway that controls a reflex. In its simplest form: sensory neuron detects stimulus, interneuron in the spinal cord processes it, motor neuron triggers a response, all before the brain is even aware of what happened.
Autonomic nervous system (ANS)
The division of the peripheral nervous system that controls involuntary bodily functions (heart rate, digestion, pupil dilation). It operates largely outside conscious awareness.
Sympathetic nervous system
The ANS branch that prepares the body for action during perceived threat or excitement. Often summarised as "fight or flight." It increases heart rate, dilates pupils, inhibits digestion, and releases adrenaline.
Parasympathetic nervous system
The ANS branch that calms the body and conserves energy. Often summarised as "rest and digest." It slows heart rate, constricts pupils, and promotes digestion.
Neuron
A nerve cell specialised for receiving, processing, and transmitting information via electrical and chemical signals. The three main types are sensory neurons, motor neurons, and interneurons.
Action potential
A brief electrical charge that travels along a neuron's axon when the neuron fires. Think of it as a rapid wave of electrical activity that moves in one direction, from the cell body to the axon terminals.
Resting potential
The electrical charge of a neuron when it is not firing, typically around -70 millivolts. The inside of the cell is negatively charged relative to the outside.
Threshold
The level of stimulation (approximately -55 mV) a neuron must reach for an action potential to fire. Below threshold, nothing happens. At or above threshold, the neuron fires fully.
All-or-none principle
The rule that a neuron either fires completely or not at all. There is no "partial" action potential. The strength of a stimulus is communicated by the rate of firing, not by bigger signals.
Synapse
The tiny gap between two neurons where chemical communication occurs. The sending neuron releases neurotransmitters into this gap; the receiving neuron picks them up.
Neurotransmitter
A chemical messenger released at the synapse that influences whether the next neuron will fire. In simple terms, these are the molecules that carry a signal across the gap between neurons.
Reuptake
The process by which a neurotransmitter is absorbed back into the sending neuron after it has done its job. Many psychiatric medications (such as SSRIs) work by blocking this process, leaving more neurotransmitter in the synapse.
Acetylcholine (ACh)
A neurotransmitter involved in muscle contraction, memory, and learning. It is the primary messenger at the neuromuscular junction: every time you move a voluntary muscle, ACh is involved.
Dopamine
A neurotransmitter linked to reward, motivation, and motor control. Too little dopamine in certain brain areas is associated with Parkinson's disease; dysregulation of dopamine pathways is implicated in schizophrenia and addiction.
Serotonin
A neurotransmitter involved in mood regulation, sleep, and appetite. Low serotonin activity is associated with depression, which is why SSRIs (selective serotonin reuptake inhibitors) are a common treatment.
GABA (gamma-aminobutyric acid)
The brain's primary inhibitory neurotransmitter. It reduces neural activity and calms the nervous system. Anxiety disorders are linked to insufficient GABA activity; benzodiazepines (like Valium) enhance GABA's effects.
Glutamate
The brain's primary excitatory neurotransmitter. It is involved in learning and memory. Too much glutamate activity can damage neurons (excitotoxicity).
Endorphins
Neurotransmitters that reduce the perception of pain and produce feelings of well-being. The "runner's high" is attributed to endorphin release. Opiate drugs (morphine, heroin) mimic endorphins by binding to the same receptors.
Norepinephrine (noradrenaline)
A neurotransmitter involved in arousal, alertness, and the fight-or-flight response. It overlaps functionally with epinephrine (adrenaline) and is part of the sympathetic nervous system's activation cascade.
The spinal cord is the communication highway between the brain and the body.
Sensory (afferent) pathways carry information from the body up to the brain.
Motor (efferent) pathways carry commands from the brain down to the muscles.
Some responses (reflexes) are handled entirely at the spinal cord level, without input from the brain. When you touch a hot pan, your hand pulls away before you consciously feel pain.
The brainstem sits at the base of the brain and handles survival basics:
The medulla controls breathing, heart rate, and blood pressure. It also manages reflexes such as coughing and sneezing.
The pons plays a role in sleep and arousal and relays signals between the cerebellum and the cerebral cortex.
The midbrain processes auditory and visual reflexes (turning your head toward a loud sound) and contains dopamine-producing neurons relevant to motor control.
The cerebellum sits behind the brainstem and contains more neurons than the rest of the brain combined.
It coordinates voluntary movements, balance, and posture.
Damage to the cerebellum does not paralyse you, but your movements become clumsy and uncoordinated (ataxia).
Recent research suggests the cerebellum also plays a role in cognitive functions like language and attention, though this is less likely to be tested at the introductory level.
The ANS is one of two major divisions of the peripheral nervous system (the other being the somatic nervous system, which controls voluntary movement).
It has two branches that typically work in opposition:
Sympathetic: activates the body's resources for emergency action. Heart rate increases, pupils dilate, digestion slows, glucose is released into the bloodstream. Think of this as the accelerator.
Parasympathetic: returns the body to baseline after the threat passes. Heart rate decreases, pupils constrict, digestion resumes. Think of this as the brake.
The two systems are not strictly "one on, one off." Both are active to some degree at all times, maintaining a balance called autonomic tone.
The adrenal glands release epinephrine (adrenaline) and norepinephrine as part of the sympathetic response, which is why the effects of sympathetic activation can linger after the threat is gone (the chemicals take time to clear).
At rest, a neuron maintains a resting potential of about -70 mV. This is maintained by the sodium-potassium pump, which pushes 3 sodium ions out for every 2 potassium ions it pulls in.
When a neuron is stimulated:
If the stimulus is strong enough to reach threshold (about -55 mV), voltage-gated sodium channels open and sodium rushes in, making the inside of the cell positive. This is depolarisation.
At the peak of the action potential (about +30 mV), sodium channels close and potassium channels open. Potassium flows out, returning the cell toward its negative resting state. This is repolarisation.
The neuron briefly overshoots the resting potential (becomes more negative than -70 mV). This is hyperpolarisation, during which the neuron cannot fire again (the refractory period).
The sodium-potassium pump restores the original ion balance.
The action potential follows the all-or-none principle. If threshold is reached, the neuron fires at full strength. If not, it does not fire at all.
Myelin, a fatty insulating layer around some axons, speeds up signal transmission. The signal "jumps" between gaps in the myelin (nodes of Ranvier) in a process called saltatory conduction.
Diseases that damage myelin (such as multiple sclerosis) slow or block neural signals, leading to motor and sensory problems.
When an action potential reaches the axon terminal, it triggers the release of neurotransmitters from vesicles into the synaptic cleft (the gap).
Neurotransmitters cross the gap and bind to receptors on the postsynaptic neuron, like a key fitting a lock.
The effect can be excitatory (making the next neuron more likely to fire) or inhibitory (making it less likely to fire).
After binding, neurotransmitters are cleared from the synapse by:
Reuptake: the sending neuron reabsorbs the neurotransmitter.
Enzymatic degradation: enzymes in the synapse break the neurotransmitter down (e.g., acetylcholinesterase breaks down ACh).
Diffusion: the neurotransmitter drifts away from the synapse.
Acetylcholine (ACh): muscle contraction, memory, learning. Involved at every neuromuscular junction. Reduced ACh activity is associated with Alzheimer's disease.
Dopamine: reward, motivation, motor control. The mesolimbic pathway (reward) and nigrostriatal pathway (movement) are the two most tested dopamine circuits.
Serotonin: mood, sleep, appetite, pain. Low activity linked to depression; SSRIs increase serotonin availability.
Norepinephrine: alertness, arousal, fight-or-flight. Closely related to the sympathetic nervous system.
GABA: the main inhibitory neurotransmitter. Calms neural activity. Enhanced by alcohol and benzodiazepines.
Glutamate: the main excitatory neurotransmitter. Critical for learning and memory. Excess glutamate can cause excitotoxicity (cell damage or death).
Endorphins: natural painkillers. Bind to the same receptors as opioid drugs. Released during exercise, stress, and injury.
Sequence of the action potential (memorise this order):
Resting state (-70 mV) → Stimulus reaches threshold (-55 mV) → Sodium channels open, sodium rushes in (depolarisation, rises to ~+30 mV) → Sodium channels close, potassium channels open (repolarisation) → Potassium overshoots (hyperpolarisation) → Sodium-potassium pump restores resting potential
Sympathetic vs. Parasympathetic at a glance:
Function | Sympathetic | Parasympathetic |
|---|---|---|
Heart rate | Increases | Decreases |
Pupils | Dilate | Constrict |
Digestion | Inhibits | Promotes |
Breathing | Faster, deeper | Slower |
Glucose release | Increases | Decreases |
The action potential and neurotransmitter system are the foundation for understanding how psychiatric medications work. SSRIs block serotonin reuptake, L-DOPA increases dopamine availability for Parkinson's patients, and benzodiazepines enhance GABA to treat anxiety. Understanding the sympathetic nervous system also explains why panic attacks produce physical symptoms like racing heart, sweating, and tunnel vision: your body is running its emergency protocol even when there is no physical threat.
"The sympathetic nervous system is bad and the parasympathetic is good." Both are essential. Sympathetic activation keeps you alive in emergencies; parasympathetic dominance at the wrong time would leave you unable to respond to danger.
"Neurons fire at different strengths depending on the stimulus." They do not. The all-or-none principle means each action potential is the same magnitude. Intensity is coded by firing rate and the number of neurons firing, not by signal size.
"Neurotransmitters only have one function." Most neurotransmitters are involved in multiple systems. Dopamine, for instance, is relevant to both reward and motor control. Serotonin affects mood, sleep, and appetite.
"Reuptake removes the neurotransmitter permanently." Reuptake recycles the neurotransmitter back into the sending neuron for future use. It is conservation, not disposal.
⚠️ Know the functions of the medulla, pons, midbrain, and cerebellum individually. Exam questions often describe a symptom and ask which structure is affected.
⚠️ Be able to distinguish sympathetic from parasympathetic effects for specific organs (heart, pupils, digestion). A table format is helpful for studying this.
⚠️ The steps of the action potential are heavily tested. Memorise the sequence: resting potential, threshold, depolarisation, repolarisation, hyperpolarisation, refractory period.
⚠️ Match each neurotransmitter to its primary functions and associated disorders. "Which neurotransmitter is most associated with..." is a common question format.
⚠️ Understand how drugs interact with neurotransmitters (agonists increase activity, antagonists decrease it, reuptake inhibitors keep the neurotransmitter in the synapse longer).
True or False: The cerebellum initiates voluntary movements.
A: False. The cerebellum coordinates and fine-tunes movements that are initiated by the motor cortex.
Fill in the blank: The resting potential of a typical neuron is approximately ________ mV.
A: -70 mV.
True or False: The parasympathetic nervous system dilates the pupils.
A: False. The sympathetic nervous system dilates the pupils; the parasympathetic system constricts them.
Fill in the blank: The brain's primary inhibitory neurotransmitter is ________.
A: GABA.
True or False: An action potential can vary in magnitude depending on the strength of the stimulus.
A: False. The all-or-none principle means the action potential is always the same magnitude once threshold is reached.
Q: A patient has damage to the medulla. What vital functions are most likely to be affected?
A: Heart rate, blood pressure, and breathing. The medulla controls these basic life-sustaining functions, and damage is often fatal.
Q: How does the sympathetic nervous system prepare the body for an emergency?
A: It increases heart rate, dilates pupils, inhibits digestion, releases glucose for energy, and triggers adrenaline release. These changes redirect resources toward muscles and away from non-essential functions.
Q: Describe the sequence of ion movements during an action potential.
A: At threshold, sodium channels open and sodium rushes into the cell (depolarisation). At peak, sodium channels close and potassium channels open, allowing potassium to flow out (repolarisation). The cell briefly becomes more negative than resting potential (hyperpolarisation) before the sodium-potassium pump restores the resting state.
Q: An SSRI blocks the reuptake of serotonin. What effect does this have at the synapse?
A: More serotonin remains in the synaptic cleft for a longer period, increasing its availability to bind to receptors on the postsynaptic neuron. This is why SSRIs are used to treat depression, which is associated with low serotonin activity.
Q: A person with Parkinson's disease has difficulty initiating smooth movements. Which neurotransmitter is most likely deficient, and in which brain pathway?
A: Dopamine, in the nigrostriatal pathway. This pathway runs from the substantia nigra (in the midbrain) to the basal ganglia and is critical for motor control.
Neural signaling and neurotransmitters connect directly to the psychoactive drugs unit in the Aware Mind chapter (drugs alter neurotransmitter activity). They also underpin the sensation and perception material (sensory neurons generate action potentials in response to stimuli) and the learning and memory material (long-term potentiation involves changes at the synapse). The autonomic nervous system reappears in discussions of stress, emotion, and psychological disorders throughout the course.
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