5.3- Neuronal communication Flashcards
Neurones- outline the reflex arc
Sensory receptor - sensory neurone - relay neurone (CNS- brain/spinal cord) - motor neurone - effector (muscle or gland)
Impulse is transmitted as along neurones as an action potential
Name and describe the functions of 3 different types of neurones
- motor neurones- carry an action potential from the CNS to an effector
- sensory neurones - carry the action potential from a sensory receptor to the CNS
- relay neurones- connect sesnory and motor neurones
Describe the general structure of neurones
- many are very long so that they can transmit the action potential over a long distance
- a cell body contains the nucleus, many mitochondria, and ribosomes
- an axon carries impulses away from the cell body
- numerous dentrites connect to other neurones- carry impulses towards the cell body
Sensory neurones- diagram, structure
- have a long dendron carrying the action potential from a sensory receptor to the cell body
- cell body is positioned just outside the CNS
- have a short axon carrying the action potential into the CNS
Relay neurones- diagram, structure
- have many short dendrites and a short axon
- the number of dendrites and the number of divisions of the axon is variable
- relay neurones are an essential part of the nervous system- conduct impulses in coordinated pathways
Motor neurones- diagram, structure
- have cell body in the CNS
- have a long axon that carries the action potential out to the effector
Name 2 categories of neurones
-Myelinated and non-myelinated
Describe myelinated neurones
- around 1/3 of the peripheral neurones in vertebrates are myelinated- insulated by an individual myelin sheath
- most sensory and motor neurones are associated with many Schwann cells- called the myelin sheath
- these Schwann cells are wrapped tightly around the neurone so the sheath consists of several layers of plasma membrane and thin cytoplasm from the Schwann cell
- at intervals of 1-3mm along the neurone there are gaps in the myelin sheath- the nodes of Ranvier- each node is very short (2-3 micrometres long)
Describe non-myelinated neurones
- also associated with Schwann cells, but several may be enshrouded in one loosely wrapped Schwann cell
Describe advantages of myelination
- myelinated neurones can transmit an action potential much more quickly than non-myelinated neurones can: 100-120 ms -1 vs 2-20 ms-1
- myelinated neurones carry action potentials from sensory receptors to the CNS and from the CNS to effectors - carry action potentials over long distances (longest neurone in humans can be around 1m)
- increased speed of transmission mean the action potential reaches the end of the neurone much more quickly - enables a more rapid response to a stimulus
- non-myelinated neurones tend to be shorter and carry action potentials over a short distance- often used in coordinating body functions such as breathing, and the action of the digestive system- increased transmission of speed not so important
Describe resting potential in neurones
- Sodium/potassium ion pump uses ATP to actively transport 3 sodium ions are pumped out for every 2 potassium ions that are pumped in
- Gated sodium ion channels are kept closed
- Some of the potassium ion channels are open
- Means the plasma membrane is more permeable to potassium ions than to sodium ions- potassium ions tend to diffuse (by facilitated diffusion) out of the cell
- The cell cytoplasm contains large organic anions (negatively charged ions)
- Means the interior of the cell is maintained at a negative potential compared with the outside
- The cell membrane is said to be polarised
- The potential difference across the cell membrane is about -60mV
- This is called the resting potential
- In myelinated neurones, the ion exchanges described only occur at the nodes of Ranvier
Describe the stages of an action potential
1:
- The membrane starts in resting state
- Polarised- inside of cell is -60mv compared to outside
- There is a higher concentration of sodium ions outside than inside
- There is a higher concentration of potassium ions inside than outside
2:
- Sodium ion channels open and some sodium ions diffuse into the cell
3:
- Depolarisation:
- becomes less negative with respect to the outside
- reaches the threshold value of -50mv
4:
- Positive feedback causes nearby voltage-gated sodium ion channels to open
- many sodium ions flood in
- as more sodium ions enter, the cell becomes positively charged compared to the outside
5:
- The potential difference across the plasma membrane reaches +40 mV
- the inside of the cell is positively charged compared to the outside
6:
- The sodium ion voltage-gated channels close
- potassium ion voltage-gated channels open
7:
- Repolarisation:
- Potassium ions diffuse out of the cell bringing the potential difference back to negative compared with the outside
8:
- The potential difference overshoots slightly, making the cell hyperpolarised
- The potassium ion voltage gated channels close
9:
- The original potential difference is restored by sodium potassium ion pumps so that the cell returns to its resting state
Describe sensory receptors
- specialised cells that can detect changes in our surroundings
- most are energy transducers that convert one form of energy to another
Describe transducers (sensory receptors)
- most are adapted to detect changes in a particular form of energy
- each change is called a stimulus- sensory receptors respond by creating a signal in the form of electrical energy- nerve impulse
Name examples of sensory receptors, their stimulus, and energy changes involved
- Light sensitive cells (rods and cones) in the retina- detect change in light intensity- convert light to electrical
- Temperature receptors in skin and hypothalamus- detect change in temperature- heat to electrical
- Pacinian corpuscles in the skin- detect change in pressure on skin- mechanical to electrical
- Vibration receptors in cochlea of ear- detect change in sound- sound to electrical
- Olfactory cells in epithelium lining in the nose, chemical receptors in taste buds on tongue- detect chemical changes in the air and food- chemical to electrical
Describe Pacinian corpuscle’s
- pressure sensor that detects changes in pressure on skin
- oval-shapes structure that consists of a series of concentric rings of connective tissue wrapped around the end of a nerve cell
- Pressure on skin causes the rings of connective tissue to deform
- Causes sodium ion channels to open- sodium ions enter the sensory neurone- generator potential
- Reaches threshold potential- sodium ion gated channels open- starts action potential
- only sensitive to changes that deform the rings of connective tissue- if pressure is constant they stop responding
What causes an action potential to move along a neurone
Local currents in the cytoplasm of the neurone