Cerebral cortex

• It receives input from the spinal cord, vestibular system (balance), visual system, and proprioceptors (muscle stretch receptors).
• The right cerebral hemisphere tends to dominate perceptual, emotional, visuospatial, and musical processing.
• The left cerebral hemisphere tends to dominate language, logical reasoning, calculation, spatial awareness, and sequential processing.
• The nerve tracts of the spinal cord together with the spinal nerves provide a two-way communication system between the brain and body parts outside the nervous system • Tracts that conduct sensory impulses to the brain are called ascending tracts • The motor impulses from the brain to the motor neurons reaching muscle and glands are called descending tracts
• The face is huge and distorted (enormous lips and tongue) whereas the back is relatively tiny. These distortions occur because the area of sensory cortex devoted to a particular body region is proportional not to the region’s absolute size, but to the number of sensory receptors it contains. Many more cortical neurons are required to process sensory information arriving from the tongue than the back.
• Upper motor neuron - Cell body lies in a CNS processing centre • Synapses on the lower motor neuron • Innervates a single motor unit in a skeletal muscle: – activity in upper motor neuron may facilitate or inhibit lower motor neuron
• Motor pathways can be divided into three main groups: pyramidal tract, extrapyramidal tract, and reticulospinal tract
• Lower motor neuron - Cell body lies in a nucleus of the brain stem or spinal cord • Triggers a contraction in innervated muscle: – only axon of lower motor neuron extends outside CNS – destruction of or damage to lower motor neuron eliminates voluntary and reflex control over innervated motor unit
• Insufficient blood supply to parts of the brain for brief periods causes transient ischemic attacks, temporary disturbances in brain functions
• Hypertension and atherosclerosis (hardening of the arteries from fatty buildup) are the major risk factors for strokes.
• Tonic receptors • Are always active • Show little peripheral adaptation • Are slow-adapting receptors • Remind you of an injury long after the initial damage has occurred
• Phasic receptors • Are normally inactive • Become active for a short time whenever a change occurs • Provide information about the intensity and rate of change of a stimulus • Are fast-adapting receptors
• The frequency and pattern of action potentials contain information about the strength, duration, and variation of the stimulus.
• Your perception of the nature of that stimulus depends on the path it takes inside the CNS.
• First-Order Neuron – Sensory neuron delivers sensations to the CNS – Cell body of a first-order general sensory neuron is located in dorsal root ganglion or cranial nerve ganglion
• Second-Order Neuron – Axon of the sensory neuron synapses on an interneuron in the CNS – May be located in the spinal cord or brain stem
• Third-Order Neuron – If the sensation is to reach our awareness, the second-order neuron synapses on a third-order neuron in the thalamus
• Posterior column pathway • Carries sensations of highly localized (“fine”) touch, pressure, vibration, and proprioception • Spinal tracts involved: – left and right fasciculus gracilis – left and right fasciculus cuneatus.
• Posterior column pathway- Axons synapse on third-order neurons of ventral nuclei of the thalamus; nuclei sort the stimulus according to the nature of the stimulus as well as the region of the body involved; processing in thalamus determines the perception of stimulus; ability to determine stimulus (from thalamus to sensory cortex).
• Tactile discs - Also called Merkel discs • Fine touch and pressure receptors • Extremely sensitive to tonic receptors • Have very small receptive fields.
• Merkel's disk- found in the basal epidermis and hair follicles.
• Meissner corpuscles • Perceive sensations of fine touch, pressure, and low-frequency vibration • Adapt to stimulation within 1 second after contact • Fairly large structures • Most abundant in the eyelids, lips, fingertips, nipples, and external genitalia.
• The cerebrum cortex is composed of six layers: the outermost is the molecular layer, which contains numerous fibres and only a few neuronal cell bodies. - Beneath this is the external granular layer, containing interneurons called granule cells, and glial cells. - The next layer, which is the thickest layer, is the external pyramidal cell layer, composed of pyramidal cells, granule cells and neuroglia. - Beneath this are three more layers of grey matter (internal granular, internal pyramidal, and multiform layer) and white matter (dendrites).
• – Lamellated corpuscles • Also called Pacinian corpuscles • Sensitive to deep pressure • Fast-adapting receptors • Most sensitive to pulsing or high-frequency vibrating stimuli
• The Spinothalamic Pathway – Provides conscious sensations of poorly localized (“crude”) touch, pressure, pain, and temperature
• The Spinothalamic Pathway:

  • First-order neurons: Axons of first-order sensory neurons enter the spinal cord and synapse on second-order neurons within posterior grey horns
  • Second-order neurons: Cross to the opposite side of the spinal cord before ascending; ascend within the anterior or lateral spinothalamic tracts:
  • The anterior tracts carry crude touch and pressure sensations
  • The lateral tracts carry pain and temperature sensations
  • Third-order neurons: Synapse in the ventral nucleus group of the thalamus; sensations are relayed to the primary sensory cortex after being sorted and processed