Non-majors A&P · Nervous System
Nervous System: Tissue, Organization & Anatomy
The whole system, from cell to structure: divisions and tissue, the brain and its regions, protection and cerebrospinal fluid, the spinal cord, and the peripheral nerves.
Watch the videoScan for the full lecture on YouTube.
Part 1
Organization and Tissue
The divisions, the neuron, its support cells, and how tissue is arranged.
Overview
What the nervous system does
- Two divisions: the central nervous system (CNS), the brain and spinal cord, and the peripheral nervous system (PNS), the nerves outside it.
- Three jobs: sense (sensory input), integrate (process), and respond (motor output).
- The PNS has a sensory (afferent) arm carrying signals in and a motor (efferent) arm carrying commands out.
- The motor arm splits into somatic (voluntary) and autonomic (involuntary).
Sense it, process it, respond to it.
Nervous system
CNSbrain + spinal cord
PNSnerves outside the CNS
Sensory (afferent)signals in
Motor (efferent)commands out
Somaticvoluntary
Autonomicinvoluntary
Sympathetic
Parasympathetic
The cell
The neuron
- The neuron signals and cannot divide.
- Cell body (soma): holds the nucleus and runs the cell.
- Dendrites receive incoming signals.
- Axon sends the signal to the next cell; the myelin sheath insulates it and speeds the impulse.
Dendrites receive, the cell body decides, the axon sends.
Structure of a neuron
Variety
Types of neurons
- By shape: multipolar (most common), bipolar, and unipolar.
- By job: sensory (toward the CNS), motor (away from it), and interneurons (connect the two inside the CNS).
Most neurons are multipolar; by job they are sensory, motor, or interneurons.
Neuron
By shape
Multipolarmost common
Bipolar
Unipolar
By function
Sensory
Motor
Interneuron
Support
The neuroglia
- Glia support neurons and, unlike neurons, can divide.
- Astrocytes support and help form the blood-brain barrier.
- Oligodendrocytes make CNS myelin; Schwann cells make PNS myelin.
- Microglia clean up debris; ependymal cells help circulate cerebrospinal fluid.
Neurons signal; glia support, insulate, and clean up.
Organization
Gray matter, white matter, and names
- Gray matter is neuron cell bodies; white matter is myelinated axons.
- A bundle of axons is a tract in the CNS and a nerve in the PNS.
- A cluster of cell bodies is a nucleus in the CNS and a ganglion in the PNS.
Gray = cell bodies, white = myelinated axons; the names change between CNS and PNS.
Part 2
The Brain
The cerebrum, the deeper regions, and the brainstem.
Cerebrum
The cerebrum and its lobes
- The wrinkled surface (cortex) folds into ridges (gyri) and grooves (sulci) to pack in more neurons.
- Four main lobes: frontal (movement, planning), parietal (touch), temporal (hearing), occipital (vision); plus the deep insula.
- The corpus callosum is the band of white matter connecting the left and right hemispheres.
One folded cortex, four lobes, two hemispheres joined by the corpus callosum.
The cerebral lobes
Cortex
Functional areas of the cortex
- Primary motor cortex (frontal lobe): commands voluntary movement.
- Primary somatosensory cortex (parietal lobe): receives touch.
- Visual cortex (occipital) and auditory cortex (temporal).
- Broca's area produces speech; Wernicke's area understands it.
Each patch of cortex has a job: move, feel, see, hear, speak.
Cortical functional areas
Deep brain
Diencephalon, cerebellum, and limbic system
- Diencephalon: the thalamus (relay station for sensory input) and the hypothalamus (homeostasis and control of the pituitary).
- Cerebellum: coordinates movement, balance, and posture (it fine-tunes what the cerebrum starts).
- Limbic system: the emotion-and-memory circuit (hippocampus, amygdala).
Thalamus relays, hypothalamus keeps balance, cerebellum coordinates, limbic feels and remembers.
The brain in section
Brainstem
The brainstem
- Three parts, top to bottom: midbrain, pons, and medulla oblongata.
- It connects the brain to the spinal cord and passes all the tracts between them.
- The medulla holds vital centers that control heart rate, breathing, and blood pressure.
The brainstem is the life-support stalk: damage to the medulla is life-threatening.
The brainstem
Part 3
Protection and CSF
The membranes and the fluid that cushion the CNS.
Coverings
The meninges
- Three membranes wrap the brain and cord, outer to inner: dura mater (tough), arachnoid mater (web-like), and pia mater (delicate, hugs the surface).
- The subarachnoid space (between arachnoid and pia) holds cerebrospinal fluid.
- Meningitis is inflammation of these membranes.
Three layers, tough to delicate: dura, arachnoid, pia.
The meninges
Fluid
Ventricles and cerebrospinal fluid
- The brain has four hollow ventricles.
- The choroid plexus in the ventricles makes CSF.
- CSF circulates through the ventricles and the subarachnoid space, cushioning the brain and cord, then is reabsorbed into the blood at the arachnoid villi.
- Too much CSF is hydrocephalus. The blood-brain barrier keeps the changing blood away from brain tissue.
The choroid plexus makes it, the fluid cushions, the arachnoid villi drain it.
Choroid plexusmakes CSF
↓
Ventriclesfour, inside the brain
↓
Subarachnoid spacecushions brain + cord
↓
Arachnoid villireabsorbed into blood
Part 4
The Spinal Cord
The cable between the brain and the body.
Spinal cord
The spinal cord
- Runs from the medulla down to about L1-L2, ending at the conus medullaris; the cauda equina of nerve roots trails below (safe spot for a lumbar puncture).
- Two enlargements (cervical and lumbar) supply the limbs.
- In cross-section: a butterfly of gray matter (cell bodies) inside, wrapped in white matter tracts, the reverse of the brain.
- White matter carries ascending (sensory, to the brain) and descending (motor, from the brain) tracts.
A gray-inside, white-outside cable ending at L1-L2, with the cauda equina below.
Spinal cord cross-section
Part 5
The Peripheral Nerves
Spinal nerves and cranial nerves.
Spinal nerves
The spinal nerves
- 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal.
- Each joins the cord by a dorsal (sensory) root, with the dorsal root ganglion of sensory cell bodies, and a ventral (motor) root; the two merge into a mixed nerve.
- A nerve is wrapped at three scales: endoneurium (each axon), perineurium (each bundle), epineurium (the whole nerve).
- Nerves regroup in plexuses so each limb draws from several spinal nerves.
31 mixed pairs: sensory in the back root, motor in the front root.
A spinal nerve
Cranial nerves
The twelve cranial nerves
Twelve pairs run straight from the brain. Some are sensory (S), some motor (M), some both (B). Mnemonic for the names: "On Old Olympus' Towering Tops, A Finn And German Viewed Some Hops."
| # | Name | Type |
|---|---|---|
| I | Olfactory | S (smell) |
| II | Optic | S (vision) |
| III | Oculomotor | M (eye) |
| IV | Trochlear | M (eye) |
| V | Trigeminal | B (face) |
| VI | Abducens | M (eye) |
| VII | Facial | B (face, taste) |
| VIII | Vestibulocochlear | S (hearing, balance) |
| IX | Glossopharyngeal | B (throat, taste) |
| X | Vagus | B (organs) |
| XI | Accessory | M (neck) |
| XII | Hypoglossal | M (tongue) |
Twelve pairs, numbered front to back; the vagus (X) is the long one that reaches the chest and abdomen.
Part 6
Reflexes
The fast, automatic loop.
Reflexes
The reflex arc
- A reflex is a fast, automatic response that does not wait for conscious thought.
- It runs from a receptor, along a sensory neuron, through an integration center in the CNS, out a motor neuron, to an effector.
Five parts, one quick loop.
Receptordetects the stimulus
↓
Sensory neuronto the CNS
↓
Integration centerCNS processes
↓
Motor neuroncommand out
↓
Effectormuscle or gland responds
Wrap-up
Key takeaways
- The nervous system is the CNS (brain and spinal cord) and PNS (nerves); it senses, integrates, and responds.
- The cerebrum has four lobes with functional areas; the diencephalon relays and regulates, the cerebellum coordinates, and the brainstem runs vital centers.
- Three meninges and the CSF (made by the choroid plexus) protect and cushion the CNS.
- The spinal cord (gray inside, white outside) ends at L1-L2 and carries sensory and motor tracts.
- The PNS is 31 pairs of mixed spinal nerves and 12 pairs of cranial nerves.