WEEK 3 - NERVOUS SYSTEM Flashcards
Parasympathetic Nervous System (PNS)
Controls homeostasis and the body at rest, is responsible for the body’s “rest and digest” function
Sympathetic Nervous System (SNS)
Controls the body’s response to perceived threat and is responsible for the “fight or flight” response
Describe and identify major components of a neuron
Cell body: biosynthetic centre of a neuron
Dendrites: main input region (receiving signals from other neurons)
- convey incoming messages towards cell body (NOT action potentials, they are short distance signals called GRADED POTENTIALS)
Axons - any long nerve fibre
- The conducting region of the neuron; generates nerve impulses and transmits them.
- Nerve impulse is generated at the junction of the axon hillock and axon
- It is conducted to the axon terminals (secretory region)
Myelin Sheath; nerve fibres are covered with a segmented myelin sheath
* Protects and electrically insulates fibres, which increases the transmission speed of nerve impulses
* Myelin sheath in PNS are formed by Schwann cells
* CNS contains both myelinated and nonmyelinated axons.
* Myelin sheath is composed of oligodendrocytes
TRUE or FALSE
Sensory (or afferent) convey impulses TO CNS
TRUE
TRUE or FALSE
Motor (or efferent) neurons convey impulses TO CNS
FALSE
Motor (or efferent) convey impulses FROM CNS to effector organs (muscles and glands)
Describe the division of CNS
Central Nervous System (CNS); consists of the brain and spinal cord.
* Occupy dorsal body cavity
Describe the division of PNS
Peripheral Nervous System (PNS); part of the nervous system outside of CNS
Describe the division of Somatic Nervous System
Somatic/ Voluntary Nervous System; composed of somatic motor nerve fibres that conduct impulses from CNS to skeletal muscles
Describe the division of ANS
Autonomic/ Involuntary nervous system (ANS); consists of visceral motor nerve fibres that regulate the activity of smooth muscles, cardiac muscles and glands
Define resting membrane potential (RMP) and recognise the contribution that ion distribution across the membrane makes to RMP
RMP: voltage across the membrane is approximately -70mV.
* The minus sign indicates that the inside of the membrane is negatively charged relative to the outside.
* -70mV is the potential difference in a resting neuron.
* The membrane is said to be polarised.
* Is 25 times more permeable to potassium
* The K+ flowing out of the cell causes the inside to be more -ve
* Na+ trickles into the cell which makes it slightly more +ve
* Because some K+ is always leaking out of the cell, some Na+ is always leaking in * Sodium-potassium pump: ejects 3 Na+ from the cell and transports 2 K+ back into the cell
When do ligand-gated channels open?
Chemically gated/ ligand-gated channels: open when the appropriate chemical (i.e. neurotransmitter) binds
When do voltage-gated channels open?
Voltage-gated channels: open and close in response to changes in the membrane potential
When do mechanically gated channels open?
Mechanically gated channels: open in response to physical deformation of the receptor (as in sensory receptors for touch and pressure)
Action potential
- Long-distance signal of axons. Aka nerve impulse
- Brief reversal of the membrane potential from -70mV to +30mV along the axon
Repolarisation
- when membrane potential is returning to resting value
Depolarisation
- when membrane potential is more positive than the RMP
Hyperpolarisation
- when membrane potential is more negative than RMP
RESTING STAGE
All gated Na+ and K+ channels are closed
* ONLY leakage channels are open (maintains RMP)
DEPOLARISATION
Na+ channels open
* Na+ rushes into the cell
* Interior becomes less and less negative
* When depolarisation reaches threshold level, it becomes self-generating (positive feedback)
REPOLARISATION
Na+ channels are inactivating, and K+ channels open
* Slow voltage-gated Ka+ channels open and K+ rushes out of the cell (following its electrochemical gradient)
* Na+ entry into the cell declines/ stops
* This restores the internal negativity of resting neuron
HYPERPOLARISATION
Some K+ channels remain open and some Na+ channels reset
* The period of increased K+ permeability lasts longer than needed
* Sodium-potassium pump redistributes the ions
Describe how neurons communicate with each other by chemical signalling molecules released at the synapse
Axon terminal: contains synaptic vesicles (containing neurotransmitters)
Describe the relevance of graded potentials such as excitatory postsynaptic potential (EPSPs) and inhibitory postsynaptic potentials (IPSPs) to synaptic transmission
- Local changes in membrane potential of postsynaptic membrane
- Changes in membrane potential are graded (vary in size) * EPSP; Excitatory Postsynaptic potential * IPSP; Inhibitory Postsynaptic potential
Why is a neurotransmitter that causes depolarisation of the postsynaptic membrane, excitatory?
It brings the membrane closer to threshold
What is a depolarising postsynaptic potential called?
excitatory postsynaptic potential (EPSP)
Does a single EPSP initiate a nerve impulse? Will it reach threshold?
No, but the postsynaptic cell does become more excitable and because it’s partially depolarised, it is most likely to reach threshold when the next EPSP occurs
What is an inhibitory neurotransmitter?
A neurotransmitter that causes hyperpolarisation of the postsynaptic membrane
Why is action potential more difficult than usual during hyperpolarisation?
The membrane potential becomes more negative and thus even farther from threshold than in its resting state
A hyperpolarising postsynaptic potential
Is termed an inhibitory postsynaptic potential (IPSP)
Explain why neurons have absolute and relative refractory periods and recognise why this means that action potential propagation can only be in one direction
- Absolute refractory period; ensures that a stimulus cannot produce a second action potential. Ensures each action potential is a separate event and enforces a one-way transmission of APs
- Ends when the voltage gated Na+ channels begin to transition from inactivated to resting/ closed state
- Relative refractory period: a second AP CAN be generated, but that’s only when the stimulus is exceptionally strong
Explain the difference between saltatory and continuous action potential propagation, and explain the role of myelin sheath (nodes of Ranvier) in determining the speed of action potential propagation
In saltatory conduction, the nerve impulse will jump between the spaces between the nodes of Ranvier. This is faster than continuous conduction, where the nerve impulse travels down the whole unmyelinated neuron
Stimulus intensity is encoded for within a single neuron by?
Action potential frequency and number of receptors stimulated.
