8.1.2. Nervous System

Question 1

Types of receptors

Answer 1

• Exteroceptor: receptors that detect external stimuli
• Interoceptor: receptors that detect internal stimuli
• Photoreceptor: receptors that detect light
• Osmoreceptor: receptors that detect changes in the concentration of solutes (osmolarity), usually sodium
• Nociceptors: pain receptors. Myelinated Type A fibers carry sensations of fast pain, unmyelinated Type C fibers carry sensations of slow pain
• Thermoreceptors: temperature receptors
• Chemoreceptors: detects changes in the chemical composition of blood
• Mechanoreceptors: receptors sensitive to stimuli that distort their plasma membranes. Proprioreceptors monitor the positions of joints and muscles. Baroreceptors detect pressure changes. Tactile receptros provide sensations of touch, pressure, and vibration.

Receptors are excitable cells. When a stimulus excites a receptor, the membrane potential of the receptor is changed (receptor potential). If potential rises above the threshold, an action potential is initiated in the nerve fibre attached to the receptor, sending information to the central nervous system

Question 2

Nervous system parts

Answer 2

Central nervous system:

• Brain
• Spinal cord
• Contains relay neurons

Peripheral nervous system:

• Cranial nerves
• Spinal nerves
• Peripheral nerves
• Contains sensory neurons and motor neurons

Question 3

Role of the nervous system

Answer 3

• Responds to environmental changes (stimuli) to maintain homeostasis

Sensory-somatic nervous system

• Sensory neurons receive information from the external environment, and relay this information to the central nervous system. Motor neurons then transmit messages about desired movements from the CNS to the muscles, making them contract in response to the external stimuli
• Some actions in this nervous system are voluntary, but other are involuntary e.g. flinching

Automatic nervous system

• Sensory neurons relay information from the internal environment to the CNS. Motor neurons then relay information to regulate physiological responses in the internal environment, like metabolic rate and heart rate. Basically, the automatic nervous systems regulates the functioning of major internal organs

Question 4

Neuron structure

Answer 4

• Nerve cells
• Elongated shape with a specific structure consisting of dendrites, axon, myelin sheath, and axon ending
• A bundle of neuronal fibres make up a nerve

Parts of a neuron:

• Dendrite: branch-like projections that connect a neuron to other cells. The branching increases surface area so that the neuron can collect incoming impulses. Dendirtes receives signal from the previous neuron
• Cell body: contains organelles including the nucleus, ER, mitochondria, and Golgi body
• Axon: long projection that conducts electrical signals away from the cell body
• Myelin sheath: insulates the axon to prevent ion flow across the neuron membrane, and promote saltatory conduction where nerve impulses jump from node to node along the axon instead of being propagated along its full length
• Axon terminals: Projections from the end of the axon that line up next to the dendrites of the next neuron or target cells, e.g. muscle cells, where the signal leaves the neuron. Synaptic vesicles (small sacs at found at the axon terminals) store neurotransmitters, which are released across the synapse when a neuron fires.

Fun fact: alcohol shrinks dendrites, but learning promotes the growth of dendrites.

Question 5

Role of neurons

Answer 5

Sensory neurons

• Carry nerve impulses from peripheral parts of the body to the brain or spinal cord
• Have specialised receptor ends at the tips of their dendrites, or dendrites attached to a sensory organ, e.g the tongue
• Axons are attached to another neuron

Interneurons

• Lie within the brain or spinal cord
• They transmit impulses from one part of the brain to another
• Link other neurons and may direct incoming information from sensory neuron for processing
• Both axons and dendrites are attached to other neurons
• Transfer impulses to motor neurons

Motor neurons

• Carry nerve impulses from out of the brain or spinal cord to effectors, e.g. muscle cells
• Stimulate muscle contraction and gland secretions
• Dendrites are connected to other neurons, but axons are connected to an effector organ

Question 6

Nerve communication

Answer 6

• All nerve cells have a difference in electrical charge across their membranes, called membrane potential
• Resting membrane potential is -70mV. There is a positive charge outside the cell and negative inside.

Reaction to stimulus

• When there is a stimulus, sodium channels open, and sodium ions come into the cell
• This reduces the negative charge of the axon (depolarisation), as Na+ ions are positively charged
• When membrane charge reaches the -55mV threshold, action potential is fired
• After action potential is fired, repolarization occurs: potassium ions enter the cell through potassium channels, increasing the negative charge again
• The positive charge that occurs in the neuron due to action potential will be passed to the next neuron as a voltage pulse
• Refractory period: as potassium ions keep entering the cell, the cell charge drops below -70mV (hyperpolarisation). The charge will then recover back to -70mV, but until this happens, the nerve cannot fire again.

Fun fact: if the threshold is not reached, the neuron will not fire and the stimulus will not be registered

Threshold: particular value of the membrane voltage that needs to be reached for an action potential to be started

Question 7

Action potential (will be in assessment)

Answer 7

• Rapid sequence of changes in the voltage across a membrane (when a neuron sends an impulse to the next neuron)
• Either gets fired or doesn’t. The magnitude of the electrical charge does not affect how big the action potential is
• Causes a nerve impulse in nerve cells and a contraction in muscle cells
• Different stimuli fire different sensory neurons, and different excitable cells have different thresholds for firing

Excitable cell: any cell which can have an action potential generated. This includes neurons and receptor cells, e.g. osmoreceptors

Question 8

Synapse

Answer 8

• Junction between two communicating neurons
• Synaptic transmission: process of crossing the synapse between two neurons. Synaptic transmission is achieved by the release of neurotransmitters from synaptic vesicles at the axon terminals
• Depressant substances suppress the release of neurotransmitters and stimulants increase it, affecting the transmission of nerve impulses across the synapses

Question 9

Neurotransmitters

Answer 9

• Nerves are transmitters of electrochemical signals because they use both electricity and biochemicals to transmit a message
• Neurotransmitters are released from the axon terminals
• Common neurotransmitters are dopamine, histamine, and endorphin
• The signal is electrical while travelling along the neuron, and is converted into a chemical signal when it reaches the synapse. When the chemicals reach the dendrites of the next cell, they cause the other neuron to fire
• Drugs e.g. depressants/stimulants alter the transmission of nerve impulses by suppressing or increasing the release of neurotransmitters

Question 10

Dendrites

Answer 10

• Receive signals from the previous neuron

Question 11

Resting potential

Answer 11

• Resting membrane potential is negative because the inside of a neuron is negative compared to the outside of a neuron
• This is because potassium ions can pass easily through the membrane, leaving the neuron, while Cl- and Na+ ions cannot cross as freely
• Negatively charged proteins inside the neuron also cannot cross the membrane, resulting in the resting potential being -70mV

Question 12

Stimulus Receptor Model - Flowchart

Answer 12

Stimulus → receptor → sensory neuron → interneuron → motor neuron → effector → response

Question 13

Stimulus meaning

Answer 13

• An external or internal environmental change that an organism can detect and respond to
• Or a chemical or physical change that activates a receptor molecule in a cell, generating a response e.g. the secretion of a hormone

Question 14

Receptor meaning

Answer 14

• Molecule in a cell mebmrane that binds and responds to specific molecules e.g. hormones and neurotransmitters, triggering a response
• Purpose is to detect a change from a stable state

Question 15

Effector meaning

Answer 15

• The organ that carries out the response, e.g. the gland that releases a hormone or the muscle that contracts

Question 16

Nerves

Answer 16

• Bundle of nerve fibres (dendrites or axons) located outside the brain
• E.g. auditory nerves in the ears and optic nerves in the eyes

Question 17

Comparing nervous and endocrine systems

Answer 17

Nature of message

• Nervous impulse: electrochemical impulse
• Endocrine system messenger is chemical

Mode of transmission

• Nervous impulse is conducted along neural pathways, with chemical or electrical transmission at the synapses
• Endocrine system: homrones travel in the bloodstream and body fluids

Speed of transmission

• Nervous impulses travel quickly. Response occurs soon after the receptor detects the stimulus
• Effect of hormones is delayed as time is needed for the release of hormones from the glands, and the transport of the hormone in the bloodstream to the target cell

Cells contacted

• Nervous impulse will only cause a response in muscle or glands that are part of the nervous pathway
• Hormones can reach every cell of the body which is contacted or near some part of the circulatory system

Lifespan of effect

• Nervous impulses produce short-lived effects that are usually reversible
• Effects of endocrine stimulation stay longer