Biology notes, chapter 12
Coordination and Response in Humans
Textbook Biology Form 4 KSSM (DLP), pages 214–243
What this chapter covers
- {'Sequence and describe the components of human coordination': 'stimulus, receptor, integration centre, effector and response (12.1.1)'}
- Identify external and internal stimuli, list the types of sensory receptors and justify the need to respond (12.1.2–12.1.4)
- Describe the organisation of the nervous system and the functions of the parts of the brain and spinal cord (12.2)
- Draw and label sensory and motor neurones and explain how an impulse crosses a synapse (12.3)
- Compare voluntary and involuntary actions and describe two-neurone and three-neurone reflex arcs (12.4)
- Describe health issues of the nervous system and the effects of drug and alcohol abuse (12.5)
- State the hormones of each endocrine gland, explain the fight or flight response and compare the nervous and endocrine systems (12.6)
- Predict the effects of hormonal imbalance on health (12.7)
12.1Coordination and response
Stimulus — a change in the environment that is detected by an organism. Receptor — a special sensory cell that detects a stimulus and converts it to nerve impulses. Integration centre — the brain (or spinal cord), which interprets impulses and coordinates a response. Effector — a muscle or gland that produces the response.
Pathway of coordination: stimulus → receptor → sensory neurone → integration centre → motor neurone → effector → response.
| Type of stimulus | Examples |
|---|---|
| External | Light, sound, smell, taste, surrounding temperature, pressure, touch |
| Internal | Blood osmotic pressure, body temperature, blood sugar level |
Types of sensory receptor
| Receptor | Detects |
|---|---|
| Photoreceptor | Light |
| Thermoreceptor | Change in temperature |
| Chemoreceptor | Chemical substances |
| Baroreceptor | Change in pressure (e.g. blood pressure in the aortic arch and carotid artery) |
| Mechanoreceptor | Touch and pressure |
| Nociceptor | Pain |
Internal stimulus: blood pressure rises during running
Baroreceptors in the aortic arch and carotid artery detect the rise → sensory neurone → cardiovascular control centre in the medulla oblongata → motor neurone → heart muscle contracts more weakly and blood vessels widen → blood pressure returns to the normal range.Responding to stimuli is needed for survival: animals move to find shelter when the climate changes, and the body must correct internal changes (e.g. a rise in body temperature) to stay in the normal range.
You slap a mosquito that bites your leg. Name the receptor, the integration centre and the effector.
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Receptor: mechanoreceptor / nociceptor in the skin. Integration centre: brain (cerebrum). Effector: muscles of the hand.
12.2The nervous system
The nervous system has two subsystems. The central nervous system (CNS) is the brain and spinal cord. The peripheral nervous system (PNS) is 12 pairs of cranial nerves, 31 pairs of spinal nerves and the sensory receptors.
| Part of brain | Function |
|---|---|
| Cerebrum | Largest part; folded surface. Controls emotions, hearing, sight, personality and voluntary actions; learning, memory, language and mathematical skills. Receives information, interprets it and decides the response. |
| Hypothalamus | Controls body temperature, water balance and blood pressure; senses hunger, thirst and fatigue. Links the nervous and endocrine systems through the pituitary gland. |
| Cerebellum | Maintains body balance and coordinates muscle contraction for body movement. |
| Medulla oblongata | Controls involuntary actions: heartbeat, breathing, peristalsis, blood pressure, vomiting, coughing, sneezing, swallowing. |
| Pituitary gland | Main endocrine gland; its hormones control other endocrine glands. |
Spinal cord
- Lies inside the vertebral column, surrounded by cerebrospinal fluid.
- Grey matter (butterfly or H shape) holds cell bodies; white matter holds axons covered in myelin sheath.
- The dorsal root carries sensory neurones in (cell bodies in the dorsal root ganglion); the ventral root carries motor neurones out.
- Functions: processes some sensory information, controls reflex actions, connects the brain to the PNS.
Peripheral nervous system
The somatic nervous system controls voluntary actions. The autonomic nervous system controls involuntary actions such as heartbeat and contraction of blood vessels. Together they connect the receptors and effectors to the CNS.
Do not mix up the cerebellum and the cerebrum. Cerebrum = thinking and voluntary actions. Cerebellum = balance and smooth movement. The medulla oblongata is not in charge of thinking; it controls the automatic actions.
Why can we not stop ourselves from sneezing?
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Sneezing is an involuntary action controlled by the medulla oblongata through the autonomic nervous system, not by the cerebrum.
12.3Neurones and synapses
| Part of neurone | Function |
|---|---|
| Dendrite | Receives impulses and sends them to the cell body |
| Cell body | Has the nucleus; integrates signals and coordinates metabolism |
| Axon | Carries impulses away from the cell body |
| Myelin sheath | Insulates the axon, protects it and supplies nutrients |
| Node of Ranvier | Gap in the myelin sheath; impulses jump from node to node, so they travel faster |
| Synaptic knob | Swelling at the end of the axon; sends signals to muscle, gland or another neurone |
| Neurone | Where | Function |
|---|---|---|
| Sensory | Dorsal root; cell body in dorsal root ganglion | Carries impulses from receptors to the CNS |
| Relay | Entirely inside the CNS (grey matter) | Connects sensory neurone to motor neurone |
| Motor | Ventral root; cell body in grey matter | Carries impulses from the CNS to effectors |
Transmission across a synapse
The synapse is the narrow gap between a synaptic knob and the dendrite of the next neurone. Impulses cross it chemically, so transmission is slower and goes in one direction only.
- The electrical impulse reaches the synaptic knob and synaptic vesicles release neurotransmitter (e.g. acetylcholine, noradrenaline) into the synapse.
- The neurotransmitter diffuses across the synapse and binds to receptor proteins on the dendrite of the receiving neurone.
- Binding starts a new electrical impulse in the receiving neurone.
Textbook Figure 12.9, p. 225.
The synaptic knob contains many mitochondria because making and releasing neurotransmitter needs energy.
Why does an impulse travel in one direction only across a synapse?
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Only the synaptic knob has vesicles of neurotransmitter, and only the dendrite of the receiving neurone has the receptor proteins.
12.4Voluntary and involuntary actions
| Voluntary action | Involuntary action | |
|---|---|---|
| Awareness | Done consciously, by our own will | Automatic, without being aware |
| Nervous system | Somatic | Autonomic |
| Controlled by | Cerebral cortex (cerebrum) | Medulla oblongata and hypothalamus |
| Effector | Skeletal muscles | Smooth muscles and glands |
Both involve a stimulus, impulses, neurones and an effector.
Reflex action and the reflex arc
A reflex action is a fast, automatic response to a stimulus that is not controlled by the brain. The nerve pathway is the reflex arc.
Three-neurone arc (pricking a finger): receptor → sensory neurone (dorsal root) → relay neurone in the spinal cord → motor neurone (ventral root) → effector muscle. Two-neurone arc (knee jerk): stretch receptor → sensory neurone → motor neurone in the spinal cord → quadriceps muscle contracts. There is no relay neurone.
Textbook Figure 12.11, p. 228.
Importance: the response is quick, so injury is reduced, and the brain is free for higher thinking. Examples: coughing, pupil reflex.
A reflex is not the same as a voluntary action: the impulse goes through the spinal cord and does not wait for the cerebrum. The brain is told afterwards, which is why you feel the pain a moment later.
State the sequence of impulse transmission in the knee jerk reflex.
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Stretch receptor → sensory neurone → motor neurone (in spinal cord) → quadriceps muscle (effector).
12.5Health issues of the nervous system
| Condition | Key point |
|---|---|
| Multiple sclerosis | Immune system attacks the myelin sheath; impulses cannot pass properly |
| Alzheimer's disease | Loss of reasoning, memory and self-care; confusion |
| Parkinson's disease | Shrinkage of the nervous system; tremors, poor posture and balance |
| ALS (Lou Gehrig) | Motor neurones die; movement such as chewing and walking is lost |
| Epilepsy | Abnormal brain activity; unconsciousness, muscle spasms |
| Cerebral palsy | Brain damage before or after birth; poor muscle control |
| ADHD | Hyperactive, unable to concentrate |
| Autism | Problems communicating and interacting |
Drugs and alcohol
| Substance | Effect |
|---|---|
| Stimulant | Increases CNS activity; excess gives euphoria followed by depression |
| Sedative (depressant) | Slows transmission of impulses; calms the mind |
| Hallucinogen | Causes hallucination; relief from pain and anxiety |
| Narcotic | Slows the normal functions of the brain |
| Alcohol | Disrupts coordination and thinking; slows transfer of impulses |
A drunk person must not drive: slower impulses mean a slower response and poor coordination, so accidents are likely.
12.6The endocrine system
Endocrine glands are ductless glands that secrete hormones directly into the blood. A hormone affects only its target cells, which have specific receptors.
Textbook Figure 12.13, p. 233.
| Gland | Hormone | Function |
|---|---|---|
| Hypothalamus | GnRH | Stimulates the pituitary to secrete FSH and LH |
| Anterior pituitary | GH | Growth, protein synthesis, fat metabolism |
| FSH | Follicle development (ovary); spermatogenesis (testis) | |
| LH | Ovulation, corpus luteum, oestrogen and progesterone (ovary); testosterone (testis) | |
| TSH | Stimulates thyroid to secrete thyroxine | |
| ACTH | Stimulates adrenal cortex | |
| Posterior pituitary | Oxytocin | Uterine contraction at birth; milk production |
| ADH | Kidney tubule reabsorbs more water | |
| Thyroid | Thyroxine | Raises metabolic rate and body temperature; growth and development |
| Pancreas | Insulin | Lowers blood glucose; glucose converted to glycogen |
| Glucagon | Raises blood glucose; glycogen converted to glucose | |
| Adrenal | Adrenaline | Raises blood sugar, respiratory rate and heartbeat; contracts blood vessels |
| Aldosterone | Kidneys reabsorb salt | |
| Ovary | Oestrogen, progesterone | Female secondary sexual characteristics, repair of uterus wall; placenta formation |
| Testis | Testosterone | Male secondary sexual characteristics; spermatogenesis |
Fight or flight
In danger, the hypothalamus sends impulses to the adrenal medulla, which secretes adrenaline and noradrenaline. Heart rate, breathing rate, blood pressure, blood glucose and metabolism all rise, so more oxygen and glucose reach the brain and skeletal muscles. Both systems work together, and the body returns to normal when the danger has passed.
Nervous system versus endocrine system
| Nervous system | Endocrine system | |
|---|---|---|
| Signal | Electrical impulses along neurones | Hormones in the blood |
| Speed of response | Fast | Slow |
| Duration of effect | Short | Long |
| Area affected | One organ | Several organs (target cells) |
TSH and ACTH are "stimulating hormones" acting on other glands. GH, oxytocin and ADH act directly on target organs.
Which gland is called the main endocrine gland, and why?
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The pituitary gland, because its hormones control the secretion of other endocrine glands.
12.7Hormonal imbalance
| Condition | Cause | Effects |
|---|---|---|
| Diabetes mellitus | Too little insulin or insulin not used | High blood glucose, frequent urination, thirst, blurred vision, slow wound healing |
| Hypoglycaemia | Too much insulin | Low blood glucose, faintness, headache, fatigue |
| Diabetes insipidus | Too little ADH | Large volume of urine, thirst, dehydration |
| Hypothyroidism | Too little thyroxine | Slow heartbeat, sensitive to cold, tiredness, weight gain |
| Hyperthyroidism | Too much thyroxine | Sweating, heat intolerance, palpitations, weight loss |
| Gigantism | Too much GH in childhood | Abnormally long bones, very tall |
| Dwarfism | Too little GH in childhood | Bones grow slowly, body stays small |
Diabetes mellitus (sugar, insulin) and diabetes insipidus (water, ADH) have different causes. Goitre (enlarged thyroid) is linked to a lack of iodine, found in seafood.
A child has too little growth hormone. Name the condition and the gland involved.
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Dwarfism; anterior lobe of the pituitary gland.