Nervous Systems (2) Flashcards
Information processing of nervous systems
- Sensory input
- Integration
- Motor output
Components of a neuron
Nucleus Cell body Axon hillock Dendrites Axon Synaptic terminals Synapse (junction between synaptic terminals and cell body)
Structural diversity of vertebrate neurons
Sensory neuron
Interneuron
Motor neuron
A nerve
Consists of many neurons
- axons
- connective tissue
- blood vessels
Glia
Supporting cells vital for structural integrity and normal function.
10-50 times more glia than neurons in the mammalian brain.
- Astrocytes
- Oligodendrocytes & Schwann cells
Astrocytes
- CNS
- Structural support
- Regulate extracellular concentration of ions and neurotransmitters
- Formation of the blood-brain barrier
Oligodendrocytes
- CNS
- Form myelin sheaths around axons
- Lipid membranes: insulator
- Have nodes of Ranvier
Schwann cells
- PNS
- Form myelin sheaths around axons
- Lipid membranes: insulator
- Have nodes of Ranvier
Gradients of ions across the membrane
The resting membrane potential is negative
- The inside membrane is negative relative to the outside (120mM Cl- ECF but 100mM A- anions cytosol)
Na+/K+-ATPase
- Pumps 3 x Na+ out of the cell
- Pumps 2 x K+ into the cell
(More Na+ in the extracellular fluid than in the cytosol, more K+ in the cytosol than in the extracellular fluid)
The membrane at rest has many open K+ channels but few open Na+ or Cl- channels.
Build up of -ve charge in neuron: limited by electrical gradient vs. chemical gradient of K+.
Eq’m potential in neuron: approx. -70mV
Process not dependent on voltage-gated ion channels- which are required for action potentials.
Excitable cells
All cells have a membrane potential.
Rapid changes in membrane potential occur in these cells:
Neurons
Myocytes
Pancreatic beta cells
Membrane potential
Voltage of inside of membrane relative to outside - equilibrium
Hyperpolarisation
Inside of membrane becomes more negative
- Opening of voltage-gated K+ channels
- K+ out
Depolarisation
Inside of membrane becomes more positive
- opening of voltage-gated Na+ ion channels
- Na+ in
Resting potential
-70mV
Threshold
-55mV
Action potentials
General info:
Magnitude is independent of the strength of the original stimulus - all or nothing response depending on if threshold is reached.
Steps:
1. Resting state:
Voltage-gated ion channels are closed
- A stimulus:
Causes a few Na+ channels to open, Na+ rushes in - Depolarisation:
If threshold is reached:
Lots of Na+ channels open
Lots of Na+ rushes in - Repolarisation
K+ channels open
K+ rushes out
Na+ channels INACTIVATED and then start to close - Undershoot
Small hyperpolarisation
Also need Na+/K+-ATPase to restore Na+ and K+ concentrations
Usually start at the axon hillock
Travels long distances by regenerating itself along the axon
Voltage-gated Na+ channels
Three states: Fast
Closed —> Depolarisation
Open
Inactivated
General anaesthetics prevent VG Na+ channel transition from inactivated to closed state
Voltage-gated K+ channels
Two states: Slower
Closed —> Depolarisation
Open
Absolute refractory period
From start of depolarisation at threshold to start of undershoot.
No action potential can be generated
- Na+ channels open for first half
- Na+ channels inactivated for the second half
Relative refractory period
From start of undershoot to the end.
AP only occur if a large enough stimulus is applied
-Because some Na+ channels are in the closed state again
Refractory periods
Limit firing frequency
The action potential can only travel in one direction
Factors affecting action potential conduction speed
Axon diameter
-Larger diameter, less resistance
Temperature
-Chemical reactions occur faster at warmer temperatures
Degree of myelination
-Insulates the axon membrane in vertebrates to enable faster conduction speed
Conduction speed is affected more by myelination than by axon diameter
Saltatory conduction and smooth conduction
Nodes of ranvier restrict sites where Na+ and K+ channels can open and close (120 m/sec) - salutatory conduction
Unmyelinated nerve fiber - smooth conduction
Presynaptic and postsynaptic neurons
Presynaptic neuron —> Postsynaptic neuron
Presynaptic neuron —> effector cell (muscle)
Electrical synapses
- at gap junctions
- direct electrical current between cells
- few synapses of this type
Chemical synapses
- release of a neurotransmitter
- neurotransmitter released by presynaptic neuron
Neurotransmitter release at chemical synapse
Action potential propagated down the axon to the presynaptic membrane which opens voltage-gated Ca2+ channels. Ca2+ bind to synaptic vessels containing neurotransmitter and release the neurotransmitter into the synapse through exocytosis of the vesicles to the presynaptic membrane.
The neurotransmitter binds to ligand-gated ion channels on the postsynaptic neuron.
These open to let in some Na+. Leads to a postsynaptic potential - graded
Excitatory postsynaptic potential
EPSP. If depolarisation at postsynaptic membrane
Inhibitory postsynaptic potential
IPSP. If hyperpolarisation at postsynaptic membrane
Temporal summation
Most postsynaptic potentials decline before they reach axon hillock (threshold).
Several EPSPs come from the same synapse just after each other.
Can reach threshold at axon hillock - action potential!
Spatial summation
Two or more EPSPs from different synapses
Postsynaptic potentials vs. Action potentials
PSP
- EPSP or IPSP
- Graded
- Local
- At the cell body or dendrites
Action Potential
- depolarisation
- ‘all or nothing’
- EPSPs can add up and cause an action potential
- Generated at the axon hillock
- Travels along the axon
Direct synaptic transmission
- Neurotransmitter opens ion channels on the postsynaptic membrane
- action via ligand-gated ion channels
- Ion channel linked receptors
Indirect synaptic transmission
- Neurotransmitter binds to a receptor on the post synaptic membrane
- Activates a signal transduction pathway
- Involves a second messenger
- Can result in EPSPs or IPSPs depending on the neurotransmitter and the receptor type
- GPCRs
Examples of amino acid neurotransmitters
GABA
Glycine
Glutamate
Aspartate
Amine neurotransmitters
Acetylcholine
Dopamine
Norepinephrine
Serotonin
Removal of neurotransmitters from the synaptic cleft
- Recycled by selective uptake by transporters
- Taken up by astrocytes
- Broken down by enzymes e.g. acetylcholinesterase
- Diffusion
Primitive nervous systems
All animals except sponges have nervous systems
Hydra, Sea star, flatworm
Nervous system of more complex vertebrates
Segmentally arranged clusters of neurons = ganglia
Ganglia = PNS
Connected to CNS
Leeches, insects
Peripheral Nervous system
Cranial nerves (12 pairs in mammals
Spinal nerves (31 pairs in mammals)
Most spinal and cranial nerves contain sensory neurons and motor neurons
Skin –> Sensory neuron (Dorsal root ganglion –> Dorsal root) –> Interneuron –> Motor neuron (Ventral root) –> muscle
The reflex arc
Hit on knee, cell body of sensory neuron in dorsal root ganglion, SN passes through gray matter of spinal chord, transmitted directly to mortor neuron of quadricep, and also to interneuron then motor neuron of hamstring
Voluntary and Involuntary motor neurons
Voluntary - Somatic nervous system
Mostly involuntary - Autonomic nervous system:
Sympathetic division
Parasympathetic division
Enteric division (don’t really have to worry about)
Sympathetic nervous system
Fight or flight Bronchi dilate Heart beats faster Glycogen to glucose Adrenaline secretion Digestion is inhibited Inhibits salivary gland secretion
Parasympathetic nervous system
Rest and digest Calming Constricts bronchi in lungs Slows heart Stimulates digestion Stimulates salivary gland secretion
Central Nervous system
Brain + Spinal chord
Cerebrospinal fluid
Protects the CNS
- Clear fluid
- 4 ventricles and central canal
- Supply of nutrients and hormones, remove waste
Gray and white matter
Gray - dendrites, unmyelinated axons, cell bodies
White - myelinated axons in trancts
Brain regions
Developed from the embryonic:
- Forebrain (cerebrum and diencephalon)
- midbrain (part of brainstem)
- hindbrain (part of brainstem, cerebellum)
The brainstem
Basic functions
Homeostasis, movement
Transfer of info to the rest of the brain
Reticular formation -selectivity
Cerebellum
- Coordination
- Motor function
- Cognitive and perceptual functions
The Diencephalon
Epithalamus
- Connects limbic system (emotional center) to the rest of the brain
- Pineal gland: melatonin (sleep)
Thalamus
- Input from sensory neurons
- Output via motor neurons
Hypothalamus
- Homeostatic regulation - hormones (via posterior and anterior pituitary glands)
- Biological clock
- Temperature regulation
- Survival - hunger, thirst…
Four lobes of the cerebral cortex
Frontal lobe
-Skeletal muscle control, decision making, planning, Broca’s area
Temporal lobe
-hearing (auditory cortex), Wernicke’s area
Parietal lobe
-Touch (somatosensory cortex), sensory association cortex
Occipital lobe
-Visual association cortex
Broca and Wernicke’s areas
Broca - speech generation
Wernicke - speech hearing
Emotions - the limbic system
Olfactory bulb Hypothalamus Thalamus Amygdala Hippocampus: mem
Memory
Short term: Hippocampus
Long term: Cerebral cortex
Remembering: communication between the cerebral cortex and the hippocampus
Long-term potentiation in vertebrates (interesting)
High frequency transmission of glutamate. Results in increase in size of postsynaptic potentials at synapse. Last for days or weeks. A fundamental process for storing memories and hence for learning.
