Nervous Coordination

The nervous system allows organisms to detect changes in their environment and respond rapidly. It relies on specialised cells called neurones that transmit electrical impulses at high speed. Understanding the structure and function of different neurone types is essential for A-Level Biology and underpins topics on reflexes, the brain, and muscle contraction.

Key Definition: A neurone is a specialised cell adapted for the rapid transmission of electrical impulses (action potentials) from one part of the body to another.

The Organisation of the Nervous System

The mammalian nervous system is divided into two main parts:

Central Nervous System (CNS) — the brain and spinal cord. The CNS acts as the coordinator, processing sensory information and initiating motor responses.
Peripheral Nervous System (PNS) — all the nerves outside the CNS, including cranial and spinal nerves. The PNS connects receptors and effectors to the CNS.

The PNS is further subdivided:

Somatic nervous system — controls voluntary actions (skeletal muscle).
Autonomic nervous system — controls involuntary actions (smooth muscle, cardiac muscle, glands). This is further divided into the sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) divisions.

Types of Neurone

There are three main types of neurone, each with a distinct role and structure:

1. Sensory Neurones

Carry impulses from receptors to the CNS.
Have a long dendron that carries the impulse from the receptor towards the cell body, and a short axon that carries the impulse from the cell body into the CNS.
The cell body is located off to one side of the neurone (in a dorsal root ganglion), not at the end.
Also called afferent neurones.

2. Relay Neurones (Interneurones)

Found entirely within the CNS (brain and spinal cord).
Connect sensory neurones to motor neurones.
Have short dendrites and short axons, forming complex interconnections.
Allow integration and processing of information.
Enable reflex arcs and higher brain functions.

3. Motor Neurones

Carry impulses from the CNS to effectors (muscles or glands).
Have a long axon that extends from the CNS to the effector, and many short dendrites that receive impulses from relay neurones.
The cell body is located within the CNS.
Also called efferent neurones.

Feature	Sensory Neurone	Relay Neurone	Motor Neurone
Direction of impulse	Receptor → CNS	Within CNS	CNS → Effector
Cell body position	Off to one side (dorsal root ganglion)	Within CNS	Within CNS
Axon length	Long dendron, short axon	Short axon	Long axon
Myelinated?	Usually yes	Usually no	Usually yes

Structure of a Typical Motor Neurone

A motor neurone has the following key structures:

Cell body (soma): Contains the nucleus, rough endoplasmic reticulum (Nissl granules for protein synthesis), mitochondria, and other organelles. Many ribosomes are present to synthesise neurotransmitters and ion channel proteins.
Dendrites: Short, branched extensions of the cell body that receive impulses from other neurones via synapses.
Axon: A single, long cytoplasmic extension that carries the action potential away from the cell body towards the effector. The cytoplasm within the axon is called axoplasm, and the membrane is called the axon membrane (axolemma).
Axon hillock: The region where the axon joins the cell body; this is where action potentials are initiated.
Axon terminal (synaptic knob): The swollen end of the axon that forms a synapse with the next neurone or an effector. Contains synaptic vesicles filled with neurotransmitter and many mitochondria to provide ATP for neurotransmitter release.

The Myelin Sheath

Many neurones in the PNS are myelinated — they are surrounded by a fatty, insulating layer called the myelin sheath.

Formation

The myelin sheath is formed by Schwann cells in the PNS (and oligodendrocytes in the CNS).
Each Schwann cell wraps tightly around a section of the axon multiple times, forming concentric layers of its cell membrane.
The cytoplasm of the Schwann cell is squeezed out, leaving behind layers of phospholipid bilayer that act as an electrical insulator.
Each Schwann cell covers approximately 1–2 mm of the axon.

Nodes of Ranvier

Between adjacent Schwann cells there are small gaps (approximately 2–3 µm wide) where the axon membrane is exposed. These gaps are called nodes of Ranvier.
At the nodes, the axon membrane contains a high density of voltage-gated sodium ion channels.
The myelin sheath prevents ion exchange across the membrane in the myelinated regions (internodes), so depolarisation can only occur at the nodes.

Saltatory Conduction

In myelinated neurones, the action potential effectively jumps from one node of Ranvier to the next. This is called saltatory conduction (from the Latin saltare, meaning to jump).
This greatly increases the speed of impulse transmission — myelinated neurones can conduct impulses at up to 120 m s⁻¹, compared with only 0.5–2 m s⁻¹ in unmyelinated neurones.
Saltatory conduction also reduces the metabolic cost of transmitting impulses because fewer sodium and potassium ions need to be actively pumped back across the membrane after each impulse.

Exam Tip: Be precise with terminology. The myelin sheath is formed by Schwann cells (PNS) or oligodendrocytes (CNS). The gaps between Schwann cells are nodes of Ranvier. The impulse 'jumps' between nodes — this is saltatory conduction.

Factors Affecting the Speed of Nerve Impulse Transmission

Several factors influence how quickly an action potential travels along a neurone:

Factor	Effect on Speed	Explanation
Myelination	Increases speed	Saltatory conduction means the impulse jumps between nodes of Ranvier
Axon diameter	Larger diameter = faster	Less resistance to ion flow in the axoplasm; the ratio of surface area to volume is smaller
Temperature	Higher temperature = faster (up to a point)	Increased kinetic energy of ions and enzyme activity; protein denaturation at very high temperatures reduces speed

The Reflex Arc

A reflex arc is the simplest nerve pathway, enabling a rapid, involuntary response to a stimulus. The pathway is:

Stimulus → Receptor → Sensory neurone → Relay neurone (in CNS) → Motor neurone → Effector → Response

Features of Reflex Arcs

Rapid — the pathway involves few synapses, so the response is fast.
Involuntary — the response occurs without conscious thought, which is important for protective reflexes (e.g., withdrawing the hand from a hot surface).
Stereotyped — the same stimulus always produces the same response.

Example: The Knee-Jerk Reflex

Tapping the patellar tendon stretches the quadriceps muscle.
Stretch receptors (muscle spindles) in the quadriceps are stimulated.
Impulses travel along sensory neurones to the spinal cord.
In the spinal cord, the sensory neurone synapses directly with a motor neurone (this is a monosynaptic reflex — no relay neurone).
The motor neurone stimulates the quadriceps to contract, causing the lower leg to kick forward.

Example: The Withdrawal Reflex

A pain receptor in the finger detects a painful stimulus (e.g., a hot object).
Impulses travel along a sensory neurone to the spinal cord.
The sensory neurone synapses with a relay neurone in the grey matter of the spinal cord.
The relay neurone synapses with a motor neurone.
The motor neurone stimulates the biceps (effector) to contract, pulling the hand away.
Simultaneously, the relay neurone sends impulses to the brain so the person becomes aware of the stimulus.

Clinical Relevance: Demyelination

Multiple sclerosis (MS) is an autoimmune disease in which the immune system attacks and destroys the myelin sheath surrounding neurones in the CNS. This disrupts saltatory conduction, slowing or blocking nerve impulse transmission. Symptoms include muscle weakness, loss of coordination, numbness, and visual disturbances. This illustrates the critical importance of the myelin sheath for normal nervous system function.

Summary

The nervous system consists of the CNS and PNS, using three types of neurone: sensory, relay, and motor.
Motor neurones have a cell body with dendrites, a long myelinated axon, and synaptic knobs.
The myelin sheath, formed by Schwann cells, insulates the axon and enables saltatory conduction at nodes of Ranvier.
Saltatory conduction dramatically increases the speed of impulse transmission.
Reflex arcs provide rapid, involuntary responses to stimuli.
Speed of transmission depends on myelination, axon diameter, and temperature.