Definition
A stimulus is any detectable change in the internal or external environment that triggers a response — examples include light (detected by the eye), heat, a sound, or a chemical change in the blood.
A neuron has dendrites (short, branching extensions that receive signals from other neurons) and an axon (a single, usually longer extension that carries the impulse away towards the next neuron or an effector) — the key structural difference being that dendrites bring signals in, while the axon sends the signal out.
Schwann cells are the supporting cells for neurons; they wrap around the axon to make the myelin sheath, an insulating layer that speeds up nerve impulse transmission along the axon.
Definition
Three types of neurons: sensory neurons carry impulses from a receptor towards the central nervous system (CNS); motor neurons carry impulses from the CNS towards an effector (muscle or gland); inter-neurons (relay neurons) connect sensory and motor neurons within the CNS itself. All three share the same basic structure (cell body, dendrites, axon) and transmit impulses the same basic way, but differ in exactly where they carry signals to and from within the nervous pathway.
Definition
A synapse is the small gap between two neurons (or between a neuron and an effector cell), across which nerve impulses pass via chemical messengers called neurotransmitters — released by the first neuron, diffusing across the gap, and binding to receptors on the next cell to continue (or sometimes block) the signal.
Definition
The forebrain includes the cerebrum (responsible for conscious thought, voluntary movement, and interpreting sensory information — different lobes handle different functions, e.g. the occipital lobe contains the regions responsible for vision and colour perception), the thalamus (a relay centre for sensory information passing to the cerebrum), and the hypothalamus (regulates body temperature, and controls sensations of hunger, thirst, and sleep, alongside linking the nervous system to the endocrine system).
The midbrain relays information between the forebrain and hindbrain and plays a role in visual and auditory reflexes.
The hindbrain includes the cerebellum (controls balance and fine coordination of movement — this is exactly why it's the part responsible for balance while riding a bicycle) and the medulla oblongata (controls vital, involuntary functions like breathing rate and heart rate — an injury here puts these basic life-support functions most at risk).
Example 1 — locating memory function
If a patient cannot remember things, which part of their brain may be affected?
- Memory formation and recall are primarily associated with regions of the cerebrum (particularly areas linked to the hippocampus, part of the forebrain's structures).
Answer: The cerebrum (forebrain), particularly memory-related regions
Definition
Meninges are three protective membrane layers surrounding the brain and spinal cord; their function is to protect these delicate structures from mechanical damage and infection, and to contain the cerebrospinal fluid that cushions them further.
Definition
The somatic nervous system controls voluntary movements (skeletal muscles, under conscious control). The autonomic nervous system controls involuntary functions (like heart rate, digestion, and gland activity) automatically, without conscious control.
Based on the axons they contain: sensory nerves contain only sensory neuron axons (carrying signals towards the CNS); motor nerves contain only motor neuron axons (carrying signals away from the CNS); mixed nerves contain both sensory and motor axons together.
Humans have pairs of cranial nerves, which connect directly to the brain (rather than the spinal cord) and mostly serve the head and neck region.
Definition
A reflex action is a rapid, automatic, involuntary response to a stimulus, not requiring conscious thought. The reflex arc is the specific, short neural pathway a reflex signal travels: receptor sensory neuron (relay neuron in the spinal cord) motor neuron effector.
Reflex actions are faster than normal (voluntary) actions because the reflex arc's signal typically only needs to travel to the spinal cord and back, bypassing the brain's more complex processing entirely — this shorter pathway with fewer synapses to cross means the response happens with minimal delay, which is exactly why the spinal cord's role in reflex actions is so important: it lets the body react to danger (like touching something hot) before the brain has even fully registered what happened.
Example 2 — a reflex action and its arc
Describe an example of a reflex action and identify its reflex arc.
- Example: quickly withdrawing a hand after touching a hot object.
- Reflex arc: heat receptors in the skin detect the stimulus a sensory neuron carries the impulse to the spinal cord a relay neuron in the spinal cord passes it to a motor neuron the motor neuron carries the impulse to the arm's muscles (effector) the muscles contract, withdrawing the hand.
Answer: Hot object receptor sensory neuron relay neuron (spinal cord) motor neuron muscle (effector) — hand withdrawn
Remember
During danger, the autonomic nervous system's sympathetic branch activates quickly, immediately triggering physical changes (increased heart rate, faster breathing, adrenaline release) that prepare the body to respond, faster than a slower, considered conscious decision could.
Definition
Hormones are the chemical messengers of the endocrine system, released directly into the blood by endocrine glands and carried to target organs/tissues elsewhere in the body, where they produce a specific effect.
The pancreas acts as both an exocrine gland (releasing digestive enzymes via a duct into the small intestine) and an endocrine gland (releasing hormones like insulin and glucagon directly into the blood) — this dual role makes it a notable exception among glands that are usually purely one or the other.
Definition
The pituitary gland (often called the 'master gland') releases several hormones, including growth hormone (regulates body growth), and others that in turn control the activity of several other endocrine glands.
Excess growth hormone in childhood causes gigantism (excessive height/growth), while too little growth hormone in childhood causes dwarfism (stunted growth).
Definition
Goitre (enlargement of the thyroid gland) is commonly caused by a deficiency of iodine in the diet, since the thyroid needs iodine to produce its hormones, and the gland enlarges as it works harder trying to compensate for insufficient hormone production.
If parathyroid hormone were missing, blood calcium levels would fall dangerously low, since the parathyroid glands are specifically responsible for raising and regulating blood calcium — without this hormone, processes depending on calcium (like nerve function and muscle contraction, including the heart) would be seriously disrupted.
Definition
The adrenal glands release adrenaline, which prepares the body for the 'fight or flight' response — increasing heart rate, blood pressure, and blood glucose availability, and redirecting blood flow to muscles, all to ready the body for immediate physical action.
Adrenaline is called an 'emergency hormone' because it's released rapidly in response to sudden stress or danger, producing an immediate, whole-body physiological response geared entirely towards handling that emergency.
Definition
The pancreas releases insulin (lowers blood glucose, by promoting its uptake into cells and storage as glycogen) and glucagon (raises blood glucose, by promoting glycogen breakdown) — together keeping blood glucose within a healthy range.
If the pancreas releases too much insulin, blood sugar falls too low (hypoglycaemia), which can cause symptoms like weakness, confusion, and, if severe, loss of consciousness.
In type 1 diabetes mellitus, the pancreas fails to produce the required insulin at all (usually due to autoimmune destruction of the insulin-producing cells), requiring lifelong insulin treatment — this differs from type 2 diabetes, where the body still produces some insulin but responds to it poorly.
Definition
Ovaries produce oestrogen and progesterone, regulating the female reproductive cycle and secondary sexual characteristics. Testes produce testosterone, regulating sperm production and male secondary sexual characteristics.
Quick Recap
- ●Neuron: dendrites (in) axon (out) Schwann cells (myelin sheath); sensory/motor/inter-neurons; synapse neurotransmitters connect neurons
- ●Forebrain: cerebrum (thought, movement, vision in occipital lobe), thalamus (relay), hypothalamus (temp, hunger/thirst/sleep). Hindbrain: cerebellum (balance), medulla (breathing/heart rate — vital)
- ●Reflex arc: receptor sensory neuron relay neuron (spinal cord) motor neuron effector — fast because it bypasses the brain
- ●Somatic (voluntary) vs autonomic (involuntary) nervous system; pairs of cranial nerves; meninges protect brain/spinal cord
- ●Hormones: chemical messengers via blood. Pancreas exocrine endocrine. Pituitary master gland (growth hormone: excess gigantism, deficit dwarfism)
- ●Thyroid/iodine deficiency goitre; parathyroid blood calcium; adrenaline fight-or-flight ('emergency hormone')
- ●Insulin (lowers glucose) glucagon (raises glucose); too much insulin hypoglycaemia; type 1 diabetes no insulin produced at all
