Neurons: MDCAT Biology notes
Neurons MDCAT notes: neuron structure and types, reflex arc, resting and action potential, Na-K pump, saltatory conduction and synaptic transmission.
Structure of a neuron
- Cell body: contains the nucleus, which every neuron has, and Nissl's granules. These are groups of ribosomes on rough ER that make proteins.
- Dendrites: short branches that receive signals and carry them toward the cell body.
- Axon: a long, thin process that carries impulses away from the cell body. Its terminals (synaptic knobs) secrete neurotransmitters.
- Myelin sheath: an insulator formed in the peripheral nervous system by Schwann (neurilemma) cells. The gaps between sections are the nodes of Ranvier.
The impulse travels dendrite → cell body → axon.
| Neuron | Carries impulse | Note |
|---|---|---|
| Sensory (afferent) | Receptor → CNS | A single long dendron; dendrite and axon differ only in length; cell body in the dorsal root ganglion |
| Associative (interneuron) | Within the CNS | Links sensory to motor neurons; most CNS neurons are multipolar |
| Motor (efferent) | CNS → effector (muscle or gland) | In the eye muscles, one motor neuron may supply only 3–5 fibres |
Reflex arc
A reflex is a rapid, automatic, involuntary response, such as the knee jerk. It does not involve the conscious brain. The pathway is receptor → sensory neuron → associative neuron (spinal cord) → motor neuron → effector. Most reflex arcs are polysynaptic. Coordination needs receptors, neurons and effectors, and effectors are the structures that produce the response.
Resting membrane potential
- At rest the neuron is polarized, at about −70 mV. The outside is positive and the inside is negative.
- $\mathrm{Na^+}$ is about 10 times more concentrated outside. $\mathrm{K^+}$ is about 30 times more concentrated inside.
- The Na–K pump moves 3 $\mathrm{Na^+}$ out for every 2 $\mathrm{K^+}$ in, using ATP.
- The resting membrane is leaky to $\mathrm{K^+}$, so $\mathrm{K^+}$ diffuses out (efflux). Large negative proteins stay inside.
Action potential
- A stimulus above the threshold opens voltage-gated $\mathrm{Na^+}$ channels.
- Depolarization: $\mathrm{Na^+}$ rushes in and the inside becomes positive (about +50 mV).
- Repolarization: $\mathrm{Na^+}$ gates close and $\mathrm{K^+}$ gates open; $\mathrm{K^+}$ leaves.
- Hyperpolarization: the $\mathrm{K^+}$ gates are slow to close, so the potential briefly overshoots to between −70 and −90 mV.
- Refractory period: the brief time when a new impulse cannot fire, while the original ion distribution and polarity are restored.
An action potential lasts about 1 millisecond. A nerve impulse is an electrochemical wave.
Speed of conduction
In myelinated fibres the impulse jumps from node to node, which is called saltatory conduction. In human myelinated fibres it reaches about 100–120 m/s. Speed rises with myelination and with greater axon diameter. So a wide, myelinated axon is the fastest.
Synapse
A synapse is the junction between two neurons, or between a neuron and a muscle cell. The gap is the synaptic cleft.
- The impulse reaches the presynaptic knob, and $\mathrm{Ca^{2+}}$ enters.
- Vesicles release neurotransmitter, which is made in the presynaptic neuron, into the cleft.
- It binds receptors on the postsynaptic membrane. $\mathrm{Na^+}$ gates open, $\mathrm{Na^+}$ diffuses in, and the membrane depolarizes.
- The transmitter is then broken down by enzymes. Acetylcholine is broken down by acetylcholinesterase, and adrenaline by monoamine oxidase.
Electrical synapses pass current directly and have no neurotransmitter.
Neurotransmitters
- Acetylcholine: the main transmitter outside the CNS. Noradrenaline is also used in the nervous system.
- Dopamine, serotonin, glutamate and GABA act mainly within the CNS.
- Endorphins are made in the brain. They act as neurotransmitters and hormones, reduce pain and give a feeling of well-being.
Autonomic nervous system
The autonomic system controls involuntary targets: visceral organs, smooth muscle, cardiac muscle and glands (for example salivation). It does not control skeletal muscle or thoughts and emotions.
Common MDCAT traps
- Depolarization is $\mathrm{Na^+}$ moving in; repolarization is $\mathrm{K^+}$ moving out.
- The pump ratio is 3 $\mathrm{Na^+}$ out to 2 $\mathrm{K^+}$ in.
- $\mathrm{Na^+}$ is 10 times higher outside and $\mathrm{K^+}$ is 30 times higher inside. Do not swap these.
- A reflex does not need the brain, but it does need the spinal cord.
- Schwann cells are found in the peripheral nervous system.
Quick revision
- Resting potential is −70 mV, with the outside positive.
- Nissl's granules are ribosomes.
- Myelinated fibres conduct at 100–120 m/s.
- Sensory neuron cell bodies are in the dorsal root ganglion.
- Acetylcholine is the main transmitter outside the CNS.