Nervous System Flashcards
What are neurons?
The basic structural and functional units of the whole nervous system
Neuron structure consists of?
Cell body,dendrites and the axon
What is the Cell body and its function
contains the nucleus
- responsible for controlling the functioning of the cell
- around nucleus, is cytoplasm containing organelles
Dendrites and its function
- short extensions of the cyotplasm of the cell body
- carry messages, nerve impulses into the cell body
Axon and its function
- sinle long extension of the cytoplasm
- caries nerve impulses away from cell body
- length varies
- at its end, it divides into small branches
- each branch terminates at the axon terminal
What is a Myelin sheath?
- a layer of fatty material that covers the axon
- nerve fibres that have a myelin sheath is called myelinated fibres
- those that don’t = unmyelinated
Nerve fibres
- any long extension of a nerve cell, usual refers to axon
Myelin sheath formation
- outside brain and spinal cord, it is formed by special cells called Schwan cells, which wrap around axon
- outermost coil of the schwan cell forms a neurilemma around myelin sheath
- allows saltatory conduction: nerve impulse jump from nodes of raniver
Nodes of Raniver
a gap in the myelin sheath of a nerve fibre
Myelin three important functions?
- it acts as an insulator
- protects axon from damage
- speeds up movement of nerve impulses along the axon
Neurilemma?
helps in the repair of injured fibres
What is a nerve impulse?
an electrochemical change that travels along the membrane of a nerve cell
4 steps of Transmission of nerve impulse?
- A single action potential occurs in one section of a membrane.
- it triggers an action potential in the adjacent section of the membrane
- an action potential does not travel along the nerve fibre – the message/nerve impulse does.
- The process is likened to a line of dominoes
4 steps of TRANSMISSION ALONG UNMYELINATED FIBRES
- Depolarisation of one area of the membrane, causes movement of sodium ions into next section.
- Results in opening of voltage-gated sodium channels
- Initiates action potential
- Process repeats along whole length of the membrane.
- less action potentials generated in given time
- no saltatory conduction
Action potentials?
rapid depolarisation and repolarisation of the membrane.
Depolarisation 1
(to Threshold)
- -70mV
-Stimulus (neurotransmitter or sensory receptor) opens ligand-gated sodium ion channels
- more sodium ions begins to flows into the cell, making intracellular fluid less negative.
- Membrane begins to become depolarised (inside of cell becomes less negative)
Delopolarisation 2
(Rising phase of action potential)
- If depolarisation reaches threshold of -55mV, voltage-gated sodium ion channels open
- sodium ion influx continues independent of the stimulus – size of the response not dependent on the size of the stimulus (all-or-none response).
- Inward movement of sodium ions results in inside of membrane becoming more positive than the outside.
- Membrane potential reaches ~ +40mV.
- Membrane said to be depolarised.
Repolarisation
(Falling phase of action potential)
- Sodium channels close stopping the influx of sodium ions
- Voltage-gated potassium channels (slower to respond than voltage-gated sodium ion channels) open.
- Flow of potassium out of the cell returns the membrane potential to negative (repolarisation)
Hyperpolarisation
(and return to resting state)
- Potassium channels open longer than needed resulting in membrane potential dropping below resting membrane potential (hyperpolarisation/undershoot).
- Extended opening of potassium channels and sodium/potassium pump action result in undershoot (hyperpolarisation).
- potassium channels close (Na+/K+ pump continues) and resting membrane potential restored.
Steps of action potential
1) Depolorisation 1
2) Depolarisation 2
3) Repolarisation
4) Hyperpolarisation
What is a Refractory period
- a short period following a stimulus during which a nerve cell or muscle fibre cannot be stimulated again
- Membrane cannot undergo another action potential from time threshold is reached until resting membrane potential restored – refractory period.
- Prevents the nerve impulse going backwards along the nerve fibre
What is a synapse?
is the very small gap between one neuron and the next.
- Due to gap, action potentials/nerve impulses cannot continue to next neuron.
- transmission across synapse allows communication between neurons, and messages transmitted
5 steps of TRANSMISSION ACROSS A SYNAPSE
- Nerve impulse reaches axon terminal activating voltage-gated calcium ion channels.
- calcium ions flow into cell at presynaptic axon terminal.
- Causes synaptic vesicles to fuse with membrane, releasing neurotransmitters into the gap
- Neurotransmitters diffuse across the gap and attach to receptors on the next neuron.
- Stimulates ligand-gated protein channels to open, allowing influx of sodium ions and initiating an action potential.
What are receptors?
- Structure that is able to detect a change in the body’s internal or external environment.
- Can be grouped together in a sense organ (E.g. eye or ear) or a specialised nerve endings in parts of the body (E.g. skin).
What are 6 types of receptors?
Thermoreceptor
osmoreceptors
Chemoreceptor
Mechanoreceptors
Nociceptors
Photoreceptors
thermoreceptors, what do they detect and where are they found
- Able to respond to heat and cold.
DETECT:
External temperature: - Detected by thermoreceptors in SKIN
- Send information to HYPOTHALAMUS
Internal temperature:
- Monitored by thermoreceptors in the hypothalamus
- detect the temperature of the blood flowing through the brain.
osmoreceptors, what do they detect and where are they found
- Located in HYPOTHALAMUS
- Sensitive to osmotic pressure and respond to very small changes
- Osmotic pressure is determined by concentration of substances dissolved in blood plasma.
- Stimulate hypothalamus so the body’s water content is maintained within very narrow limits.
chemoreceptors, what do they detect and where are they found
- Stimulated by particular chemicals
- Found in the MOUTH AND NOSE
- give sensitivity to taste, smell.
- internal chemoreceptors are sensitive to composition of body fluids.
- E.g. In BLOOD VESSELS – sensitive to pH and CO2/O2 concentrations, involved in regulation of the heartbeat and breathing.
MECHANORECEPTORS, what do they detect and where are they found
- Found mainly in the SKIN
- detect changes in pressure
Different types:
1) Close to surface - sensitive to light touch. - More numerous in sensitive areas (E.g. lips, fingertips)
2) Deeper in the skin - sensitive to pressure and vibrations. - The ear contains specialized mechanoreceptors which detect vibrations caused by sound.
NOCICEPTORS, what do they detect and where are they found
aka pain receptors
- Stimulated by damage to tissues such as :
- a cut/heavy bump
- poor blood flow to a tissue
- excessive stimulation from stimuli such as heat or chemicals
- Concentrated in SKIN and mucous membranes (lining of internal structures).
- Not present in the brain.
- Don’t adapt quickly.
- Therefore pain continues, or worsens, while the stimulus is present – protective function.
PHOTORECEPTORS, what do they detect and where are they found
- Respond to light
- Two types in the eye – rods and cones
- Large amounts and tightly packed in the RETINA
- Responsible for converting light into signals that are sent to the brain
Reflex arc
- Pathway nerve impulse follows
1) Receptor (either ending of sensory neuron or associate specialised cell)
– reacts to change in internal or external environment by initiating nerve impulse in sensory neuron.
2) Sensory neuron – carries impulse from receptor to CNS.
3) Synapse – at least one. - Usually one or more interneuron but may be passed directly to motor neuron.
4) Motor neuron – carries impulse to effector
5) Effector – receives nerve impulse and carries out response.
Nerve
A bundle of nerve fibres held together by connective tissue
Axon terminal
forms synapse with next neuron
3 types of functional neurons
Sensory neurons (or afferent/receptor neuron)
- Carry messages from receptors in the sense organs, or skin, to the CNS (brain and spinal cord)
Motor neurons (or efferent/effector neuron)
- Carry messages from the CNS to the muscles and glands, the effectors.
Interneurons neurons (or relay/association/connector)
- Link between sensory and motor neurons.
What are two classifications of neurons
Functional and structural
4 types of structural neurons
Multipolar neurons
- one axon and multiple dendrites extending from the cell body.
- most common
- includes most of the interneurons in the brain and spinal cord as well as the motor neurons that carry messages to the skeletal muscles.
Bipolar neurons
- One axon and one dendrite
- both have many branches from either end
- Occur in eye, ear + nose where they take impulses from receptors to other neurons.
Unipolar neurons
- One extension only, an axon,
- These types of neurons are not found in humans or other vertebrates.
- found in insects.
Pseudounipolar neurons
- single axon from the cell body, then separates into two extensions.
- One extension
connects to dendrites, while the other ends in axon terminals.
- cell body lies to one side of the main axon.
- Most sensory neurons that
carry messages to the spinal cord are of this type.
speed of nerve impulses in fibres can be varied by?
- Impulses in unmyelinated fibres travel at maximum speed of 7km/h compared with up to 500km/h in myelinated fibre.
- Due to “jumping conduction” between noes of Ranvier.
- Diameter of fibre