The Nervous System
The nervous system has two divisions. The central nervous system (CNS) consists of the brain and spinal cord; it integrates and processes the information sent by nerves and is the body's control centre. The peripheral nervous system (PNS) consists of the nerves that carry sensory messages to the CNS and motor messages from the CNS out to the muscles and glands (the effectors).
The PNS divides into the somatic (voluntary) and autonomic (involuntary) systems; the autonomic system further divides into the sympathetic and parasympathetic divisions.
- Somatic nervous system - relays information between the skin, skeletal muscles and CNS. This pathway is under conscious control (the organism decides whether to move) - except for reflexes.
- Autonomic nervous system - relays information from the CNS to the organs; it is entirely involuntary.
- Sympathetic division - takes control in times of stress, as in the fight-or-flight response.
- Parasympathetic division - controls the body in times of rest.
NEURONS AND NERVES
A neuron is a single nerve cell, the basic structural and functional unit of the nervous system. Neurons generate electrical messages called action potentials, which allow fast transmission of information throughout the body. A nerve is a group of neurons organised into a tissue. Nerves belong to the PNS, spreading from the brain and spinal cord into every other part of the body.
THE STRUCTURE OF A NEURON
All neurons share four common features:
- Dendrites - short, branching terminals that receive impulses and relay them to the cell body.
- Cell body - contains the nucleus and is the site of the cell's metabolic reactions.
- Axon - conducts impulses away from the cell body; it varies in length from 1 mm to 1 m. Many axons are wrapped in a myelin sheath (made by Schwann cells), which protects the axon and allows the impulse to 'skip' along it far more rapidly.
- Axon terminals - button-like endings through which the axon makes synaptic contact with other nerve cells or with effector cells.
Impulses travel in one direction only: dendrites → cell body → axon → axon terminals.
THREE TYPES OF NEURON
| Neuron | Function | Structure |
|---|---|---|
| Sensory | carries impulses from sensory receptors to the CNS | cell body with no dendrites attached and only one branch extending from it; some have a receptor cell attached |
| Interneuron | found in the CNS — the link between sensory and motor neurons; processes incoming input and relays the outgoing response | large cell body with many dendrites; axon short in the spine, long in the brain, with extensively branching terminals |
| Motor | carries impulses from the CNS to effectors (muscles, glands, organs) | multiple dendrites; a long axon terminating at the effector — the longest neurons in the body, up to 1 m |
The three types work in sequence. For example: receptors in the eye detect a glass of water (stimulus) → sensory neurons transmit the message to the CNS → interneurons in the brain sort and interpret it and determine a response → motor neurons carry the response message to the bicep (effector) → the arm reaches out and lifts the glass.
SYNAPSES AND NEUROTRANSMITTERS
Neurons do not touch one another. Between them is a microscopic gap, the synapse, which the electrical message cannot cross; the signal crosses chemically instead. Four steps are involved:
- The arriving impulse triggers the release of neurotransmitters (chemical messengers) by exocytosis into the synapse.
- The neurotransmitters diffuse across the gap and are detected by a specific receptor on the postsynaptic neuron, re-starting the impulse.
- The neurotransmitter is then reabsorbed into the presynaptic neuron by a specific neurotransmitter reuptake pump (a transport protein), ready for reuse.
- The junctions between motor neurons and their target effectors (neuromuscular junctions) use neurotransmitters in the same way.
Neurotransmitter is released by exocytosis, binds its specific receptor on the postsynaptic neuron, and is then recycled through the reuptake pump.
| Neurotransmitter | Role | Imbalance linked to |
|---|---|---|
| Dopamine | movement, attention, learning, emotion; reward pathway (pleasure, addiction) | excess → schizophrenia; deficiency → Parkinson's disease |
| Acetylcholine (ACh) | muscle action, memory, learning | Alzheimer's disease — ACh-producing neurons are damaged |
| Serotonin | mood, hunger, sleep, arousal | low levels → depression |
| Glutamate | excitatory; memory | excess → overstimulation, seizures, migraines |
| GABA | inhibitory | low levels → insomnia, seizures |
THE REFLEX ARC
A reflex is a rapid, predictable and involuntary response to a stimulus - for example sneezing, coughing, blinking, or withdrawing the hand from a hot pan. The reflex arc is the direct route that produces it: from a sensory neuron, to an interneuron in the spinal cord (not the brain), to a motor neuron and an effector. Because the brain is bypassed, the response is fast; the brain is informed only after the response has occurred (and records the event).
The reflex arc: receptor → sensory neuron → interneuron in the spinal cord → motor neuron → effector.
There are two categories of reflex:
- Somatic reflexes - activate skeletal muscles (e.g. withdrawing the hand from a hot surface).
- Autonomic reflexes - regulate smooth muscle, the heart and blood pressure, glands, and digestion.