brodidit for students
Home / Notes / Class 12 / Biology / Nervous coordination
Nervous coordination: Class 12 Biology
The organisation of the human nervous system, the neurone, resting and action potentials, synapses and neurotransmitters, reflex arc, the brain and spinal cord, autonomic nervous system, the eye and ear, and common nervous disorders.
Key points
Coordination is the working together of body parts. The nervous system gives fast, short lived responses by electrical impulses, while the endocrine system gives slower, longer lasting control by hormones. The nervous system has the central nervous system (brain and spinal cord) and the peripheral nervous system (nerves to and from the CNS).
The neurone is the unit of the nervous system. It has a cell body with the nucleus, short branched dendrites that receive impulses, and a long axon that carries impulses away. Many axons are wrapped in a fatty myelin sheath made by Schwann cells, with gaps called nodes of Ranvier. Myelin makes the impulse jump from node to node (saltatory conduction), which is much faster.
Types of neurones:
- Sensory neurones carry impulses from receptors to the CNS.
- Relay (intermediate) neurones lie in the CNS and connect sensory and motor neurones.
- Motor neurones carry impulses from the CNS to effectors, which are muscles and glands.
A resting neurone is polarised. The inside is negative compared with the outside, with a resting potential of about −70 mV. The sodium potassium pump moves 3 Na⁺ out for every 2 K⁺ in, and the membrane is more permeable to K⁺, so a negative charge builds up inside.
The action potential, in order:
- A stimulus makes sodium channels open. If the threshold (about −55 mV) is reached, many more open and Na⁺ rushes in. This is depolarisation, and the inside rises to about +30 mV.
- Sodium channels close and potassium channels open. K⁺ leaves, which is repolarisation.
- The potential briefly goes below resting level (hyperpolarisation) and then returns to −70 mV. This is the refractory period, when a new impulse cannot start, so impulses travel one way.
- The action potential follows the all or none law: it is always the same size, and a stronger stimulus only gives more frequent impulses.
A synapse is the junction between two neurones. The gap is the synaptic cleft, about 20 nm wide. When an impulse reaches the end of the presynaptic neurone, calcium ions enter and vesicles release neurotransmitter, such as acetylcholine, into the cleft. It diffuses across and binds to receptors on the postsynaptic membrane, causing a new impulse. The neurotransmitter is then broken down (acetylcholine by acetylcholinesterase) or taken back. Synapses make impulses go in one direction only.
A reflex action is a fast, automatic response to a stimulus that does not need thought. The path is the reflex arc: receptor, sensory neurone, relay neurone in the spinal cord, motor neurone, and effector. Examples are the knee jerk and pulling a hand from a hot object. Reflexes protect the body from harm.
The brain has three main parts. The cerebrum is the largest, with two hemispheres, and controls thinking, memory, speech and voluntary movement. The cerebellum controls balance and coordination of movement. The medulla oblongata in the brainstem controls breathing, heart rate and other involuntary actions. The hypothalamus controls temperature and links the nervous and endocrine systems. The spinal cord carries impulses between the brain and body and controls spinal reflexes. The brain is protected by the skull, three membranes called meninges, and cerebrospinal fluid.
The autonomic nervous system controls involuntary actions of heart, gut and glands. Its sympathetic part prepares the body for emergency (fight or flight): it raises heart rate and widens pupils. The parasympathetic part works in rest and digestion: it slows the heart and increases digestion. The two have opposite effects on most organs.
Sense organs contain receptors. The eye focuses light on the retina, where rods work in dim light and cones give colour vision. The optic nerve takes impulses to the brain. The ear detects sound with the cochlea and balance with the semicircular canals. Nervous disorders include Parkinson's disease, with lack of dopamine and tremors, and epilepsy, which is sudden abnormal electrical activity in the brain.
Study tip: Draw a motor neurone, a reflex arc and a synapse with every label, and learn the graph of an action potential with the voltage values and the ion movements at each phase. In long answers use the correct order of events and name the ion and the channel. Do not mix up hormonal and nervous control in comparison questions.
Definitions and flashcards
Say the meaning first, then open the card to check.
Neurone
A nerve cell that carries electrical impulses.
Myelin sheath
A fatty covering of an axon that speeds up impulse conduction.
Resting potential
The electrical difference across the membrane of an unstimulated neurone, about −70 mV.
Action potential
A brief reversal of the membrane potential that is the nerve impulse.
Synapse
The junction between two neurones.
Neurotransmitter
A chemical that carries the signal across a synapse.
Reflex arc
The pathway of a reflex from receptor to effector.
Effector
A muscle or gland that responds to nerve impulses.
Refractory period
The short time after an impulse when the neurone cannot fire again.
Short questions with model answers
These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.
What is the role of the myelin sheath? (2 marks)
Myelin insulates the axon and has gaps at the nodes of Ranvier. The impulse jumps from node to node, which makes conduction much faster and saves energy.
Define a reflex action and give one example. (2 marks)
A reflex action is a rapid, automatic and involuntary response to a stimulus that does not involve the thinking part of the brain. An example is the withdrawal of the hand from a hot object.
Name the parts of a reflex arc in order. (3 marks)
Receptor, sensory neurone, relay neurone in the spinal cord, motor neurone, effector. The stimulus is detected by the receptor, and the effector produces the response.
Explain why impulses move in one direction across a synapse. (3 marks)
Neurotransmitter vesicles are present only in the presynaptic terminal, and the receptors are only on the postsynaptic membrane. The transmitter can therefore pass only from the first neurone to the second. The transmitter is then removed by enzymes, so the signal ends.
Describe how a nerve impulse is transmitted along an axon. (5 marks)
At rest the inside of the axon is negative, about −70 mV, because of the sodium potassium pump and the leaky potassium channels. A stimulus opens sodium channels and Na⁺ enters, which depolarises that part to about +30 mV if the threshold is reached. This causes the next part of the membrane to depolarise, so the action potential passes along. Behind it the sodium channels close and the potassium channels open, K⁺ leaves, and the resting potential is restored. During the refractory period no new impulse can start, so the impulse goes in one direction. In myelinated axons it jumps between nodes of Ranvier.
Compare the sympathetic and parasympathetic systems. (3 marks)
The sympathetic system prepares the body for danger. It speeds the heart, widens the pupils, and slows digestion. The parasympathetic system acts at rest. It slows the heart, narrows the pupils, and increases digestion. They work in opposition to keep a balance.
Common mistakes
- Saying that sodium ions leave the axon during depolarisation. They enter.
- Writing that the action potential gets bigger with a stronger stimulus. It is all or none, and only the frequency changes.
- Confusing sensory and motor neurones, or the dendrite with the axon.
- Saying the neurotransmitter travels along the next neurone. It crosses the cleft and binds to receptors.
- Mixing up the cerebrum (thinking) and the cerebellum (balance).