Nervous system Flashcards
What is the Nervous system?
- Communication and control centre.
- Allows reaction to stimulus and control in the environment.
- Reactions are automatic and involuntary.
Difference between neurons, nerve fibres and nerves?
Nerve fibres: Long extension from cell body (axon).
Neuron: A nerve cell.
Nerve: A collection of neurons bundled together.
Dendrites
Short extensions/branches off cell body. Receive messages from sensory receptors and carry them towards cell body.
Cell body
Contains nucleus, cytoplasm with other organelles.
Axons
Single long extension from cell body. End divides into branches, each ending at an axon terminal. Carries impulse away from cell body.
Mylein sheath
Layer of lipid material covering axon. Axons with myelin sheath and myelinated, ones without are unmyelinated. Formed by Schwann cells which wrap around axon. Functions: Insulation, Protection, Speeds up nerve impulses.
Nodes of Ranvier
Gaps in myelin sheath.
Neurilemma
Outermost coil of Schwann cell. Helps repair injured fibres.
Grey matter
Grey areas of neurons. Unmyelinated fibres.
White matter
White areas due to lipid myelin. Myelinated fibres.
Structural types of neurons
Multipolar - Motor, Interneurons - One axon and multiple dendrites.
Bipolar - Sensory - Cell body in middle, one axon, one dendrite.
Unipolar - Sensory - One extension, an axon. Not found in humans.
Pseudounipolar - Sensory - Cell body is to one side of axon.
Functional types of neurons
Afferent - Carry messages from receptors in sense organs/ skin towards CNS.
Interneurons, Relay, Connector, Association - Located in CNS. Link between Sensory and Motor neurons. Motor,
Efferent - Carry messages away from CNS towards effectors in muscles and glands.
Nerves in a resting state
A group of separated positive and negative charges have potential to come together and release energy. This potential can be measured in Volts.
In intracellular and extracellular fluids there are positively and negatively charged ions.
Extracellular fluid - High concentration of Sodium ions and Chlorine ions.
Intracellular fluid - High concentration of Potassium ions and various negative ions.
Potential difference
Difference between concentrations of positive and negative ions either side of cell membrane. Potential difference is called Membrane potential. Membrane potential of unstimulated nerve cell is -70 mV.
How does cell membrane maintain potential difference?
1. Sodium potassium pump - Active transport. Moves 3 Na+ ions out of cell and 2 K+ ions into cell using Na+ and K+ protein pumps.
2. Cell membrane is not equally permeable to all ions (many negatively charged ions trapped inside cell). - Highly permeable to K+, Slightly permeable to Na+, Impermeable to various other large negative ions.
Polarised membrane
Inside membrane is negative, outside is positive. While K+ is positive, there is not enough inside cell to counteract large number of negative ions.
Depolarisation
Occurs if level of stimulation exceeds a certain threshold (-15mv), lifting resting membrane potential to -55mv. Movement of Na+ is too great to be balanced by an outward movement of K+. Inside membrane becomes positive, and outside becomes negative.
Repolarisation
Inside cell returns to an overall negative charge, outside cell returns positive.
Hyperpolarisation
K+ channels remain open longer than what is needed, results in potential dropping lower than resting (-70mv).
Action potential
Rapid depolarisation and repolarisation across a membrane.
Steps of an action potential
Step 1 - A sufficiently strong stimulus occurs (15mV, reaching resting membrane potential to -55mv). All or none response, action potential commences.
Step 2 - Sodium channels in axon open. Na+ diffuses into cell. Depolarisation occurs, Intracellular fluid becomes positive, extracellular becomes negative. At 15mV sodium channels close, stopping movement of Na+ into cell.
Step 3 - At 15mV potassium channels open. K+ diffuses out of cell. Repolarisation occurs, intracellular fluid becomes negative, extracellular becomes positive. At -70mV potassium channels close, and K+ stop moving out. However, some K+ leaks through membrane, causing hyperpolarisation.
Step 4 - Sodium-Potassium Pump restores resting membrane potential. It moves 3 Na+ molecules back outside cell and 2 K+ molecules back inside cell.
Refractory period
Period between threshold being reached -55mv and resting membrane potential being stored -70mv. Occurs for a short period after an action potential, so part of nerve fibre cannot be stimulated again. Ensures impulse travels in only one direction.
Conduction along unmyelinated fibres
Depolarisation of one channel stimulates depolarisation in adjacent channel (next to original stimulus). Continuous propagation.
Saltatory conduction
Action potential cannot occur in myelin sheath. It diffuses along axon through cytoplasm until next node of ranvier. Action potentials are said to ‘jump’ from node to node. Less distance to cover = much faster
