Nervous Tissue, Organization & Anatomy

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Organization
- CNS: brain and spinal cord (integration and command)
- PNS: cranial and spinal nerves and ganglia
- Sensory (afferent) toward the CNS; motor (efferent) away from it
- Somatic: voluntary, skeletal muscle; autonomic: involuntary, organs
Autonomic
- Sympathetic: fight or flight; raises heart rate, dilates pupils, mobilizes energy
- Parasympathetic: rest and digest; slows the heart, stimulates digestion
- They balance each other to keep organs tuned to the situation
Neuron parts
- Dendrites: receive signals toward the soma
- Soma: cell body and integration center
- Axon: carries the signal away
- Axon terminal: releases neurotransmitter
- Myelin and nodes of Ranvier: insulation and the gaps where the signal jumps
Conduction
- Myelin: fatty insulation that speeds conduction
- Nodes of Ranvier: gaps in the myelin
- Saltatory conduction: the signal jumps node to node, which is much faster than unmyelinated conduction
Structural classes
- Multipolar: many dendrites, one axon (most neurons)
- Bipolar: one dendrite, one axon (special senses)
- Pseudounipolar: one process that splits (most sensory neurons)
Functional classes
- Sensory (afferent): toward the CNS
- Motor (efferent): from the CNS to an effector
- Interneuron: connects neurons within the CNS; about 99 percent of neurons
CNS glia
- Astrocyte: blood-brain barrier and potassium buffering
- Oligodendrocyte: myelinates CNS axons
- Microglia: immune defense in the CNS
- Ependymal: makes and circulates CSF
PNS glia
- Schwann cell: myelinates PNS axons and supports regeneration
- Satellite cell: surrounds and supports cell bodies in ganglia
Matter
- Gray matter: cell bodies, dendrites, and synapses (cortex and nuclei)
- White matter: myelinated axons in tracts; white because of the myelin
Brain regions
- Cerebrum: cognition, sensation, voluntary movement
- Diencephalon: thalamus, hypothalamus, epithalamus
- Brainstem: vital reflexes (midbrain, pons, medulla)
- Cerebellum: coordination, balance, motor learning
Lobes
- Frontal: voluntary motor, executive function, Broca speech
- Parietal: somatosensation and spatial orientation
- Temporal: hearing, language comprehension, memory
- Occipital: vision
- Insula: taste and interoception
Diencephalon
- Thalamus: sensory relay for everything except smell
- Hypothalamus: homeostasis and hormone control via the pituitary
- Pineal gland: melatonin and circadian rhythm
Brainstem
- Midbrain: visual and auditory reflexes; substantia nigra
- Pons: bridge to the cerebellum; helps control breathing
- Medulla: cardiac, vasomotor, and respiratory centers; motor tracts cross here
Spinal cord
- Extent: medulla to L1 or L2; 31 pairs of spinal nerves
- Dorsal horn: sensory input; ventral horn: motor neuron cell bodies
- Enlargements: cervical and lumbar, for the limbs
- Conus medullaris and cauda equina: the tapered end and the nerve roots hanging below it
Coverings and CSF
- Dura, arachnoid, pia (outer to inner); CSF sits in the subarachnoid space
- Choroid plexus makes CSF in the ventricles
- Lateral, then third ventricle
- Cerebral aqueduct to the fourth ventricle
- Subarachnoid space, then arachnoid villi into venous blood
Barrier
- Built by: tight junctions between brain capillary cells plus astrocyte end-feet
- Lets through: lipid-soluble molecules, oxygen, carbon dioxide, glucose
- Keeps out: most drugs, toxins, and pathogens
Clinical reasoning
- Multiple sclerosis: loss of CNS myelin slows and blocks conduction
- Stroke: symptoms localize to the damaged region (speech, vision, movement)
- Hydrocephalus: blocked CSF flow raises pressure in the skull
- Meningitis: inflamed meninges, sampled through CSF