6.5 Nerves, Hormones & Homeostasis
6.5.1 State that the nervous system consists of the central nervous system (CNS) and peripheral nerves, and is composed of cells called neurones that can carry rapid electrical impulses
Organization Of The Human Nervous System
Our brain and spinal cord are our central nervous system (CNS). These two structure receive sensory information from various receptors and then interpret and process that sensory information.
If a response is needed, some portion of the brain or spinal cord initiates a response which is called a motor response. The cells that carry this information are called neurones.
Sensory neurones bring information to the CNS and motor neurones carry response information to muscles. Together they make up peripheral nerves.
A neurone is an individual cell which carries electrical impulses from one point in the body to another and does so very quickly. When many individual neurones group together into a single structure, the structure is called a nerve.
There are two categories of peripheral nerve:
- · Spinal nerves – 31 pairs (left and right) of these emerge directly from the spinal cord. They are mixed nerves, some of the neurones within them are sensory and some are motor.
- · Cranial nerves – 12 pairs of these emerge from an area of the brain known as the brainstem.
6.5.2 Draw and label the structure of a motor neurone
6.5.3 State that nerve impulses are conducted from receptors to the CNS by sensory neurones, within the CNS by relay neurones, and from the CNS to effects by motor neurones
Nerve impulses are conducted from receptors to the CNS by sensory neurones, within the CNS by relay neurones, and from the CNS to effects by motor neurones.
6.5.4 Define resting potential and action potential (depolarization and repolarization)
Resting Potential
The state of being where an area of a neurone is ready to send an action potential (but is not currently sending) is called the resting potential and this area of the neurone is said to be polarized.
Action Potential
A self-propagative wave of ion movements in and out of the neurone membrane. The movement of ions isn’t along the length of the axon, but instead consists of ions diffusing from outside the axon to the inside and from inside the axon to outside.
6.5.5 Explain how a nerve impulse passes along a non-myelinated neurone
When a nervous system pathway is traced, the form of an electrical impulse known as an action potential is being followed.
Resting Potential
The resting potential is characterized by the active transport of sodium ions and potassium ions in two different directions. The vast majority of the sodium ions are actively transported out of the axon cell into the intercellular fluid and the majority of the potassium ions are transported into the cytoplasm.
There are negatively charged organic ions permanently located in the cytoplasm of the axon. This collection of charged ions leads to a net positive charge outside the axon membrane (positive in relation to the inside) and a net negative charge inside the axon membrane.
Action Potential
The resting potential requires active transport (protein channels and ATP) to set up a concentration gradient of both sodium and potassium ions. Since sodium ions are actively transported to the outside of the membrane, they diffuse in when a channel open for this purpose. Soon after, a channel opens for potassium ions and they diffuse out of the axon.
This diffusion of sodium ions in and potassium ions out is the impulse or action potential. It’s a nearly instantaneous event that occurs in one area of an axon and is called depolarization.
This area of the axon then initiates the next area of the axon to open up the channels for sodium, then potassium and thus the action potential continues down the axon.
This is the self-propagating part of an action potential; once you start an impulse at the dendrite end of a neurone, that action potential will self-propogate itself to the far axon-end of the cell.
Returning To Resting Potential
When one area of an axon has opened a channel to allow sodium ions to diffuse in and potassium ions to diffuse out, that area can’t send another action potential until the sodium and potassium ions have been restored to positions characteristic of the resting potential.
Diffusion can’t do this so active transport is required to pump these two ions to their resting potential positions. This is called repolarization.
The time it takes for any one neurone to send an action potential and then repolarize so it can send another is called the refractory period of that neurone.
6.5.6 Explain the principles of synaptic transmission
If a neurone’s axon is touched with an electric probe, an action potential is begun which travels in both directions along the axon. However, the action potential is continued to the next neurone only in the direction in which synaptic transmission can occur.
The sensory neurones of the pathway are lined up so that the terminal end of the axon of the first neurone adjoins the dendrites of the next neurone. The first neurone is called the presynaptic neurone and the second is the postsynaptic neurone. This is because a chemical communication called a synapse occurs between these two neurones.
Mechanism Of Synaptic Transmission
At the far end of axons are swollen membranous areas called terminal buttons. Within these terminal buttons are many small vesicles filled with a chemical called a neurotransmitter. The term ‘neurotransmitter’ is used for any chemical that’s used for synaptic transmission.
When an action potential reaches the area of the terminal buttons, it initiates the following sequence of events:
- Calcium ions diffuse into the terminal buttons
- Vesicles containing neurotransmitter fuse with the plasma membrane and release neurotransmitter
- Neurotransmitter diffuses across the synaptic gap from the presynaptic neurone to the postsynaptic neurone
- Neurotransmitter binds with a receptor protein on the postsynaptic neurone membrane
- This binding results in an ion channel opening and sodium ions diffusing in through this channel
- This initiates the action potential to begin moving down the postsynaptic neurone because it’s depolarized
- Neurotransmitter is degraded (broken into two or more fragments) by specific enzymes and is released from the receptor protein
- The ion channel closes to sodium ions
- Neurotransmitter fragments diffuse back across the synaptic gap to be reassembled into the terminal buttons of the presynaptic neurone.
