Neural signalling

A **nerve cell** — the cell specialised to carry **electrical impulses** around the body.

**Dendrites → cell body → axon → axon terminals** (always one direction).

They **receive incoming signals** and carry them towards the cell body.

It carries the **nerve impulse away from the cell body** towards the axon terminals.

It is a **fatty layer** that **insulates the axon** and **speeds up** the nerve impulse.

The **gaps between segments of the myelin sheath**, where the impulse 'jumps' so it travels faster.

It carries impulses **from receptors TOWARDS the CNS** (it brings information in from the senses).

It carries impulses **from the CNS TO effectors** (muscles and glands), producing a response.

By **direction**: sensory carry impulses **TO** the CNS, motor carry impulses **FROM** the CNS.

A **muscle or gland** that carries out a response (by contracting or by releasing a secretion).

The **brain** and the **spinal cord**.

**All the nerves outside the brain and spinal cord**, connecting the CNS to the rest of the body.

The voltage across the membrane of a neuron that is **not** conducting an impulse — the inside is about **−70 mV** (negative) relative to the outside.

About **−70 mV** (the inside is negative relative to the outside).

**Negative** — about −70 mV compared with the outside.

A membrane protein that uses **ATP** to move **Na⁺ out** of the neuron and **K⁺ in**, against their concentration gradients.

**3 sodium ions (Na⁺) out** and **2 potassium ions (K⁺) in**.

The pump moves **3 positive ions out for every 2 in**, so more positive charge leaves than enters; **K⁺ leaking back out** makes it more negative still.

It moves ions **against their concentration gradients** — this is **active transport**, which requires energy from ATP.

From **cell respiration** (in the neuron's mitochondria).

Movement of a substance across a membrane **against** its concentration gradient, requiring energy from **ATP**.

It is **lost** — no respiration → no ATP → the pump stops → the ion gradients run down.

Some **K⁺ leaks back out** down its concentration gradient, making the inside **more negative**.

**Outside** the axon (the pump keeps Na⁺ high outside and low inside).

**Inside** the axon (the pump keeps K⁺ high inside and low outside).

About **−70 mV**, with the inside of the axon **negative** compared with the outside.

A rapid, temporary **reversal of the membrane potential** (from about −70 mV up to +40 mV and back) that travels along the axon as a nerve impulse.

Voltage-gated **Na⁺ channels open** and **Na⁺ ions rush IN**, so the inside becomes positive and the membrane potential rises to about +40 mV.

**K⁺ channels open** and **K⁺ ions move OUT**, so the inside becomes negative again and the membrane potential falls back towards −70 mV.

**Sodium (Na⁺)**, moving **INTO** the axon.

**Potassium (K⁺)**, moving **OUT of** the axon.

The membrane potential a stimulus must reach to trigger an action potential. Below it nothing fires; at or above it a full action potential fires.

An action potential fires **fully or not at all** — every one is the **same size**, regardless of how strong the stimulus is.

It makes the neuron fire action potentials **more frequently** — it does **not** make each action potential bigger.

Each region **depolarises the next region**, regenerating the impulse so it stays the **same size** all the way along.

The region just behind the impulse is briefly **recovering (refractory)** and cannot fire again straight away, so the impulse moves forward only.

**Depolarisation** — Na⁺ ions entering the axon (membrane potential rising towards +40 mV).

**Repolarisation** — K⁺ ions leaving the axon (membrane potential falling towards −70 mV).

The **Na⁺/K⁺ pump**, which pumps Na⁺ out and K⁺ in to re-establish the resting ion balance.

**Myelination**, **saltatory conduction** (nodes of Ranvier), and **axon diameter**.

A **fatty insulating layer** that wraps around the axon of some neurons.

A **gap in the myelin sheath** where the axon membrane is exposed and the action potential is regenerated.

Conduction in which the impulse **'jumps' from one node of Ranvier to the next**, instead of moving continuously along the membrane.

The myelin **insulates** the axon, so the impulse only forms at the nodes and **jumps** between them — much faster than continuous conduction.

**Only at the nodes of Ranvier** — the gaps in the myelin; the sheath insulates the rest of the axon.

A **wider** axon conducts **faster** because it has **less internal resistance** to the flow of charge.

A **myelinated** axon — it uses fast saltatory conduction; an unmyelinated axon conducts slowly and continuously.

The **thin, unmyelinated** one — no saltatory conduction and high internal resistance.

The **wide, myelinated** one — saltatory conduction plus low internal resistance.

It comes from the Latin for **'to jump'** — the impulse leaps from node to node.

The fatty myelin sheath acts like the plastic coating on a wire, **insulating** the axon so the impulse only forms at the bare nodes.

The **junction (gap)** between two neurons, where a signal passes from one to the next using a **chemical** (neurotransmitter).

The **narrow gap** between the two neurons that the neurotransmitter **diffuses across**.

The **chemical messenger** released into the synaptic cleft that carries the signal across the gap.

**Neurotransmitter** — ready to be released from the presynaptic neuron.

The two neurons are separated by the cleft; electricity cannot cross the gap, so a **neurotransmitter** bridges it.

**Calcium ions (Ca²⁺) entering** the presynaptic neuron when the impulse arrives.

By **exocytosis** — vesicles fuse with the presynaptic membrane and empty their contents into the cleft.

**Ca²⁺ enters** → **vesicles fuse** with the presynaptic membrane → neurotransmitter **released by exocytosis** into the cleft.

**Ion channels open**, **Na⁺ enters**, and the postsynaptic membrane **depolarises** (an EPSP).

A **depolarisation** of the postsynaptic membrane caused by neurotransmitter binding — it makes a new impulse **more likely**.

**Presynaptic** = holds vesicles and **releases** neurotransmitter; **postsynaptic** = carries receptors and **receives** it.

It gives a **short diffusion distance**, so the neurotransmitter crosses **quickly** and transmission is **fast**.

**Sodium (Na⁺)** — it enters through channels opened by the neurotransmitter.

A **fast, automatic response** to a stimulus that needs **no conscious thought**.

The fixed nerve pathway a reflex follows: **stimulus -> receptor -> sensory neuron -> relay neuron (CNS) -> motor neuron -> effector -> response**.

The signal takes a **short cut through the spinal cord** instead of travelling to the brain and back.

It **detects the stimulus** and starts a nerve impulse (e.g. a sensory nerve ending in the skin).

A **muscle or gland** that carries out the response (e.g. a muscle that contracts).

A **sensory nerve ending** in the skin.

A **muscle** that contracts to pull the body part away.

**Inside the CNS** — in the grey matter of the **spinal cord**.

The **sensory neuron** (S = sending in).

The **motor neuron** (M = moving out).

**Touch, pressure, texture, stretch or vibration.**

**Temperature** (heat and cold).

**Chemicals** — this is how **taste and smell** work.

**Light** (e.g. the brightness of light entering the eye).

The **iris muscles** — they **contract to make the pupil smaller**, reducing the light entering the eye.