In one breath
The nervous system has three divisions: the central nervous system (brain and spinal cord), the peripheral nervous system (somatic nerves to skin, muscle and bone) and the autonomic nervous system (nerves to the viscera). The brain is forebrain, midbrain and hindbrain, and the midbrain, pons and medulla together make the brainstem. Whatever it does, the system works in four steps: detect a stimulus, process it, issue a command and carry out the response.
Leads to: Neurons and glial cells · Sensation: general principles · Movement and organization of the motor system
The three divisions
| Division | What it is |
|---|---|
| Central (CNS) | Brain and spinal cord |
| Peripheral (PNS) | Somatic nerves from the cord to skin, muscles, bones |
| Autonomic (ANS) | Fibres from cord and brainstem to the viscera |
The brain
The CNS is one continuous organ: the brain inside the skull, carrying on down the vertebral canal as the spinal cord. The brain has three parts.
| Part | Subdivision | Structures in it |
|---|---|---|
| Forebrain (prosencephalon) | Telencephalon | The two cerebral hemispheres |
| Diencephalon | Thalamus, hypothalamus, metathalamus, subthalamus | |
| Midbrain (mesencephalon) | None | Corpora quadrigemina, cerebral peduncles, substantia nigra, tegmentum, midbrain nuclei |
| Hindbrain (rhombencephalon) | Metencephalon | Pons and cerebellum |
| Myelencephalon | Medulla oblongata |
Brainstem = midbrain + pons + medulla.
What each part does
- Telencephalon: perceiving sensations, cognition, learning and memory, and planning and programming responses.
- Diencephalon: mainly relays information on to the cortex (see Thalamus: nuclei, functions and thalamic syndrome), and controls autonomic and endocrine functions.
- Midbrain:
- the central pattern generator for locomotion;
- the nuclei for the righting reflexes;
- a part of the reticular formation.
- Brainstem as a whole: controls cardiovascular and respiratory functions, motor activities, sleep and wakefulness, and visceral functions.
- Worth knowing, though not in your pages: the diencephalon also includes the epithalamus (pineal gland and habenula), and the metathalamus is the medial and lateral geniculate bodies.
Draw it: parts of the CNS
Draw a side view of the brain running on into the spinal cord. Bracket the cerebral hemispheres and the thalamus region as forebrain, the short stalk below as midbrain, and the pons, cerebellum and medulla as hindbrain. Add a second bracket from midbrain to medulla labelled brainstem, and end the cord at L1.
The spinal cord
- Starts at the base of the skull as the downward continuation of the medulla and ends at the body of the first lumbar vertebra (L1). So in adults it is shorter than the vertebral column.
- Has 31 segments. Each has a motor root and a sensory root, and the roots join to form 31 pairs of spinal nerves, one of each pair on either side.
- Carries all the ascending and descending tracts.
- Conducts both ways:
- up: sensory input that arrives from the periphery by somatic nerves is passed to the brain;
- down: signals from the brain’s motor and autonomic areas are passed to their target structures.
- Your book says the space beyond the end of the cord, especially between L1 and L2, is where CSF is drawn off in lumbar puncture.
- Worth knowing, though not in your pages: the 31 pairs are 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. Below L1 the lower roots run down the canal as the cauda equina.
A slip in your book: the lumbar puncture level
The needle is not put in at L1–L2, where the tip of the cord can still lie. It goes in below the cord, at the L3–L4 or L4–L5 interspace, where only the cauda equina floats in CSF. A line joining the tops of the iliac crests crosses about L4, and that is how the space is found. If you are asked, write L3–L4 or L4–L5.
Peripheral nervous system
- Made up of the somatic nerves that leave the spinal cord to supply skin, muscles, bones and other structures.
- Carries information both ways:
- sensory axons collect it from the periphery and bring it to the CNS;
- motor neurons take signals from the CNS out to muscles and related structures.
- Worth knowing, though not in your pages: the 12 pairs of cranial nerves also belong to the PNS.
Autonomic nervous system
- Nerve fibres that begin in the spinal cord and brainstem and supply the viscera. They reach the organs by travelling with the somatic and cranial nerves.
- Two divisions, sympathetic and parasympathetic, which act reciprocally to regulate visceral functions.
- Worth knowing, though not in your pages: sympathetic outflow is thoracolumbar (T1–L2) and parasympathetic outflow is craniosacral (cranial nerves III, VII, IX and X, and S2–S4). The enteric nervous system of the gut wall is sometimes counted as a third division.
Functions of the nervous system
Its three big jobs:
- Communication between parts of the body, and integration of the functions of different systems.
- Regulation of body functions.
- Coordinated interaction between the body and its surroundings.
Neural control is the main controlling mechanism for many body processes. It works through neurons, which are built to carry information fast from one part of the body to another.
Four processes
| Step | What happens |
|---|---|
| 1. Recognition | Stimulus detected; sent to the CNS as nerve impulses |
| 2. Processing | CNS processes it; the sensation is perceived |
| 3. Command | CNS generates a suitable command signal |
| 4. Response | Effectors respond as the command directs |
Recognising the stimulus
This is how we come to know our surroundings, through the sensations.
- Sensation is the most basic recognition of a stimulus. A stimulus is any change in the physical energy around us that can be perceived, and sensory detection begins when one is applied.
- Receptors transduce that energy into action potentials in sensory neurons. The body has receptors of different modalities, each detecting its own kind of change.
- Stimuli they detect: mechanical, chemical, photic (light), auditory (sound), thermal and electrical.
- Sensory neurons carry the action potentials from the receptors to the CNS along sensory pathways (see Sensation: general principles and Sensory receptors and their classification).
Processing and the command signal
- The sensory signal is first relayed in the thalamus, then processed in the sensory cortex, and then turned into other signals in the brain’s sensory networks.
- The processed signals are integrated into command signals by detailed planning and programming.
- The command goes to the effector organs, which carry out the plan.
- The brain learns from sensory input and stores what it learns in memory for later use.
Responses
- Motor: movement of body parts.
- Autonomic: a change in visceral function.
- Behavioural: a change in how the person behaves.
A response can be internal (a change in cognition or behaviour) or external (movement).
Exam-answer skeleton: "Name the parts of the CNS and give their functions" (short note)
- The three divisions of the nervous system: CNS, PNS and ANS.
- CNS = brain + spinal cord. The brain’s three parts and their subdivisions, as a table.
- Forebrain: telencephalon and diencephalon, with what each does.
- Midbrain: its structures; the locomotor pattern generator and righting reflexes.
- Hindbrain: pons, cerebellum and medulla; the brainstem and its functions.
- Spinal cord: its extent (to L1), 31 segments and pairs of spinal nerves, all the tracts, two-way conduction.
- A labelled diagram of the parts of the CNS.