Chapter 12: Nervous Tissue Flashcards
The nervous system
Responsible for all our behaviours, memories, and movements.
It helps to maintain homeostasis
3 basic functions of nervous system
1) Sensory function - sensory input from receptors detect changes inside/outside the body.
2) Integrative function - sensory input is interpreted and coordinated with an appropriate motor response.
3) Motor function - motor output is brought to an effector organ (such as a muscle or gland)
2 divisions of the nervous system
1) Central nervous system - contains the brain and spinal cord
2) Peripheral nervous system - contains the cranial and spinal nerves, ganglia, and sensory receptors
Subdivisions:
a) sensory division
b) motor division
Sensory division
Part of the PNS which delivers sensory information from various sensory receptors throughout the body to the CNS.
Carries info such as smell, hearing, vision, taste, touch, temp, pain…etc.
Motor division
Part of the PNS where motor neurons pass down impulses from CNS to effectors (muscles/glands) in order to produce movements.
Subdivisions:
i) Somatic nervous system
ii) Autonomic nervous system
Somatic nervous system
Part of the motor division of the PNS which is under voluntary control.
Motor neurons will conduct impulses from CNS to skeletal muscles.
Autonomic nervous system
Part of the motor division of the PNS which is under involuntary control.
Motor neurons conduct impulses from CNS to smooth and cardiac muscles
Subdivisions:
Sympathetic - fight or flight
Parasympathetic - rest of digest
Neurons
Nerve cells which are the functional units of the nervous system. They will generate action potentials (impulses). They cannot undergo mitosis.
Parts of a neuron:
Soma - hold the nucleus, organelles, nissl bodies, neurofibrils
Dendrites - carry info towards the cell body of the neuron
Axons - carry info away from cell body of the neuron
Neuroglia
Supportive/protective cells that aid neurons.
They are smaller and more numerous than neurons, and are capable of mitosis.
6 types:
- astrocytes, oligodendrocytes, microglia, ependymal cells (CNS)
- Schwann cells, satellite cells (PNS)
Axons
Can be
a) Myelinated - covered by layers of neuroglial membrane and have nodes of ranveir.
If in CNS = wrapped by oligodendrocytes
If in PNE = wrapped by schwann cells
b) Unmyelinated - surrounded by a thin Schwann cell membrane that enclosed several axons
Grey matter
Consists of neuron cell bodies, dendrites, unmyelinated axons, axon terminals, neuroglia
White matter
Consists of neuron processes, mainly myelinated axons (myelin sheath)
Nucleus
Cluster of neuron cell bodies in the CNS
Ganglion
Cluster of neuron cell bodies in the PNS
Tract
Bundle of neuron processes in the CNS
Nerve
Bundle of neuron processes the PNS
Ion channels
Ions can diffuse down an electrochemical gradient through a channel protein. They travel from an area that is charged, to an area that is oppositely charged.
4 kinds:
- leakage channels
- ligand-gated channels
- mechanically-gated channels
- voltage-gated channels
Leakage channels
The gates of the channel will randomly open and close
There are more K+ leakage channels than Na+ leakage channels in the cell membrane so more + leaks out causing a more negative charge inside the cell
Ligand-gated channels
Channel will open and close in response to a certain chemical stimulus
Mechanically-gated channels
Channel will open or close in response to a mechanical stimulus (such as vibration, touch, pressure)
Voltage-gated channel
Channel will open in response to a change in membrane potential (voltage)
Resting membrane potential
There is difference in electrical charge that exists across the cell membrane.
Resting membrane potential = -70mV,
Outside of the cell (extracellular fluid) there is a higher concentration of Na+, and Cl-
Inside the cell (intracellular fluid) there is a higher concentration of K+ and negative proteins.
Na/K pump
The sodium potassium pump maintains the diffusion of Na+ and K+.
3Na+ bind to the pump and ATP donates its phosphate which will change the shape and expel the 3Na outside the cell. While it’s still open to the outside, 2K+ bind to the pump, the phosphate detaches which changes the shape of the pump, and the 2K+ and released into the cell.
Why can’t equilibrium be reached in a neuron?
Neurons need the difference in ion concentration in order to generate a nerve impulse / action potential.
Threshold stimulus
The smallest amount of stimulation or force that is needed to initiate a response.
Must get the membrane potential to -55mV to make an action potential
(Essentially it triggers an action potential)
Steps of generating an action potential
- Resting state
- Depolarizing phase
- Repolarizing phase
- Hyperpolarizing phase (if necessary)
- Refractory period
Resting state
All the voltage gated Na+ K+ channels are closed. The membrane is at a resting membrane potential (-70mV).
There is a buildup of negative charges inside the cell.
Depolarizing phase
The voltage-gated Na+ channels rapidly send in Na+ ions, which makes the inside less negative (depolarizes it).
The membrane potential went from -70mV to -55mV (threshold needed to make action potential) to +30mV.
Repolarizing phase
The Na+ channels close and the K+ channels start to slowly open and it becomes repolarized as the K+ ions are expelled from the neuron, leaving the negative charges in it.
The membrane potential goes back to -70mV as the ion distributions of the resting state are restored
Hyperpolarizing phase
(Doesn’t always happen)
When the K+ channels close too slowly, too much K+ could leave the neuron.
The membrane potential could get to -90mV, but will eventually get back to -70mV.
Refractory period
A time period where a second action potential cannot be generated as it is resetting from the last one. Makes sure there is only 1 action potential going on at a time.
2 types:
Absolute refractory period - an action potential is not possible at all
Relative refractory period - a stronger stimulus is required to generate an action potential
Conducting of an action potential
When the action potential goes towards the axon terminals.
2 types:
Continuous conduction - occurs along unmyelinated axons (much slower)
Saltatory conduction - occurs along myelinated axons (much faster).
The action potential jumps from node to node regenerating itself. (Depolarization happens at each node)
Synapse
A junction between 2 neurons
Can be between a neuron and its effector
or
Between 2 neurons:
Axodentritic - axon terminal to dendrite
Axosomatic - axon terminal to soma
Axoaxonic - axon terminal to axon terminal
The first neuron is presynaptic, the second is postsynaptic. They are separated by the synaptic cleft.
Transmission across synapses
An action potential can’t cross the synaptic cleft because there are ligand gates channels it must go through, so it must be converted to a chemical signal using neurotransmitters.
Removal of the neurotransmitter
Can occur by:
1. Breakdown via enzymes in the synaptic cleft
2. Re-uptake into presynaptic neuron
3. Uptake into neighbouring neuroglia cells
4. Diffusion away from cleft
Divergence
The presynaptic neuron may branch many times to synapse with many postsynaptic neurons
Convergence
Many presynaptic neuron endings may synapse with a single postsynaptic neuron
Astrocytes
The largest, most numerous neuroglial cells in the CNS
Function to maintain chemical environment around the neurons
Microglial cell
Smaller neuroglial cells in the CNS
Function to clean up cellular debris / invaders of the neurons
Ependymal cells
Bottom layer of neuroglial cells in the CNS
Function to produce cerebrospinal fluid and secrete it
Oligodendrocytes
Round (alien-looking) neuroglial cells in the CNS
Function by wrapping themselves around neurons to speed up the conduction of nerve impulses (form myelin sheaths)
Schwann cells
In the PNS
Wrap around part neuron to speed up nerve impulses
Satellite cells
In the PNS
Function to help in structural support, and mediate the exchange of things passing in and out of the neuron.
