Topic 6.5 Nerves, Hormones and Homeostasis

Question 1

6.5.1 State that the nervous system consists of the _______ _______ ______ (CNS) and __________ ______, and is composed of cells called _______ that can carry rapid electrical impulses.

Answer 1

The nervous system consists of the central nervous system (CNS) and peripheral nerves, and is composed of cells called neurons that can carry rapid electrical impulses.

Question 2

6.5.2 Draw and label a diagram of the structure of a motor neuron.

Answer 2

• cell body
  
  • nucleus
  • dendrites (projections off cell body)
• axon
• myelin sheath
• notes of Ranvier
• motor end plates
• skeletal muscle fibres

Question 3

6.5.3 State that nerve impulses are conducted from receptors to the CNS by _______ neurons, within the CNS by _____ neurons, and from the CNS to effectors by _____ neurons.

Answer 3

Nerve impulses are conducted from receptors to the CNS by sensory neurons, within the CNS by relay neurons, and from the CNS to effectors by motor neurons.

Question 4

6.5.4 Define resting potential and action potential (depolarization and repolarization).

Answer 4

Resting potential: the electrical potential across the plasma membrane of a cell that is not conducting an impulse (-70 mV); maintained by the sodium-potassium pump; inside of neuron develops a net negative charge because of chloride and other negatively charged ions ∴ there’s an electrical potential/voltage across the membrane.

Action potential: the reversal and restoration of the
electrical potential across the plasma membrane of a cell, as
an electrical impulse passes along it (depolarization and
repolarization).

Question 5

6.5.5 Explain how a nerve impulse passes along a non-myelinated neuron.

Answer 5

• action potential in one part of neuron causes action potential to begin in next section of neuron, due to diffusion of sodium ions between region w/ action potential & region w/ resting potential; ion movements reduce resting potential, and voltage-gated channels will open if potential rises above threshold level
• resting potential = -70 mV
• threshold level = -55 mV
• for every 3 Na⁺ ions expelled from neuron, 2 K⁺ enter

1. at resting potential, net - charge inside neuron, + outside
2. reversal: when impulse passes along neuron, Na⁺ and K⁺ can diffuse across membrane through voltage-gated ion channels; sodium channels open first; electrical potential is reversed (depolarized) (+ inside membrane due to entrance of many Na+ ions, net - charge outside)
   
   • reduced electrical potential causes adjacent voltage-gated Na⁺ channels to open, generating wave of depolarization
3. restoration (repolarization): potassium channels open after short delay; K⁺ ions diffuse out of neuron (down conc’n gradient), creating net - charge inside membrane, + charge outside; electrical potential is restored (repolarized)
   
   • hyperpolarization (electrical potential drops to -75 mV) briefly occurs when too much K⁺ is expelled
4. before the neuron can fire again, resting potential must be restored; the original distribution of ions (Na⁺ out, K⁺ in) is re-established by active transport
   
   • ​​the inability to propagate another action potential during this time (refractory period) ensures nerve impulses only travel in one direction

Question 6

6.5.6 Explain the principles of synaptic transmission.

Answer 6

• junction between two neurons is called a synaptic cleft; forms a physical gap between the pre-synaptic and post-synaptic neurons
• an action potential (electrical signal) cannot cross the synaptic gap, so it triggers the release of chemicals (neurotransmitters) to continue the signal

Synaptic Transmission

• When an action potential reaches the end of the presynaptic neuron, it triggers the opening of voltage-gated calcium channels
• Calcium ions (Ca²⁺) diffuse into the presynaptic neuron and promote the fusion of vesicles (containing neurotransmitters) with the plasma membrane
• The neurotransmitters are released from the presynaptic neuron by exocytosis and cross the synaptic cleft
• Neurotransmitters bind to appropriate receptors on the postsynaptic membrane, opening transmitter-gated ion channels
• Sodium and other positively charged ions diffuse into postsynaptic neuron, causing depolarization of postsynaptic membrane
• The depolarization passes on down the postsynaptic neuron as an action potential
• The combination of chemical messengers received by dendrites determines whether the threshold is reached for an action potential in the postsynaptic neuron
• Neurotransmitter molecules released into the synaptic cleft are either recycled (by reuptake pumps) or broken down (by enzymatic activity)
• Calcium ions are pumped out of presynaptic neuron into synaptic cleft

Question 7

6.5.7 State that the endocrine system consists of glands that release ________ that are transported in the blood.

Answer 7

The endocrine system consists of glands that release hormones that are transported in the blood.

Question 8

6.5.8 State that homeostasis involves maintaining the internal environment between limits, including ______ __, ______ ________ ____________, _____ _______ ____________, ____ ___________ and _____ _______.

Answer 8

Homeostasis involves maintaining the internal environment between limits, including blood pH, carbon dioxide concentration, blood glucose concentration, body temperature and water balance.

The Composer Plays Great Waltzes

body Temperature

Carbon dioxide concentration

blood pH

blood Glucose concentration

Water balance

Question 9

6.5.9 Explain that homeostasis involves monitoring levels of variables and correcting changes in levels by negative feedback mechanisms.

Answer 9

• Most homeostatic control mechanisms operate through a negative feedback loop
• When specialised receptors detect a change in an internal condition, the response generated will be the opposite of the change that occurred
  
  • e.g. rise in level (of body temp, pH, etc.) will generate response that lowers level; fall in level will generate response that increases level
  • this accounts for the small fluctuations about a set point
• When levels go too far from set point, the level of a product feeds back to control the rate of its own production, generating a response that will be opposite to the change that occured
  
  • e.g. rise in level of product feeds back to decrease production and reduce level; fall in level feeds back to increase production and raise level
• When levels have returned to equilibrium, the effector ceases to generate a response

Question 10

6.5.10 Explain the control of body temperature, including the transfer of heat in blood, and the roles of the hypothalamus, sweat glands, skin arterioles and shivering.

Answer 10

• body temp at 36.8°C
• if too high or too low, hypothalamus sends messages to different parts of body to stabilize temp.

Responses to Overheating

• skin arterioles dilate (widen) so more blood flows to skin; temp. of skin rises, so more heat is lost to environment
  
  • blood transfers heat from core of the body
• skeletal muscles relax to generate less heat
• sweat glands secrete sweat making skin surface damp; water evaporates from damp skin and has a cooling effect

Responses to Chilling

• skin arterioles constrict (narrow) to decrease blood flow to skin, so less heat is lost to environment
• skeletal muscles do many small rapid contractions (shivering) to generate heat
• sweat glands don’t secret sweat and skin remains dry

Question 11

6.5.11 Explain the control of blood glucose concentration, including the roles of glucagon, insulin and α and β cells in the pancreatic islets.

Answer 11

• blood glucose level usually 4-8 millimoles per dm³ of blood
• pancreas cells monitor glucose levels
• controlled by negative feedback

High glucose levels trigger…

• β cells in pancreas islets to produce insulin
• ​insulin increases permeability of cells to glucose so glucose can enter cells
  
  • ​insulin stimulates liver and muscle cells to absorb glucose and store it as glycogen in the cytoplasm
• this lowers blood glucose level (negative feedback)
• if glucose is too high, it’s converted to and stored as fats

Low glucose levels trigger…

• α cells in pancreas islets to release glucagon
• glucagon stimulates liver and muscle cells to break down glycogen into glucose and release glucose into the blood
• this raises blood glucose levels

Question 12

6.5.12 Distinguish between type I and type II diabetes.

Answer 12

Type I

• early onset (childhood)
• β cells producing insufficient insulin
• auto-immune disease
  
  • own body attacks β cells
• need injections to regulate insulin levels
• can’t be controlled solely by diet

Type II

• later onset
• target cells become insensitive to insulin
• injections usually not needed
• usually, low carbohydrate diets can control condition