19.1 Behavioral Neuroscience: Neuroanatomy & Function

19.1 Behavioral Neuroscience: Neuroanatomy & Function

Quick Answer: The nervous system divides into the central nervous system (CNS) — brain and spinal cord — and the peripheral nervous system (PNS), which links the CNS to the body. Knowing which branch a structure belongs to matters because PA-CAT Behavioral Sciences items test where a lesion produces a given deficit.

The PNS splits into the somatic division (voluntary: afferent sensory plus efferent motor to skeletal muscle) and the autonomic division (involuntary). The autonomic nervous system further divides into the sympathetic (thoracolumbar outflow, "fight-or-flight") and parasympathetic (craniosacral outflow, "rest-and-digest") branches. A third autonomic component, the enteric system, governs gut motility and secretion semi-independently.

Brain Structures and Functional Localization

The cerebrum is divided into four lobes whose primary functions are testable:

• Frontal lobe — motor control (precentral gyrus, Brodmann area 4), expressive language (Broca's area, area 44, left-dominant), and executive functions (planning, working memory, judgment, impulse control).
• Parietal lobe — somatosensory cortex (postcentral gyrus, areas 1, 2, 3) and spatial integration.
• Temporal lobe — auditory cortex, Wernicke's receptive language area (area 22), and medial temporal memory circuit.
• Occipital lobe — vision (primary visual cortex V1, area 17, at the calcarine sulcus).

This lobar map is the cortical homunculus: motor and sensory representations are inverted, with disproportionately large areas devoted to the hands, face, and tongue — the parts requiring fine control.

The limbic system — hippocampus, amygdala, mammillary bodies, anterior thalamic nuclei, cingulate gyrus — supports emotion, memory consolidation, and motivated behavior. The hippocampus converts short-term to long-term declarative memory; bilateral damage produces anterograde amnesia (patient H.M. is the classic case). The amygdala assigns emotional valence to stimuli and is central to fear conditioning.

The brainstem comprises the midbrain, pons, and medulla. It contains the reticular activating system (arousal and consciousness), cranial nerve nuclei, and autonomic centers controlling respiration and cardiovascular function. The cerebellum coordinates movement timing, balance, and motor learning; it compares intended with actual movement and corrects error. The basal ganglia (caudate, putamen, globus pallidus, subthalamic nucleus, substantia nigra) gate voluntary movement via direct (facilitatory, D1 receptor) and indirect (inhibitory, D2 receptor) pathways; dopamine from the substantia nigra pars compacta balances these loops. Degeneration produces hypokinetic (Parkinsonism) or hyperkinetic (Huntington, chorea) disorders.

Spinal Cord Tracts

A Brown-Séquard (cord hemisection) lesion causes ipsilateral loss of fine touch/proprioception and motor control (dorsal column + corticospinal, not yet crossed) with contralateral loss of pain/temperature (spinothalamic, already crossed at entry). This crossed pattern is a classic localization question.

Cranial and Peripheral Nerves

The 12 cranial nerves serve the head, neck, and some viscera:

• Purely sensory: I olfactory, II optic, VIII vestibulocochlear
• Motor: III oculomotor, IV trochlear, VI abducens, XI accessory, XII hypoglossal
• Mixed: V trigeminal, VII facial, IX glossopharyngeal, X vagus

The vagus (CN X) is the longest parasympathetic pathway, supplying the heart, lungs, and gut to the splenic flexure. Peripheral nerves are organized into plexuses (cervical, brachial, lumbar, sacral); the phrenic nerve (C3–C5; "C3, 4, 5 keeps the diaphragm alive") is clinically critical because cord injury above C3 paralyzes breathing.

The Neuron

Meninges, Ventricles, and Blood Supply

The CNS is wrapped in three meninges: dura mater (tough outer), arachnoid (web-like middle, with the subarachnoid space holding CSF), and pia mater (delicate inner, adherent to brain). The ventricular system produces and circulates cerebrospinal fluid (CSF): lateral ventricles → foramen of Monro → third ventricle → cerebral aqueduct → fourth ventricle → subarachnoid space (via foramina of Luschka and Magendie) → reabsorbed at the arachnoid granulations. CSF is produced by the choroid plexus at about 500 mL/day. Obstruction anywhere along this route causes hydrocephalus.

Cerebral blood supply comes from the internal carotid arteries (anterior circulation) and vertebral arteries (posterior circulation, joining as the basilar artery). The circle of Willis connects them. The anterior cerebral artery (ACA) supplies medial frontal and parietal cortex (leg motor/sensory area — ACA stroke causes leg-predominant weakness); the middle cerebral artery (MCA) supplies lateral frontal, parietal, and temporal cortex (face and arm areas, plus Broca's and Wernicke's in the dominant hemisphere — MCA stroke is the most common and causes face/arm weakness and aphasia); the posterior cerebral artery (PCA) supplies the occipital lobe and medial temporal lobe (PCA stroke causes visual deficits and memory impairment).

Spinal Cord Organization

The spinal cord has 31 segments (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal), each giving rise to a pair of spinal nerves. The cord terminates as the conus medullaris at L1–L2 in adults; below this, nerve roots form the cauda equina ("horse's tail"). Lumbar puncture is performed at L3–L4 or below to avoid the cord. The cord has central gray matter (butterfly-shaped: anterior horn motor neurons, posterior horn sensory interneurons, lateral horn sympathetic preganglionics in T1–L2) surrounded by white matter tracts.

The Neuron

The neuron is the functional unit: a cell body (soma), dendrites (input), and an axon (output), insulated by a myelin sheath (CNS oligodendrocytes; PNS Schwann cells) that speeds conduction via saltatory propagation between nodes of Ranvier. Action potentials follow the all-or-none principle; conduction velocity scales with axon diameter and myelination. The synapse uses neurotransmitters to cross the cleft; common examples include glutamate (excitatory), GABA (inhibitory), dopamine, serotonin, norepinephrine, acetylcholine, and histamine. Synaptic plasticity — the ability of synapses to strengthen or weaken — is the cellular foundation of learning and memory (covered in section 19.4).