87. CCA Study Notes: Nervous System Structure and Function

The Big Picture

The nervous system controls, directs, and coordinates many body functions. It also carries messages between different parts of the body and the brain.

The nervous system has two main divisions. The central nervous system, or CNS, includes the brain and spinal cord. The peripheral nervous system, or PNS, includes nerves throughout the body.

To understand this system, focus on three big ideas. Know how nerve signals travel, know the major parts of the brain and spinal cord, and know how the nerves outside the CNS connect the body with the brain.

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Course Notes: Key Ideas You Need to Know

What the Nervous System Does

The nervous system controls, directs, and coordinates body functions. It sends information through nerves using signals called nerve impulses.

The system has two main divisions:

Division	Main Parts
Central nervous system, CNS	Brain and spinal cord
Peripheral nervous system, PNS	Nerves throughout the body

The nerves in the body connect with either the spinal cord through spinal nerves or the brain and brain stem through cranial nerves.

Two Types of Nerve Tissue Cells

Nerve tissue contains two main types of cells.

• Neuroglia nourish, protect, and insulate neurons.
• Neurons transmit information as electrical nerve impulses.

A nerve impulse is a signal that travels along a neuron’s membrane, beginning where the impulse is applied. Nerve impulses pass from one cell to another in a chain-like way.

The Three Main Parts of a Neuron

Each neuron has three main parts that take part in moving nerve impulses.

Dendrite

The dendrite contains receptors that receive stimuli involved in sensations such as:

• pain
• cold
• pressure
• heat

The dendrite carries this information toward the cell body. Dendrites usually branch from the neuron’s cell body, although this does not always happen.

Cell Body

The cell body contains smaller cell parts called organelles, including the nucleus.

The electrical nerve impulse passes through the cell body.

Axon

The axon carries information away from the cell toward the axon terminals, which are the ends of the axon.

A simple way to remember the direction is:

1. Dendrite receives information.
2. The impulse passes through the cell body.
3. The axon carries information away from the cell.

How Nerve Signals Move

Nerve impulses have both electrical and chemical features.

Within a neuron: an electrical impulse carries the signal.

Between neurons: chemical processes carry the signal from one neuron to another. Chemical signalling also occurs between a neuron and a muscle cell.

The gap between neurons is called the synapse. An electrical impulse reaching the end of an axon has travelled as an action potential, which is a pulse of electricity involving a temporary reversal of the cell’s polarity.

Neurotransmitters and the Synapse

Neurons do not form one continuous cell network because synapses separate them. Chemical communication allows signals to move across these gaps.

Axon terminals produce and store chemicals called neurotransmitters.

The presynaptic membrane, meaning the membrane before the synapse, releases neurotransmitters. They move across the gap and attach to receptors on the postsynaptic membrane, meaning the membrane after the synapse.

This interaction changes the receiving cell’s membrane. It can slow or speed up the impulse and allows charged particles called ions to move into and out of the receiving cell.

Myelin and Nerve Fibres

Nervous tissue is easily damaged and has a limited ability to regenerate ⓘ.

Some nerve fibres have a protective covering called a myelin sheath. The myelin sheath also insulates the nerve fibre.

Nerve fibres with myelin conduct impulses faster than fibres without this covering.

The Central Nervous System

The central nervous system, or CNS, consists of the brain and spinal cord.

The skull contains and protects the brain. The brain has three main parts:

• cerebrum
• cerebellum
• brain stem

Cerebrum

The cerebrum is the largest part of the brain. It is the centre of thought and intelligence.

It has two halves called hemispheres:

• The right hemisphere controls movement and activities on the body’s left side.
• The left hemisphere controls the body’s right side.

The outer layer of the cerebrum is the cerebral cortex. It contains billions of unmyelinated neurons, also called grey matter.

The cerebral cortex controls high-level brain functions, including:

• reasoning
• memory
• consciousness
• speech
• voluntary ⓘ muscle movement
• vision
• hearing
• sensation
• other activities

Cerebellum

The cerebellum regulates and coordinates body movements.

It helps control:

• smooth voluntary muscle movements
• balance

Injury to the cerebellum results in jerky movements, loss of coordination, and muscle weakness.

Brain Stem

The brain stem connects the cerebrum with the spinal cord.

Important structures within the brain stem include:

• midbrain
• pons
• medulla

The midbrain and pons relay messages between the medulla and cerebrum.

The medulla lies directly below the pons. It controls:

• heart rate
• breathing
• blood vessel size
• swallowing
• coughing
• vomiting

The brain connects with the spinal cord at the lower end of the medulla.

Spinal Cord

The spinal cord lies inside the spinal column, which protects it.

The spinal cord is about 45 cm (18 in.) long. It contains pathways that carry messages to and from the brain and spinal nerves.

Meninges and Cerebrospinal Fluid

Three layers of connective tissue called the meninges cover and protect the brain and spinal cord.

Dura Mater

The dura mater is the tough outer layer. It lies next to the skull and partly protects the brain from movement.

Arachnoid

The arachnoid is the middle layer.

Pia Mater

The pia mater is the inner layer. It adheres directly to the brain and contains blood vessels surrounding the brain.

The arachnoid space lies between the middle and inner layers. It contains cerebrospinal fluid.

Cerebrospinal fluid circulates around the brain and spinal cord. It protects the CNS and cushions shocks that could injure the brain and spinal cord.

The Peripheral Nervous System

The peripheral nervous system, or PNS, includes:

• 12 pairs of cranial nerves
• 31 pairs of spinal nerves

Cranial Nerves

Cranial nerves carry impulses between the brain and the head, neck, chest, and abdomen.

They carry impulses connected with:

• smell
• vision
• hearing
• pain
• touch
• temperature
• pressure
• voluntary muscle control
• involuntary ⓘ muscle control

The vagus nerve is the only cranial nerve with extensive association throughout the entire body.

Spinal Nerves

Different spinal nerves handle sensations and muscle activity in different areas of the body.

These sensations include:

• touch
• pain
• temperature
• pressure

Spinal nerves carry impulses from the skin, extremities, and internal body structures that cranial nerves do not supply.

An extremity is a limb of the body, especially a hand or foot.

The Autonomic Nervous System

Peripheral nerves with special functions form the autonomic nervous system.

This system controls involuntary muscles and certain body functions that occur automatically, including:

• heartbeat
• blood pressure
• intestinal contractions
• glandular secretions

Sympathetic and Parasympathetic Nervous Systems

The autonomic nervous system has two divisions:

Sympathetic Nervous System	Parasympathetic Nervous System
Associated with the “fight or flight” response	Maintains the body’s normal or relaxed condition
Generally tends to speed up functions	Makes processes such as digestion and normal cell metabolism a priority
Stimulated when you are angry, frightened, or excited	More active during the remaining time or after the sympathetic system has been stimulated too long

These two divisions balance each other. The sympathetic system generally, but not always, tends to speed up body functions. The parasympathetic system supports energy conservation.