3.8: Nervous system

Stimulus

Stimulus

A detectable change in the internal or external environment.

Receptor

Specialised sensory cells, which are transducers, converting energy to electrical impulses which travel along neurons. Examples are pressure sensors in skin and complex sense organs such as the ear and the eye.

Nervous impulses

Electrical energy created by receptors and passed along neurons.

Effector

Initiates a response, can be a muscle or a gland.

Response

Action taken in reaction to a stimulus.

Central

Nervous system comprised of the brain and the spinal cord. Processes information provided by a stimulus, and protected by tough protective membranes known as meninges.

Central canal

Centre of the spinal cord, filled with cerebral spinal fluid.

Grey

Matter made up of the nerve fibres of relay neurons, and the cell bodies of relay and motor neurons. Darker in colour.

White

Matter made up of nerve fibres surrounded by fatty myelin. Lighter in colour.

Dorsal

Root that contains a sensory neuron. Electrical impulses move into the spinal cord through it.

Gangelion

In the dorsal root. A swell that contains the body of a sensory neuron.

Ventral

Root that contains a motor neuron. Electrical impulses move out of the spinal cord through it.

Vertebra

Bone which surrounds a part of the spinal cord.

Meninge

Tough protective membrane which protects the brain and spinal cord.

Peripheral

Nervous system comprised of peripheral nerves. Has two parts, the somatic and autonomic nervous system.

Somatic

A part of the peripheral nervous system, and is comprised of pairs of nerves that originate in the brain or spinal cord, and their branches. Carries sensory impulses from receptors to the CNS, and carries impulses away from the CNS to effectors. Controls the reflex arc and voluntary movement.

Autonomic

Nervous system which provides unconscious control of the functions of internal organs, such as digestion.

Reflex arc

The simplest type of response to a stimulus. Is rapid, automatic, involuntary, and generally protective in nature. The decision making areas of the brain are not involved. Part of the somatic nervous system.

Sensory

Neuron which carries impulses from sense receptors into the CNS.

Relay

Neuron which receives impulses from sensory or relay neurons, and transmits them to motor or other relay neurons.

Motor

Neuron which carries impulses from the CNS to effectors, such as muscles or organs.

Granular

How cytoplasm within the neuron cell body is described due to the high concentration of ribosomes.

Nissl

Granules comprising groups of RER. In the cell body of neurons.

Dendrites

Thin fibres which carry impulses towards the cell body - neurons can have several.

Axon

A thin fibre which carries impulses away from the cell body - neurons can only have one.

Myelin sheath

Speeds up electrical transmission while protecting the nerve fibres. It has two parts, the Schwann cells and nodes of Ranvier.

Schwann

Cells which make up the myelin sheath. They form in embryos, wrapping 20-30 times around developing axons and then withdrawing cytoplasm, leaving a multi-layer myelin sheath containing phospholipids, which are good insulators. They surround and protect nerve fibres. Their nuclei are facing outwards.

Nodes of Ranvier

Part of myelin sheath. 1um gaps in the sheath where adjacent Schwann cells meet, exposing the axon membrane. This allows impulses to be transmitted rapidly.

Synaptic end bulb

Swelling in the axon terminal, where neurotransmitters such as acetylcholine are synthesised.

Axon terminal

Located at the end of the neuron, have synaptic end bulbs where neurotransmitters are synthesised to be transported to adjacent neurons.

Nerve nets

Simplest form of nervous system, with two cell types; ganglion and sensory. In Hydra, they are located in the ectoderm (outside layer) to sense external stimuli such as light. In response they can perform locomotion and hunt. However, it is quite slow and they cannot detect direction of stimuli.

Ganglion

Cells which provide connections in many directions in a nerve net. Cells group into a ganglia, but don’t form a brain.

Sensory

Cells which detect stimuli within a nerve net.

Resting

The potential of an axon when not conducting an impulse, around -40 mV. Caused due to negative ions of large proteins, organic phosphates and mainly the uneven distribution of K+ and Na+

Sodium potassium exchange pumps

Term used for the transport protein that actively transports Na+ and K+ ions across the axon cell membrane. Brings in 2 K+ ions for every 3 NA+ ions that are pumped out.

Voltage gated

Term to describe the sodium pumps which open and close based on voltage. During resting potential, most are shut, but stimulus energy causes them to open allowing more Na+ in, which makes the axon more positive, allowing in more Na+. This constantly increases membrane permeability.

Action

Type of axon potential which occurs when an electrical impulse travels through an axon. Generated once the threshold value is reached by a stimulus, causing voltage gated channels to open and the potential difference to reach +40 mV.

Oscilloscope trace

Measures the changes in voltage across membranes over time using microelectrodes. Able to detect magnitude, transmission speeds and patterns of impulses in different parts of the nervous system and in different situations.

Threshold

Value at which the voltage-gated sodium channels open and generate an action potential, -55 mV.

Depolarisation

When the potential difference is reversed temporarily so the inside is more positive than outside. Occurs when voltage-gated sodium channels open, allowing in more Na+, which increases positivity and opens more channels.

Repolarisation

Occurs once the action potential of +40 mV is reached. The voltage-gated sodium channels close and K+ ions diffuse out via the concentration gradient. This rapidly decreases the potential difference.

Hyperpolarisation

Occurs when K+ is diffused out faster than Na+ diffuses in, causing the potential difference to drop below the -70 mV resting potential. This causes the sodium-potassium exchange pumps to begin functioning, restoring resting potential.

Electrical

Type of synapse with 2 nm gap, small enough for electrical impulses to be directly transmitted across neurons.

Chemical

Type of synapse with a 20 nm gap, which is too large for the impulse to jump. Makes up the majority of synapses. Axon branches lie close to dendrites of other neurons, and use neurotransmitters, which diffuse across in order to transmit the impulse.

Synaptic cleft

The gap between two neurons, where neurotransmitters are diffused across. Is where acetylcholine is broken down using acetylcholinesterase.

Calcium

Ion that voltage-dependent channels open to once an action potential reaches the axon terminal. Moves in from the synaptic cleft, and triggers synaptic vesicles. Actively transported out once an action potential has been generated at the post-synaptic neuron.

Synaptic vesicles

Contains neurotransmitters, such as acetylcholine, and releases them into the synaptic cleft via exocytosis.

Acetylcholine

Neurotransmitter released via exocytosis from synaptic vesicles into the synaptic cleft. It then diffuses towards receptors in the post-synaptic membrane. To stop it, it is actively transported back into the pre-synaptic neuron, calcium ions are actively transported out to stop vesicle action and it is hydrolysed in the synaptic cleft.

Receptors

Located in the post-synaptic membrane. Has two sites, for acetylcholine, and the molecules show co-operative binding when they attach. This causes the shape of their protein to change to allow Na+ ions to enter, which depolarises the post-synaptic neuron.

Acetylcholinesterase

Enzyme which hydrolyses acetylcholine, forming ethanoic acid and choline. These diffuse back into the pre-synaptic cleft to reform acetylcholine.

Ethanoic acid and choline

Products of the hydrolysation of acetylcholine via the enzyme acetylcholinesterase. Diffuse back into the pre-synaptic neuron to reform acetylcholine.

AcCoA and choline

Combine together to form the products acetylcholine and CoA.

Absolute

Refractory period where voltage-gated sodium channels are unable to open until resting potential is reestablished. This is done to prevent the action potential from moving backwards.

Relative

Refractory period that occurs with the hyperpolarisation phase, for 5 ms - 10 ms. Strong enough impulses can restart the action potential, occurs while sodium-potassium transfer pumps restore resting potential.

All or nothing

Law of the nervous system which states all action potentials will be the same size,+40 mV - with increases in intensity represented by increased action potential frequency and will only be triggered if the -55 mV threshold value is surpassed. This is to prevent the brain from being overloaded by minor stimuli.

Temporal

Type of neuron summation where multiple action potentials are passed on from the same pre-synaptic axon to the same post-synaptic axon. This builds up to reach the threshold value.

Spatial

Type of neuron summation where several pre-synaptic neurons contribute to one post-synaptic neuron, which builds up to an action potential.

Sedatives

Drugs which inhibit the nervous system by creating fewer action potentials, such as alcohol.

Agonists

AKA stimulants. Drugs that stimulate the nervous system by allowing more action potentials to be produced, such as amphetamines.

Nicotine

Na+ ions within this drug mimic acetylcholine, which can cause them to bind simultaneously to receptors in the post-synaptic bulb. This increases impulse frequency as it is not hydrolysed. The body becomes habituated to this, which causes withdrawal once it is stopped, and tolerance.

Organophosphates

AKA phosphate esters. Drugs which inhibit acetylcholinesterase, which prevents the hydrolysation of acetylcholine. This causes repeated action potential triggering, resulting in long term health damage, and in nerve gases can cause neuromuscular junction issues, such as antagonistic muscle pairs moving and breaking bones. Other chemicals include insecticides and herbicides.

Psychoactive

Class of drugs which affect different neurotransmitters and their receptors, altering brain functioning and therefore perception, mood consciousness and behaviour. This can be therapeutic drugs such as Prozac and recreational drugs such as nicotine. Due to pleasant and beneficial effects, abuse and dependence can occur.

Saltatory

Type of conduction that occurs in myelinated axons, where impulses travel between the nodes of Ranviers across the axon. Decreases ATP needed as only these areas experience the polarisation stages, and is faster.

Spinal nerve

Where the motor and sensory neurons meet before splitting into the ventral and dorsal roots.

Inter-vertal disc

In between each vertebra, and prevents them from rubbing against each other.

Excitable

Term for cells that can change their potential and transmit electrical impulses. Uncommon.