Neural Control and Coordination | ISC Class 11 Biology Notes

This note covers neural coordination, neurons and nerves, divisions of the human nervous system, resting and action potentials, impulse conduction, synaptic transmission, the brain, spinal cord, visceral control, reflex pathways and selected nervous-system disorders.

How does neural coordination bring organs together?

Definition: Coordination is the process in which two or more organs interact and complement one another’s functions. Homeostasis means maintaining a relatively stable internal environment.

The neural system, or nervous system, provides organised connections for rapid communication between different parts of the body. Its specialised cells, called neurons, detect, receive and transmit stimuli. A stimulus is a change capable of producing a response.

What happens during physical exercise?

During exercise, greater muscular activity increases the demand for energy and oxygen. The rate of respiration, heart rate and blood flow increase. Muscles, lungs, heart, blood vessels and kidneys therefore participate in a coordinated response, rather than functioning independently.

After exercise stops, the activities of nerves, lungs, heart and kidneys gradually return to their normal conditions. The word gradually matters: coordination includes adjustment as the body’s requirements change.

How do neural and hormonal coordination relate?

The endocrine system consists of hormone-secreting glands and cells. Hormones are chemical messengers that help integrate organ activities. The neural and endocrine systems jointly coordinate the body; neural connections provide rapid communication between particular points.

Neural organisation varies among animals. Hydra has a network of neurons. Insects have a brain, neural tissues and ganglia, groups of nerve-cell bodies. Vertebrates, animals with a vertebral column, possess a more developed neural system.

How is the human nervous system organised?

The human nervous system has two main divisions. The central nervous system (CNS) comprises the brain, the central information-processing organ, and spinal cord, the elongated nervous structure continuous with it, and processes information. The peripheral nervous system (PNS) comprises the nerves associated with the brain and spinal cord.

A nerve is a bundle of nerve fibres in the peripheral nervous system, held together by connective tissue. A nerve fibre is an axon, a neuron’s long process that carries impulses away from its cell body, with associated coverings. A nerve therefore contains parts of many neurons.

Which way do impulses travel?

A nerve impulse is an electrical signal carried by a neuron. Afferent fibres carry impulses from tissues or organs towards the CNS. Efferent fibres carry regulatory impulses from the CNS towards peripheral tissues or organs.

What are the somatic and autonomic divisions?

The somatic nervous system relays impulses from the CNS to skeletal muscles, the muscles associated with movement of the skeleton. The autonomic nervous system carries impulses to involuntary organs and smooth muscles, which work without conscious direction.

The autonomic system has sympathetic and parasympathetic divisions, two branches involved in regulating internal organs. The visceral nervous system includes pathways carrying information both to and from the internal organs. These classifications describe different aspects of the same connected system.

Note: Afferent and efferent specify direction relative to the CNS. Central and peripheral specify anatomical divisions. Do not treat these two pairs of terms as interchangeable classifications.

How does a neuron’s structure support communication?

A neuron is the structural and functional unit of the nervous system. Its three major parts are the cell body, containing the nucleus and cytoplasm; dendrites, short branching processes; and the axon, a long process that conducts impulses away from the cell body.

What does each part contain?

The nucleus contains the cell’s genetic material. Cytoplasm is the cell material outside the nucleus. The cell body contains the usual cell organelles, specialised internal structures, and Nissl’s granules, granular structures also present in dendrites.

Dendrites branch repeatedly and carry impulses towards the cell body. The far end of the axon branches into terminals. Each terminal branch ends in a bulb-like synaptic knob. These knobs contain synaptic vesicles, small membrane-bound sacs holding chemical messengers called neurotransmitters.

Axons carry impulses towards a synapse, a junction between neurons, or a neuromuscular junction, a junction between a neuron and a muscle. This organisation links reception at one region of the neuron with transmission from another.

How do dendrites and axons differ?

Schwann cells are cells associated with peripheral nerve fibres. A myelin sheath is an insulating covering around an axon. A node of Ranvier is a gap between adjacent myelin-covered regions. Their arrangement affects how an impulse travels.

The drawing shows branching dendrites around a cell body containing a nucleus and Nissl’s granules. A long axon has labelled Schwann cells, myelin sheath and a node of Ranvier. Its lower end branches into axon terminals with synaptic knobs.

See Fig. 18.1 in your NCERT textbook

How do neuron types and nerve-fibre coverings differ?

Neurons can be classified by their processes. Multipolar means one axon and two or more dendrites; bipolar means one axon and one dendrite. A unipolar neuron has a cell body with one axon only and is found usually in the embryonic stage, during early development.

A pseudounipolar neuron has a single process leaving its cell body, which divides into peripheral and central branches. The peripheral branch extends towards the body’s tissues; the central branch extends towards the CNS. Both branches have the structural character of an axon.

Where are the types found?

Sensory neurons carry incoming information towards the CNS. Motor neurons carry outgoing instructions towards responding tissues. This functional classification differs from the structural classification based on the number and arrangement of processes.

What distinguishes myelinated and unmyelinated fibres?

A myelinated fibre has a myelin sheath around its axon. In peripheral myelinated fibres, Schwann cells form this sheath. An unmyelinated fibre lacks the sheath; it can still be enclosed by a Schwann cell that does not form myelin around it.

Myelinated fibres occur in cranial and spinal nerves. Cranial nerves are associated with the brain, while spinal nerves are associated with the spinal cord. Unmyelinated fibres are commonly found in autonomic and somatic systems. Neither functional division should be identified entirely with one covering type.

The distinction between a cell and its covering is essential. Myelin describes the insulating material; a Schwann cell is the cell associated with the peripheral axon. The absence of myelin therefore does not mean that the axon lacks an associated Schwann cell.

Why is the resting axonal membrane polarised?

Polarisation means an electrical difference across the membrane, with the inside negative relative to the outside in the resting state. The resting potential is the electrical potential difference across the membrane when the neuron is not conducting an impulse.

An ion is an electrically charged particle. K⁺ denotes a positively charged potassium ion, and Na⁺ a positively charged sodium ion. The superscript plus sign indicates positive charge. Axoplasm is the cytoplasm inside an axon.

How are the ions distributed?

The resting membrane is comparatively more permeable to potassium ions and nearly impermeable to sodium ions. Permeability describes how readily a substance crosses a membrane. The membrane is also impermeable to the negatively charged proteins within the axoplasm.

The axoplasm has a high concentration of potassium ions and negatively charged proteins but a low sodium-ion concentration. Outside the axon, potassium-ion concentration is low and sodium-ion concentration is high. A concentration gradient is this difference in concentration between regions.

How is the resting condition maintained?

1. The axonal membrane contains selective ion channels, pathways that allow particular ions to pass through it.
2. Different permeability to sodium and potassium helps maintain unequal ion distributions across the resting membrane.
3. The sodium-potassium pump, a membrane transport mechanism, moves three sodium ions out for every two potassium ions moved into the cell.
4. This active transport, transport requiring energy, maintains the ionic gradients. The resting membrane has a positive outer surface and a negative inner surface.

Keep the pump’s transport directions separate from the direction of an impulse. The pump transports ions across the membrane, while the impulse is conducted along the axon. The ratio is three sodium ions out to two potassium ions in.

Note: “Nearly impermeable” does not mean completely impermeable. The resting membrane’s relative permeability must be distinguished from the much greater sodium permeability during excitation.

How is an action potential generated and conducted?

Depolarisation is the reversal of the resting polarity at an excited region of the membrane. The electrical potential difference there is the action potential, the nerve impulse. Repolarisation restores the resting polarity after excitation.

What sequence carries the impulse along an axon?

1. A stimulus acts at site A, meaning the first region of axonal membrane being considered. Sodium permeability rises there, allowing rapid sodium-ion entry.
2. The inner surface at A becomes positive and the outer surface negative. This local reversal produces an action potential.
3. Site B, the adjacent region ahead of A, remains polarised initially. Current flows internally from A towards B and externally from B towards A.
4. The membrane at B depolarises and develops an action potential. Repetition of this sequence conducts the impulse along the axon.
5. The rise in sodium permeability is extremely short-lived. Increased potassium permeability follows, and potassium ions move out, restoring the resting potential at the excited region within a fraction of a second.

The fibre becomes responsive to further stimulation after recovery. The impulse advances through successive changes in membrane polarity; it does not require the sodium ions entering at the first region to travel the whole length of the axon.

Two drawings show regions A and B, sodium-entry arrows and positive and negative charge signs. Curved arrows in the lower drawing show local current between the excited region and the neighbouring region.

See Fig. 18.2 in your NCERT textbook

How does myelin change conduction?

In an unmyelinated fibre, successive adjacent membrane regions generate action potentials. This is continuous conduction. In a myelinated fibre, action potentials are regenerated at the nodes of Ranvier, while local current spreads beneath the myelin between nodes.

This is saltatory conduction, an apparent jumping of excitation from node to node. Myelin insulates the intervening membrane, so action potentials need not be regenerated at every successive part of it. Myelinated conduction is faster than unmyelinated conduction for fibres of comparable diameter.

Both modes depend on electrical changes across the axonal membrane. Myelin changes where action potentials are regenerated; it does not replace the impulse with a chemical messenger. Chemical transmission becomes important at the chemical synapse between cells.

How does an impulse pass across a synapse?

A synapse involves the membrane of a presynaptic neuron, the neuron sending the signal, and a postsynaptic neuron, the neuron receiving it. The membranes may or may not be separated by a gap called the synaptic cleft.

How do electrical and chemical synapses differ?

At an electrical synapse, the membranes lie very close together and electrical current passes directly between neurons. At a chemical synapse, neurotransmitters convey the signal across a fluid-filled synaptic cleft.

Electrical synapses are rare in humans. A chemical synapse requires a sequence of release, recognition and membrane response. A receptor here is a specific protein on the receiving membrane that binds a neurotransmitter.

What happens at a chemical synapse?

1. An action potential reaches the presynaptic axon terminal, which contains vesicles filled with neurotransmitter.
2. The vesicles move towards and fuse with the presynaptic membrane, releasing neurotransmitter into the synaptic cleft.
3. The released neurotransmitter binds to specific receptors on the postsynaptic membrane.
4. Receptor binding opens ion channels, allowing ions to enter and generate a new postsynaptic potential.
5. The new potential may be excitatory, favouring excitation, or inhibitory, opposing excitation. Transmission need not produce another action potential.

A bulb-shaped axon terminal contains labelled synaptic vesicles. The drawing identifies the presynaptic membrane, synaptic cleft, postsynaptic membrane, neurotransmitters and receptors. Vesicles are shown releasing their contents near the membrane facing the cleft.

See Fig. 18.3 in your NCERT textbook

Distinguish conduction along an axon from transmission between neurons. In chemical transmission, the electrical signal reaching the terminal leads to chemical release, and receptor binding then produces an electrical change in the receiving cell.

How are the brain and forebrain organised?

The brain is the central information-processing organ. It controls voluntary movement, balance, involuntary organ activities and thermoregulation, the regulation of body temperature. It also processes vision, hearing, speech, memory, intelligence, emotions and thoughts.

What protects the brain?

The skull protects the brain. Inside it, cranial meninges, three protective membranes, surround the brain. From outside inward these are the tough dura mater, the very thin arachnoid, and the pia mater, which contacts brain tissue.

The brain has three major regions: the forebrain, comprising cerebrum, thalamus and hypothalamus; the midbrain, situated between forebrain and hindbrain; and the hindbrain, comprising pons, cerebellum and medulla. Each contributes to coordinated control.

What are the main features of the cerebrum?

The cerebrum forms the major part of the human brain. A deep longitudinal cleft separates its left and right cerebral hemispheres, the two halves. A band of nerve fibres called the corpus callosum connects them.

The folded outer cellular layer is the cerebral cortex. Its many neuronal cell bodies give it a greyish appearance, hence grey matter. The inner region contains myelinated nerve fibres and appears whitish, hence white matter.

The cortex contains motor areas, concerned with movement, and sensory areas, concerned with sensory information. Large association areas are neither clearly sensory nor motor. They support complex functions such as linking sensory information, memory and communication.

What do the thalamus and hypothalamus do?

The thalamus, around which the cerebrum lies, is a major coordinating centre for sensory and motor signalling. The hypothalamus lies at its base and contains centres regulating temperature, eating and drinking.

The hypothalamus also contains neurosecretory cells, neurons that secrete hormones. Its hormones connect neural activity with endocrine control. The brain also controls circadian rhythms, biological rhythms occurring over 24 hours, and the activity of several endocrine glands.

The limbic system comprises inner parts of the hemispheres and associated deep structures, including the amygdala and hippocampus. These are components of this brain network. Together with the hypothalamus, it regulates sexual behaviour, emotional reactions such as fear and pleasure, and motivation.

Within the hindbrain, the pons contains connecting fibre tracts, the cerebellum is a highly folded structure, and the medulla continues into the spinal cord. The cerebral aqueduct is a canal through the midbrain. These structures can be identified in a sagittal, or lengthwise, section.

This lengthwise section shows a large folded cerebral hemisphere, corpus callosum, thalamus and hypothalamus. Labels also identify the midbrain, cerebral aqueduct, pons, cerebellum, medulla and spinal cord. Brackets group structures into forebrain and hindbrain.

See Fig. 18.4 in your NCERT textbook

What do the midbrain, hindbrain and brain stem do?

The midbrain lies between the thalamus and hypothalamus above and the pons below. The pons is a hindbrain region containing fibre tracts that connect different brain regions. A tract is a bundle of nerve fibres within the CNS.

What are the features of the midbrain?

A canal called the cerebral aqueduct passes through the midbrain. Its dorsal portion, meaning the region towards the back, consists mainly of four rounded swellings called the corpora quadrigemina. The midbrain receives and integrates visual, tactile and auditory inputs.

Visual information concerns sight, tactile information concerns touch, and auditory information concerns hearing. Integration means bringing these inputs together for coordinated processing. The midbrain is therefore more than a connecting passage between larger regions.

How do hindbrain regions differ?

The cerebellum is the folded hindbrain structure involved in coordinating movement and balance. Its convoluted surface provides additional space for many neurons. Posture means the position in which the body is held.

The medulla oblongata, or medulla, connects with the spinal cord. Cardiovascular reflexes are automatic responses involving the heart and blood vessels. Gastric secretions are substances released by glands in the stomach.

The brain stem consists of the midbrain, pons and medulla oblongata and connects the brain with the spinal cord. Do not equate it with the hindbrain: the cerebellum belongs to the hindbrain but is not one of these three brain-stem regions.

How does the spinal cord conduct signals and organise reflexes?

The spinal cord is the elongated part of the CNS continuous with the medulla and lying within the vertebral canal, the passage formed by the vertebral column. Vertebrae, the bones of that column, help protect it.

The cord is also surrounded by meninges. Cerebrospinal fluid (CSF) is the fluid within the brain’s cavities and around the brain and spinal cord. It helps cushion these structures. A narrow central canal containing this fluid runs through the cord.

What does a transverse section show?

A transverse section, a cut across the cord, shows central grey matter resembling an H or butterfly, surrounded by white matter. Grey matter contains neuronal cell bodies; white matter contains tracts carrying information between spinal levels and the brain.

The dorsal horns are the posterior projections of grey matter, and the ventral horns are its anterior projections. Posterior means towards the back; anterior means towards the front. Ventral horns contain cell bodies of somatic motor neurons.

The dorsal root carries sensory fibres into the cord. Its swelling, the dorsal root ganglion, contains sensory neuron cell bodies, including pseudounipolar neurons. The ventral root carries motor fibres out. The roots unite to form a mixed spinal nerve containing sensory and motor fibres.

How does a reflex pathway work?

A reflex action is an automatic response to a stimulus. Its pathway is a reflex arc. A sensory receptor detects the stimulus; an effector, a muscle or gland, carries out the response. A receptor organ’s role differs from neurotransmitter binding at a membrane receptor.

1. A sensory receptor detects a stimulus and initiates a signal in an afferent pathway.
2. A sensory neuron carries the impulse towards the spinal cord through a dorsal root.
3. Within the spinal cord, communication passes to a motor neuron directly or through an interneuron, a neuron connecting other neurons in the CNS.
4. The motor neuron carries an outgoing impulse through a ventral root towards the effector.
5. The effector produces the response. Information can also pass to the brain through ascending pathways.

Ascending tracts carry information towards the brain; descending tracts carry instructions from it. The cord therefore serves both as a conducting connection and as a centre for spinal reflexes. Reflex integration in the cord does not imply that the brain receives no information.

How does the visceral nervous system regulate internal organs?

Viscera are the internal organs. The visceral nervous system includes the nerves, fibres, ganglia and plexuses through which impulses travel between the CNS and viscera. A plexus is an interlacing network of nerves or nerve fibres.

Why are both incoming and outgoing pathways needed?

Information travels from the viscera to the CNS through afferent pathways. Regulatory impulses return through efferent pathways. Thus, visceral communication includes both information about an organ and instructions affecting its activity; it is not restricted to outgoing signals.

The autonomic nervous system supplies involuntary organs and smooth muscles. Smooth muscle is non-striated muscle found in many internal organs; non-striated means lacking the visible transverse bands seen in skeletal muscle. Autonomic output also regulates cardiac muscle, the muscle of the heart, and glands, structures that produce and release secretions.

How do sympathetic and parasympathetic effects compare?

The sympathetic division generally supports increased activity during demanding situations. The parasympathetic division generally supports maintenance activities such as digestion. Where both supply an organ, their effects often oppose one another. This does not mean every organ has identical dual control.

For the heart, sympathetic activity increases heart rate, whereas parasympathetic activity decreases it. In the digestive tract, sympathetic activity generally reduces movement and secretion, while parasympathetic activity generally promotes them. These effects help match internal activity to changing demands.

Keep the levels of organisation distinct. Visceral identifies communication with internal organs; autonomic identifies regulation of involuntary activity; and sympathetic and parasympathetic identify divisions of autonomic control. The terms are related but should not be substituted without considering the pathway being described.

What are some disorders of the nervous system?

Nervous-system disorders can disturb signalling, movement or protective tissues. A disorder’s name should be connected with the structure or process affected. Different disorders do not all arise from damage to a nerve or from failure of the same brain region.

How do these examples differ?

• Epilepsy is a condition characterised by a tendency to recurrent unprovoked seizures. A seizure results from abnormal excessive or synchronised electrical activity in the brain. Its effects depend on the brain regions involved.
• Parkinson’s disease is a progressive nervous-system disorder associated with loss of dopamine-producing neurons. Dopamine is a neurotransmitter. Features can include slowness of movement, muscular stiffness and tremor, an involuntary rhythmic shaking.
• Meningitis is inflammation of the meninges. Inflammation is a tissue response to injury or infection. Meningitis can be caused by infections, including bacterial or viral infections, and affects protective membranes surrounding the CNS.

These examples distinguish abnormal neural electrical activity, loss of particular neurons and inflammation of protective coverings. The terms describe different biological problems; a single symptom such as altered movement does not by itself identify one particular disorder.

Glossary

• Coordination — Interaction between organs through which their functions complement one another and support integrated activity.
• Neuron — Specialised nerve cell that detects, receives and transmits signals within the nervous system.
• Afferent fibre — Nerve fibre carrying impulses from peripheral tissues or organs towards the central nervous system.
• Efferent fibre — Nerve fibre carrying regulatory impulses from the central nervous system towards peripheral organs.
• Myelin sheath — Insulating covering around an axon that changes the pattern and speed of impulse conduction.
• Node of Ranvier — Gap between adjacent myelin-covered regions where action potentials are regenerated during saltatory conduction.
• Resting potential — Electrical potential difference across the neuronal membrane when it is not conducting an impulse.
• Depolarisation — Reversal of resting membrane polarity at an excited region following rapid sodium-ion entry.
• Repolarisation — Restoration of resting membrane polarity after excitation, associated with potassium ions moving outwards.
• Synaptic cleft — Space between presynaptic and postsynaptic membranes across which neurotransmitters pass at chemical synapses.
• Neurotransmitter — Chemical messenger released at a chemical synapse that binds to specific postsynaptic receptors.
• Corpus callosum — Tract of nerve fibres connecting the left and right cerebral hemispheres of the brain.
• Reflex arc — Neural pathway linking a stimulus detected by a receptor with an automatic effector response.
• Pseudounipolar neuron — Neuron with one process leaving its cell body and dividing into peripheral and central branches.
• Visceral nervous system — Peripheral pathways carrying impulses between the central nervous system and the body’s internal organs.

Common errors and misconceptions

• Misconception: A nerve is one long neuron. Correct: A peripheral nerve is a bundle of nerve fibres with connective tissue. A neuron is an individual cell with a cell body and processes.
• Misconception: Unipolar neurons occur exclusively in embryos. Correct: They are found usually in the embryonic stage. “Usually” must not be strengthened to “exclusively”.
• Misconception: A resting membrane is completely impermeable to sodium. Correct: It is nearly impermeable to sodium and comparatively more permeable to potassium.
• Misconception: The sodium-potassium pump carries two sodium ions out and three potassium ions in. Correct: It transports three sodium ions outwards for every two potassium ions transported inwards.
• Misconception: Every chemical synapse excites the receiving neuron. Correct: The postsynaptic potential may be excitatory or inhibitory. Release of neurotransmitter does not guarantee a new action potential.
• Misconception: The brain stem is the same as the hindbrain. Correct: The brain stem comprises midbrain, pons and medulla. The hindbrain comprises pons, cerebellum and medulla.
• Misconception: Spinal grey matter surrounds white matter. Correct: Spinal grey matter lies centrally, with white matter outside. In the cerebrum, the grey cortex lies outside the white matter.
• Misconception: Visceral pathways carry impulses only from the CNS. Correct: They carry impulses in both directions between the CNS and internal organs.

Exam-style questions with model answers

Q1. Distinguish afferent and efferent nerve fibres by their direction of impulse transmission relative to the central nervous system. [2 marks]

1. Afferent nerve fibres carry impulses from peripheral tissues or organs towards the central nervous system.
2. Efferent nerve fibres carry regulatory impulses from the central nervous system towards peripheral tissues or organs.

Q2. Describe the process arrangement in unipolar, bipolar, multipolar and pseudounipolar neurons. [4 marks]

1. A unipolar neuron has a cell body with a single axon arising from it; this type is found usually during the embryonic stage.
2. A bipolar neuron has one axon and one dendrite associated with its cell body.
3. A multipolar neuron has one axon and two or more dendrites arising from the cell body.
4. A pseudounipolar neuron has one process leaving its cell body, which divides into peripheral and central branches.

Q3. Explain resting membrane polarisation under three headings: selective permeability, ion distribution, and sodium-potassium pump transport. State the pump’s transport ratio. [3 marks]

1. The resting membrane is comparatively more permeable to potassium ions and nearly impermeable to sodium ions. It is impermeable to the negatively charged proteins inside the axon.
2. The axoplasm contains high potassium and negatively charged protein concentrations but low sodium concentration. The external fluid has low potassium and high sodium concentrations.
3. The sodium-potassium pump maintains the ionic gradients by transporting three sodium ions out for every two potassium ions in. The resting inner surface is negative relative to the outer surface.

Q4. A stimulus excites site A on an unmyelinated axon. Site B is the immediately adjacent region ahead and is initially resting. Explain action-potential generation at A, propagation to B and recovery at A. [5 marks]

1. The stimulus increases sodium permeability at site A. Sodium ions enter rapidly through the axonal membrane, initiating a local change in its electrical condition.
2. At A, the inner membrane surface becomes positive and the outer surface negative. This reversal of resting polarity is depolarisation and produces an action potential.
3. Site B is still resting, with its outer surface positive and inner surface negative. Local current flows internally from A to B and externally from B to A.
4. The membrane at B depolarises and generates an action potential. Repetition at successive regions carries the impulse forwards along the axon.
5. The sodium-permeability rise at A is extremely short-lived. Potassium permeability then rises, potassium ions move out, and resting polarity is restored at that region.

Q5. Explain chemical synaptic transmission from the arrival of an impulse at the axon terminal to the possible postsynaptic effects. [5 marks]

1. An action potential arrives at the presynaptic axon terminal. This terminal contains synaptic vesicles filled with neurotransmitter, the chemical messenger used for transfer between the neurons.
2. The impulse stimulates vesicles to move towards and fuse with the presynaptic plasma membrane. Their neurotransmitter is released into the fluid-filled synaptic cleft.
3. The released neurotransmitter reaches the postsynaptic membrane and binds to its specific receptors. These receptors recognise the chemical signal coming from the presynaptic terminal.
4. Receptor binding opens ion channels in the postsynaptic membrane. Ion entry can generate a new electrical potential in the receiving neuron.
5. The resulting postsynaptic potential may be excitatory or inhibitory. Chemical transmission therefore does not necessarily trigger another action potential in the postsynaptic neuron.

Q6. Name the brain region principally associated with each function: coordinating sensory and motor signalling; regulating temperature and hunger; controlling respiration and gastric secretions. Explain each association briefly. [3 marks]

1. The thalamus is a major coordinating centre for sensory and motor signalling. It is a forebrain structure around which the cerebrum is arranged.
2. The hypothalamus contains centres controlling body temperature and the urges to eat and drink. It lies at the base of the thalamus.
3. The medulla oblongata contains centres controlling respiration and gastric secretions, as well as cardiovascular reflexes. This hindbrain structure is connected to the spinal cord.

Q7. Compare the spinal cord’s dorsal and ventral roots, and explain how they relate to a mixed spinal nerve and a spinal reflex pathway. [4 marks]

1. The dorsal root carries sensory impulses towards the spinal cord, providing the incoming pathway for information from peripheral receptors.
2. The dorsal root ganglion contains sensory neuron cell bodies, including pseudounipolar neurons whose processes divide into peripheral and central branches.
3. The ventral root carries outgoing motor impulses from the spinal cord towards effectors, including muscles that carry out a response.
4. The roots unite to form a mixed spinal nerve. In a spinal reflex, incoming sensory information is linked within the cord to outgoing motor activity, directly or through interneurons.

Q8. Explain why conduction is termed saltatory in a myelinated axon and continuous in an unmyelinated axon. Compare fibres of similar diameter. [2 marks]

1. In a myelinated axon, action potentials are regenerated at nodes of Ranvier, so excitation appears to jump between nodes. This saltatory conduction is faster for fibres of comparable diameter.
2. In an unmyelinated axon, successive adjacent membrane regions generate action potentials along the fibre, producing continuous conduction.

Key takeaways

• The central nervous system processes information, while peripheral pathways carry incoming sensory information and outgoing regulatory impulses.
• Neuron shape, fibre covering and impulse direction are different bases of classification and must be kept separate.
• Resting polarity depends on unequal ion distributions, selective membrane permeability and maintenance of gradients by the sodium-potassium pump.
• Sodium entry produces depolarisation, while the subsequent outward movement of potassium helps restore resting membrane polarity.
• Myelinated fibres regenerate action potentials at nodes of Ranvier; unmyelinated fibres conduct through successive adjacent membrane regions.
• Chemical synapses use neurotransmitters and specific receptors, and the resulting postsynaptic potential may be excitatory or inhibitory.
• Forebrain, midbrain and hindbrain structures have distinct roles in processing information and coordinating voluntary and involuntary functions.
• The spinal cord contains conducting tracts and reflex pathways, while visceral pathways connect internal organs with central control.

Test yourself

What are the three major parts of a neuron?

They are the cell body, dendrites and axon. Dendrites carry impulses towards the cell body, while the axon carries impulses away.

Does the absence of myelin imply the absence of a Schwann cell?

No. An unmyelinated peripheral fibre can be enclosed by a Schwann cell that does not form a myelin sheath around its axon.

Which ions move inwards during depolarisation and outwards during repolarisation?

Sodium ions enter during depolarisation. The subsequent increase in potassium permeability allows potassium ions to move out during repolarisation.

How many sodium and potassium ions does the pump transport, and in which directions?

The pump transports three sodium ions out of the cell for every two potassium ions transported into it.

Why does neurotransmitter release not guarantee another action potential?

The postsynaptic potential may be excitatory or inhibitory, so chemical transmission does not necessarily excite the receiving neuron.

Which structures form the brain stem?

The midbrain, pons and medulla oblongata form the brain stem, which connects the brain with the spinal cord.

Where are grey and white matter found in a transverse section of the spinal cord?

Grey matter lies centrally in an H-shaped region, while white matter surrounds it and contains conducting tracts.

What does the visceral nervous system include?

It includes nerves, fibres, ganglia and plexuses carrying impulses both from the CNS to internal organs and from those organs to the CNS.