Nervous System Flashcards
What are the two components of the Nervous system and what do they consist of?
Central Nervous System:
- Brain
- Spinal Cord
Peripheral Nervous System:
- consists of all nerves outside the brain and spinal cord which transmit signals between the CNS and receptors and effectors in all other parts of the body:
- 12 pairs of cranial nerves
- 31 pairs of spinal nerves
The peripheral Nervous system is divided into two components. Name these components and describe their functions.
Somatic Nervous System:
- voluntary movement using skeletal muscles
- single neuron between CNS and skeletal muscle cells
- neuron have large diameter, myelinated fibres, no synapses
- innervates skeletal muscles
- can lead only to muscle cell excitation
Autonomic Nervous System:
- involuntary actions e.g. heart rate, blood vessel diameter
- Two neuron chain between CNA and effector organ - 2 neurons means greater control as can stop the response in the middle if necessary. The neurons are connected by a synapse in the ganglion. The nerve leaving the CNS is the preganglionic nerve fibre and that leaving the ganglion is the postganglionic fibre.
- innervates smooth and cardiac muscle glands, GI neurons, NOT skeletal muscle
- Can be excitatory or inhibitory
What is the difference between the afferent and efferent division of the peripheral nervous system
Afferent: transmits information from sensory receptors towards the central nervous system
Efferent: transmits information away from the central nervous system to the muscles and glands
The autonomic nervous system has two divisions: the sympathetic and parasympathetic divisions. What is the difference?
Sympathetic:
- quickly prepares body for action - ‘fight or flight’ e.g. increase heart rate, dilating blood vessels.
Parasympathetic:
- relaxes the body and inhibits or slows many high energy functions - ‘rest and digest’ e.g. decreased heart rate, constricting blood vessels, stimulating digestion
Give the main function of the autonomic Nervous System.
- Innervates secretory glands (salivary, sweat, tear, mucous)
- Innervates heart and blood vessels to control blood pressure and blood flow
- Innervates smooth muscle of the bronchi to help regulate oxygen supply to the body
- Regulates digestive and metabolic functions of gut, liver and pancreas
- Influences renal and bladder function
- Essential for sexual responses in genitals and reproductive organs
What are the three functional regions of a nerve cell?
- Soma (cell body) - contains nucleus and organelles e.g. ribosomes needed to synthesis proteins.
- Dendrites - Carry nerve impulses towards the cell body. Branchlike structures that extend from the cell body and receive signals from the axons of other neurons.
- Axons - Carry nerve impulses away from the cell body to the target cell. Long tail like structure which extends from the soma and branches at the axon terminal to make contact with other nerve cells or effectors at junctions called synapses. Schwann cells surround the axon, protecting it and providing electrical insulation. Many neurons are covered by myelin sheath made up of membranes of Schwann cells.
How does nervous transmission occur?
Nerve cells generate electrical signals that move along their length. The stimulate target cells by secreting a chemical transmitter or neurotransmitter
Explain how an action potential passes along a myelinated axon.
The fatty sheath of myelin acts as an electrical insulator preventing action potentials from forming. Every 1-3mm there are breaks in this insulation called Nodes of Ranvier, where sodium ion channels are concentrated and action potentials can occur. The localised circuits therefore arise between adjacent nodes of Ranvier and the action potential ‘jumps’ from node to node in a process called saltatory conduction.
What is the difference between afferent neurons, interneurons and efferent neurons?
Afferent neurons:
- From tissues/organs to CNS.
- Cell body outside CNS
Interneurons:
- Connect neurons within the CNS
- Entirely within the CNS
- 99% of neurons
Efferent neurons:
- Away from CNS to effector organs
- Cell body and dendrites inside CNS
How is a negative resting potential (-70mV) established in a neurone?
- the inside of the cell is more negative compared to the outside
- Sodium potassium pump uses ATP to actively transport 3 sodium ions out for every 2 potassium ions in.
- So there is more positive charge outside the inside creating a negative resting potential.
- Also results in high potassium concentration inside the cell and high sodium ion concentration outside the cell. Most potassium ion channels are open whilst sodium ion channels are closed so potassium ions diffuse back out decreasing the resting potential even further.
What two gradients drive ions across membranes?
- Concentration gradient
- Electrical gradients (+ve attracted to -ve)
What is the equilibrium potential for potassium?
The balance between the concentration gradient pushing it out and the electrical gradient pulling it in.
Describe how the Na-K ATPase pump works.
- The transporter, with an associated molecule of ATP, binds 3 NA+ at high affinity sites on the intracellular surface of the protein. Two binding sites also exist for K+ but at this stage they are in a low affinity state.
- Binding of the NA+ activates the enzymic activity of the ATP causing phosphorylation of the surface of the transporter and releasing ADP.
- The phosphorylation causes a conformational change in the transporter protein exposing the sodium ions to the extracellular fluid and reducing the affinity of the binding sites for sodium so they are released.
- The conformation of the protein is now high affinity for K+ allowing 2 K+ to bind to the transporter on the extracellular surface. This binding results in dephosphorylation of the transporter so it returns to its original shape with low affinity for K+ so they are released into the intracellular fluid.
Describe the initiation of an action potential.
- At resting potential, some potassium ion channels are open and sodium ion channels are closed.
- A stimulus causes some sodium ion channels to open so sodium ions diffuse down their electrical and concentration gradients into the axon, making the inside less negative.
- If the potential difference reaches the threshold (-55mV), more sodium ion channels open causing an even greater influx of sodium ions. Potassium ion channels open and K+ begins to leave the cell down its concentration gradient but hampered by its electrical gradient.
- Once a potential difference of about +40mV has been established, the sodium ion channels close.
- Potassium ions continue to leave the cell causing the membrane potential to return to resting level. K+ exit is now driven by both concentration and electrical gradients.
- Hyperpolarisation - Potassium ion channels are slow to close so there’s a slight overshoot where too many potassium ions diffuse out of the neurone and the potential difference becomes more negative than the resting potential.
- Potassium ion channels close and the sodium-potassium pump returns the membrane to its resting potential
Describe the propagation of an action potential.
- At resting potential there is a high concentration of NA+ outside and K+ inside. Overall, it is more positive outside.
- Stimulus causes an influx of sodium ions and depolarisation across the membrane.
- The difference between adjacent potentials causes some of the sodium ions to diffuse sideways causing sodium ion channels in the next region of the neurone to open and the resulting influx of sodium ions in this region causes depolarisation.
- Behind the new region of depolarisation, the sodium ion channels close and the potassium ion channels open and the axon begins the process of returning to its resting potential.
- The action potential is propagated in the same way along the axon so a wave of depolarisation travels along the axon.
